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Venting Today - A Complex Subject

by Tim McElwain

Part 1 of 2

The days of the old galvanized flue pipe vented into a masonry chimney lined or unlined are quickly fading away. It will not be long unit even Type “B” double wall gas vent will be a thing of the past.

With increased efficiencies in equipment the requirements for “special venting” are now in place, and venting requirements for 90+ efficiency products are likely to continually change.

Warnings like this in Installation and Operation manuals are commonplace:

WARNING

FAILURE TO VENT THIS BOILER IN ACCORDANCE WITH THESE INSTRUCTIONS COULD CAUSE FLUE GAS TO ENTER THE BUILDING RESULTING IN SEVERE PROPERTY DAMAGE, PERSONAL INJURY, OR DEATH:

• Do not attempt to vent this boiler with galvanized, PVC, or any other vent system not listed in the instructions.
• Do not attempt to mix components from different approved vent systems.
• Do not obtain combustion air from within the building.
• Do not install a barometric damper or drafthood on this equipment.

CAUTION

Moisture and ice may form on the surface around the vent termination.

In talking to manufacturers reps about problems they discover on new installations they tell me that most are improperly vented due to failure to read the installation directions. We all know what INSTALLATION INSTRUCTIONS are…we use them to set our coffee cups and soda cans on so we don’t mark the equipment!

I am not going to attempt to teach Category IV 90+ Modulating Condensing venting here. I simply want to point out some of the typical things to watch out for. It is first of all imperative that directions be followed. Here are some of the things most manufacturers have in their instructions.

A lot of equipment today will list several different ways to vent their product:

1. Horizontal Side Wall Concentric Vent – the vent exits the building through the outside wall. Concentric pipe is a “pipe within a pipe”. Flue gas exits through the inside pipe, air for combustion is drawn in around the space between the inner and outer pipe.

2. Horizontal Side Wall Double Pipe (Separate flue gas pipe out and another pipe for air in)

3. Vertical Double Pipe vent exits the building through the roof. (Separate flue gas pipe out and another pipe for air in)

In some cases they may be direct vent or they may be classified mechanical exhausting. There are different requirements for those as to termination outside the building. We will cover some of those later in one of the other parts of this article.

Two basic categories:

1. Concentric

2. Double Pipe (Separate flue gas pipe out and another pipe for air in)

There are usually requirements for maximum vent and air intake lengths as well as in some cases minimum lengths.

Vent terminals vary with manufacturers and are not interchangeable from manufacturer to manufacturer.

The most difficult and often the most troublesome is the vent termination itself. This will be covered next month in Part 2 of Venting Today – A Complex Subject.

These technical manuals are now available:

• Circuitry and Troubleshooting Volume I and Volume II. $75.00 for Volume I and $35.00 for Volume II + $10.00 S/H.

• New Edition of FUNDAMENTALS OF GAS VOLUME I. $75.00 + $10.00 S/H.

• FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. $75.00 + $10.00 S/H.

We also conduct seminars on the following topics and many others:

• Fundamentals of Gas

• Circuitry and Troubleshooting

• Hydronic Controls

• Electric Ignition Systems

• Advanced Electric Ignition Systems

• Powerpile Systems

• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

For information call Tim McElwain at 401-437-0557, email him at gastc@cox.net, or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915

Venting Today - A Complex Subject

by Tim McElwain

Part 1 of 2

One of the most difficult and often troublesome aspects of the new venting when venting out the side of buildings or even through the roof is vent termination or vent location. When we go to the code books it gets very detailed for example in the National Fuel gas Code ANSI Z223.1/NFPA 54 section 12.3.5 for direct vent appliances and concerning termination of direct vent section 12.9.3.

For Mechanical Exhausting the rules for termination are different as per section 12.4.3.6 concerning no vent terminating less than 7 feet above grade where located adjacent to public walkways.

Many times the Direct Vent rules and the Mechanical Exhausting rules get mixed up and incorrect venting termination occurs. Section 12.9 Through the Wall vent Termination covers all aspects of both direct vent (12.9.3) and mechanical exhausting (12.9.1 and 12.9.2) We will address those in Part Three of this article.

Some of the typical requirements:

Vent terminal must be at least 1 foot from any door, window, or gravity inlet into the building.
The double pipe (Example: The vent and air intake terminals must be at the same height and their center lines must be between 12 and 36 inches apart. Both terminals must be on the same wall)
All terminal bottoms must be 12 inches above normal snow line or no less than 12 inches above grade. (Note: it is often difficult to determine normal snow line)
7 feet above public walkway
Do not install directly above windows or doors
The bottom of the vent terminal must be at least 3 feet above any forced air inlet located within 10 feet.
A horizontal distance of at least 4 feet between the vent terminal and gas meters, electric meters, regulators and relief equipment. Do not install vent terminal over this equipment dues to condensate.
Do not locate vent under decks.
Top of vent terminal must be at least 5 feet below eves, soffits, or overhangs. Maximum depth of overhang is 3 feet.
Vent terminal must be 6 feet from an inside corner.
Be aware that condensate may freeze and cause damage to structures nearby.
Install vent termination away from prevailing winds in excess of 40 MPH.
Air intake must not be near possible combustion air contaminants.

These are some of the termination rules for sidewall venting. When vertical termination is allowed there are other rules these will be covered in Part Three in next month’s Sales Bytes.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.
We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling.

We also conduct seminars on the following topics and many others:

Fundamentals of Gas
Circuitry and Troubleshooting
Hydronic Controls
Electric Ignition Systems
Advanced Electric Ignition Systems
Powerpile Systems
NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Venting Today - A Complex Subject

by Tim McElwain

Part 3 of 5

This is the final article on the minimum venting including requirements and exclusions.

As was mentioned in Part two we want to address the code requirements for vent termination. We are looking at the National Fuel Gas Code ANSI Z223.1/NFPA 54 2006 version and also the International Fuel Gas Code 2006 version.

This section will cover some of the code requirements, which must be addressed by manufacturers when setting down the rules for installation. The code rules can be added to but never made less severe in their application. It is however important note that manufacturers installation requirements have precedence.

In the NFPA 54 code this is addressed in Section 12.9 on page 54-102. In the IFGC section 503.8 page 85. There is also a reference to and Appendix “C” which is an illustration showing the application of some of these rules.

To keep it simple we will refer to the NFGC section 12.9, keep in mind the international code in this instance is exactly the same as the national. I have also removed the metric measurements for simplicity.

12.9 Through the Wall Vent Termination.

Section 12.9 provides requirements for separation of the termination point of the venting systems from the building openings for venting systems that terminate through the side of the building. The concern is for recirculation of products of combustion back into the building. There is also the possibility of cross contamination of the fresh intake air for combustion to the equipment.

12.9.1 A mechanical draft venting system shall terminate at least 3 feet above any forced air inlet located within 10 feet.

Exception No. 1: This provision shall not apply to the combus¬tion air intake of a direct-vent appliance.

The intent of 12.9.1 is to prevent gases from being drawn back into the building. This requirement recognizes that vent gases are lighter than air. Exception No. 1 recognizes that direct vent appliance inlets do not communicate with air in a building.

Exception NO.2: This provision shall not apply to the separation of the integral outdoor air inlet and flue gas discharge of listed outdoor appliances.

Exception No. 2 to 12.9.1 prevents confusion in the installation of outdoor gas appliances. Some authorities have misinterpreted the code to prohibit such appliances or to require them to be modified in the field, which is not the intent of 12.9.1. An example of this type of appliance is a packaged rooftop air conditioner, which incorporates a gas vent and a circulation air inlet used for building air supply.

12.9.2 A mechanical draft venting system of other than direct-vent type shall terminate at least 4 feet below, 4 feet horizontally from, or 1 foot above any door, operable window, or gravity air inlet into any building. The bottom of the vent terminal shall be located at least 12 inches above grade.

A question that often comes up concerning section 12.9.2 is do the separation requirements for exit terminals apply to windows that do not open? In the 1992 revision to the code the word “window” was replaced with “operable window”. This came about as a question pertaining to picture windows, which do not open. So the section does not apply to any windows, which cannot be opened.

12.9.3 The vent terminal of a direct-vent appliance with an input of 10,000 Btu/hr or less shall be located at least 6 inches from any air opening into a building, and such an appliance with an input over 10,000 Btu/hr but not over 50,000 Btu/hr shall be installed with a 9 inch vent termination clearance, and an appli¬ance with an input over 50,000 Btu/hr shall be at least a 12 inches vent termination clearance. The bot¬tom of the vent terminal and the air intake shall be located at least 12 in. (300 mm) above grade.

Sections 12.9.1 and 12.9.2 are concerned with preventing equipment combustion products from being drawn into a building through fresh air inlets, including operable windows (windows which can be opened).

Subsection 12.9.3 permits the vent terminals of direct vent appliances to be located much closer to air inlets than is provided for with mechanical draft equipment. There is often a mis application of rules for mechanical draft to direct vent equipment. The vent gases from direct vent equipment disperse rapidly upon leaving the vent terminal, even when the terminal is located under an open window. However, a window is unlikely to be open when heat is needed.

12.9.4 Through-the-wall vents for Category II and Category IV appliances and noncategorized condensing appliances shall not terminate over public walkways or over an area where condensate or vapor could create a nuisance or hazard or could be detrimental to the operation of regulators, relief valves, or other equipment. Where local experience indicates that condensate is a problem with Category I and Category III appliances, this provision shall also apply.

Subsection 12.9.4 provides for the protection of persons and equipment, including gas meters. It places responsibility on the installer to locate vent termination for Category II and Category IV appliances away from walkways and gas equipment. It also recognizes that any appliance can present a condensation problem in a cold climate.

High-efficiency condensing appliances have a seasonal efficiency of 90 percent or higher, which reduces vent gas temperatures to a point where the water vapor produced as a product of combustion condenses to liquid water in the appliance or in the vent. These condensing appliances carry a vented appliance category of Category IV. This type of appliance produces much cooler vent gases, resulting in water condensing in the vent. Venting must be accom¬plished with a fan, because the vent gases are not hot enough to operate the natural draft vent. Water will condense in the vent and will dissolve some of the gases produced during combustion, which are slightly acidic. The vent materials used with these appliances must be able to resist the acidic condensate. For many of these Category IV appliances, plastic vent material is acceptable and preferred for corrosion reasons. The appliance manufacturer specifies the vent material for use with Category IV appliances.

The advantage of high-efficiency appliances is that they significantly reduce the amount of gas consumed with no loss in output. A mid-efficiency appliance uses one-third less gas than a conventional appliance, and a condensing appliance uses only one-half of the gas of a conventional appliance. The savings in fuel are offset by higher first cost and the higher maintenance requirements of high-efficiency appliances, as well as the added cost of the electricity to operate the fan.

In the next part (Part 4) we will address other requirements for venting in particular vertical venting.

In part 5 we are going to address the venting categories mentioned in this article.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.
We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling.

We also conduct seminars on the following topics and many others:

• Fundamentals of Gas
• Circuitry and Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Venting Today - A Complex Subject

by Tim McElwain

Part 4

With this continuing discussion on venting of Modulating/Condensing equipment it is once again not the purpose of this article to define exact installation requirements. The attempt here is to address some typical types of requirements for venting this equipment.

When vertical venting is an option on this equipment it is important to follow manufacturers rules as to EQUIVALENT LENGTH as it relates to air intake of vent. An example would be:

90-degree concentric elbow 4.5 feet
45-degree concentric elbow 4 feet
A 3” 90 degree elbow 5.5 feet
A 3” 45 degree elbow 4 feet

Many times failure to observe these equivalent lengths in calculating the maximum length of the vent can result in inadequate air supply or problems with the pressure switch not making. It is often a good idea if the system will not work with the vent connected to remove the vent temporarily and see if the equipment will operate with it disconnected. If it will and it will not work with it connected you obviously have a problem with the vent. One of the things to address is length and also equivalent lengths of fittings. There may also be other problems such as blockage or damage to the vent.

There may also be other requirements addressed in the following examples:

Permitted Terminals for Vertical Venting - A straight termination is installed in the end of the vent pipe. The air inlet terminal consists of a 180-degree elbow (or two 90 degree elbows) with a rodent screen. Vent manufacturer part numbers for these screens vary.

Vertical Vent Terminal Locations - Observe the following limitations on the location of all vertical vent terminals.

• The top of the vent pipe must be at least 2 feet above any object located within 10 feet.

• The vertical distance between top of the vent and air inlet terminal openings must be at least 12".

• The bottom of the air inlet terminal must be at least 12" above the normal snow accumulation that can be expected on the roof.

• The air intake terminal must be located on the roof and must be no further than 24" horizontally from the exhaust pipe.

Wall thimbles - Wall thimbles are often required where single wall vent pipe passes through combustible walls.

Pitch of Horizontal Piping - Pitch all horizontal piping so that any condensate, which forms in the piping, will run towards the boiler:

• Pitch horizontal concentric venting 5/8" per foot

• Pitch Stainless steel venting 1/4" per foot.

Supporting Pipe - Vertical and horizontal sections of pipe must be properly supported:

• Support concentric venting near the female end of each straight section of pipe.
• Support stainless steel venting as called for by the vent manufacturer's instructions.

The next article on this topic will cover some of the code requirements, which must be addressed by manufacturers when setting down the rules for installation. The code rules can be added to but never made less severe in their application. It is however important to note that manufacturers installation requirements have precedence.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.

We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling.

We also conduct seminars on the following topics and many others:

• Fundamentals of Gas
• Circuitry and Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Venting Today - A Complex Subject

by Tim McElwain

Part 5

You can use the chart below to determine the equipment’s venting category. Look at the equipment’s rating plate and then proceed down that column to determine requirements and perimeters for venting that particular category. For example, a listed Category I appliance can be vented using a Type B gas vent, chimney, single-wall metal pipe, chimney lining system that is listed for gas venting, or a special gas vent listed for the appliance. The chart refers to appliance Category I through Category IV. The categories are based on vent temperature and pressure.

The term non-positive vent (negative pressure) means that even if fans or blowers are used in an appliance or vent system, venting is accomplished by natural draft. Natural draft is created by temperature difference (delta T) and the height of the vent. (The vent pressure is lower than atmospheric).

The term positive vent pressure means that fans, blowers or other means are used to propel vent gases through the vent at above atmospheric pressure.

Note that the definitions reference "a vent gas temperature that may cause excessive condensate production in the vent.") A specific temperature is not provided, because it is not the same for all appliances. The ANSI Z21 standards for appliance categorization can be referenced for this information. Annex L contains a complete list of the ANSI Z21 standards. Note that the installer should not need this information, since the appliance vent category is included in the appliance manufacturer's installation instructions and on the appliance nameplate. The criteria in the ANSI Z21 standards are based on a flue loss of 17 percent of total energy. The 17 percent flue loss is the same flue loss built into the vent sizing tables for fan-assisted appliances. In this way, the standards ensure that the appliance will work properly with the vent system. The term nonpositive vent pressure means that the pressure in the vent will be lower than the surrounding atmosphere if the vent system meets the requirements of the National Fuel Gas Code Chapter 12 and Chapter 13. The incorporation of a fan into the appliance does not always mean that the vent pressure is positive. If unsure, check the appliance nameplate or manufacturer's instructions for the venting category, or check the vent pressure with a manometer or other pressure gauge when the appliance is operating.

VENTING CATEGORIES:

GAS-FIRED EQUIPMENT

Operating

characteristics

Category I

Category II

Category III

Category IV

Pressure in the vent

Negative

Negative

Positive

Positive

Temperature of

Above

Below

Above

Below

vent gas (4)

275°F

275°F

275°F

275°F

Annual efficiency

Below 84%

Above 84%

Below 84%

Above 84%

Condensation

Not acceptable

Possible

Possible

In heat

(1)

(in vent)

(3)

exchanger

Design requirements

Gas (air) tight vent

No

No

Yes

Yes

Corrosion-resistant

No (1)

Yes

Possible

(3)

Yes

vent (water tight)

Vent into

Permitted

No

No

No

masonry chimney

(1) and (2)

Combined venting

Permitted

No

No

No

Condensate drain

No

Ask

Possible

(3)

Yes

manufacturer

(At

equipment)

Source of

N.F.G.C.

Manufact.

Manufact.

Manufact.

information

Fuel gas code,

literature

literature

literature

heating equipment

and vent system

manufacturers

NOTE 1 Usually, there is no problem when high vent gas temperature equipment is vented into double-wall vent or into a lined masonry chimney; but condensation could occur if mid-efficiency (80% to 84%) mechanical draft equipment is vented into a vent that has highly conductive walls, cold walls, or massive walls. In this case, design a vent system that minimizes the wall losses (use double-wall pipe for the whole run and avoid long runs through cold spaces).

NOTE 2 Install either a rigid or flexible metal liner inside of the masonry chimney and use a double-wall connector when venting mid-efficiency (80% to 84%), me¬chanical draft equipment.

NOTE 3 Condensation in the vent is posing on the ambient temperature and the conductivity of the vent walls. In this case, design a vent system that minimizes the wall losses (use insulated pipe and avoid long runs through cold spaces). A corrosion-resistant flue and a drain may be required if condensation cannot be prevented - refer to the manufacturer's recommendations.

NOTE 4 The dewpoint of the vent gas depends on the fuel (natural or LP gas), the amount of excess air and the amount of dilution air. The limiting case occurs when the dewpoint of the vent gas is at a maxi¬mum, which is about 135°F. This maximum is produced when natural gas is burned with no excess air or dilution air. There¬fore 275°F = 135°F dewpoint + 140°F

It is a good rule of thumb when looking at vent temperature that any temperature from 300° (F) down is a concern for condensing and needs to be addressed in the case of Category I and III venting.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.

• Fundamentals of Gas
• Circuitry and Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Venting Today - A Complex Subject December 2007 issue

by Tim McElwain

Part 6

Plastic Piping:

• The Code
• Product Recall
• The Effects of Temperature on PVC pipe

National Fuel Gas Code specifies that plastic piping used for venting appliances listed for use with such venting materials shall be approved.

Before the introduction of high-efficiency (90 + percent efficiency) gas utilization equipment, plastic piping was prohibited as a vent material. High-efficiency (Category IV) appliances reduce vent temperatures, resulting in condensate formation. As accumulation of condensate can become a source of corrosion of metal vents, plastic piping became the preferred material. Paragraph 12.5.2 of the National Fuel Gas Code requires that plastic vent materials be used for listed gas utilization equipment only when specified in the manufacturer's instructions.

Section 12.5.3 of the National Fuel Gas Code Special Gas Vent

Special gas vent shall be listed and installed in accordance with the special gas vent manufacturer's installation instructions.

All special gas vents are listed vent materials. Special gas vents are listed in accordance with UL 1738, Standard for Venting Systems for Gas-Burning Appliances, Categories II, III and IV. Installation instructions for special gas vents include limitations on operating temperature, categories of appliance to be used with each vent, clearance to combustible materials, types of fittings and joint sealant to be used, and vent termination requirements.

Special attention should be given to the following areas:

1. Proper support for the special gas vent to prevent sagging and to allow for expansion, contraction, and condensate drainage
2. Proper cutting and cleaning of joints and fittings, and the use of recommended joint sealants (substitutes are not usually permitted)
3. Construction of a condensate trap (see the appliance manufacturer's instructions for special requirements)
4. Wall penetrations (the pipe should not be secured at a thimble, because the pipe must be allowed to move to accommodate expansion and contraction)
5. Insulation [the vent pipe or the fittings of the inside of a wall thimble must not be insulated when polymeric (nonmetallic) vent materials are used]

Product Recall

More than 15 years ago, a class of special gas vent known as "high temperature plastic vent" (HTPV) was introduced to the market for use with mid-efficiency appliances. Field experience has shown that these vent systems are prone to failure. The failure may occur because of improper installation practice and/or corrosion from acidic condensate. At this time an active product recall is still under way, with the cooperation of the U.S. Consumer Product Safety Commission, appliance manufacturers, and the vent pipe manufacturers. The product recall covers furnaces that are horizontally vented, as well as all boiler installations. Those who encounter one of these vent systems should call (800) 758-3688 for information on how to proceed. This number is operated by the product manufacturers and will be in operation until the recall is substantially complete. If the number is not in operation, questions can be referred to the furnace, boiler, or water heater manufacturer.

To determine whether the installation has an HTPV pipe system that is subject to this program, the vent pipes attached to the natural gas or propane furnaces or boilers should be checked. Vent pipes subject to this recall program can be identified as follows:

• The vent pipes are plastic.
• The vent pipes are colored gray or black.
• The vent pipes have the names "Plexvent," "Plexvent II," or "Ultravent" stamped on the vent pipe or printed on stickers placed on pieces used to connect the vent pipes together.

The location of those vent pipes should also be checked. For furnaces, only' HTPV systems that have vent pipes that go through the side walls of structures (horizonta1 systems) arc subject to this program. Other plastic vent pipes, such as white PVC or CPVC are not involved in this program.

The Effects of Temperature on PVC Pipe

Polyvinyl Chloride (PVC) is a thermoplastic, and as such, its physical properties change with temperature variations Dimensions, pressure capacity, and stiffness are all affected by temperature changes. The published dimensions and performance ratings for PVC pipe and conduit products are usually applicable only for 73°F. The following will help to explain how PVC pipe and conduit products are affected by operating temperatures other than 73°F.

Dimensions

Like all materials, PVC expands with increasing temperatures and contracts with decreasing temperatures.

The coefficient of thermal expansion for PVC is: 3.0 x 10-5 in/in/°F

Because the length-to-diameter ratios of PVC pipe and conduit products are generally very large, length change from temperature variation is the most noticeable. A good rule of thumb in design of PVC pipe and conduit systems is to allow 3/8" length variation for every 100 feet of pipe for each 10°F change in temperature. (This rule is independent of pipe size.) Table 1 can also be used to determine the effects of temperature changes on the length of PVC pipe and conduit.

NOTE:

THE MAXIMUM RECOMMENDED OPERATING TEMPERATURE FOR PVC PRESSURE PIPE IS 140° (f)

FOR CPVC IT IS 200° (F)

This is reason enough to pay very close attention to flue gas temperatures on a lot of 90+ condensing equipment.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.
We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling.

We also conduct seminars on the following topics and many others:

• Fundamentals of Gas
• Circuitry and Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Venting Today - A Complex Subject January 2008 issue

by Tim McElwain

Part 7

More Plastic Piping:

As I continue my research on venting materials and venting issues I find more and more a concern of the use of plastics for venting. This discussion was to be only a few articles when I started but has surely grown as I look further into this issue of Special Venting.

I solicited some comments from Glenn Stanton the Manager of Technical Development for Burnham Hydronics and U.S. Boiler Co., Inc. His comments are included here as to PVC venting.

The concerns expressed lie with several factors. The first of these being that PVC and ABS Pipe and fitting manufacturers do not necessary express approval for using these products for venting gas byproducts and the ASTM standards hold no mention regarding the use of venting and PVC. One or two companies that manufacture PVC pipe and fittings have put out statements to the effect that they do not recommend using it for venting gas combustion byproducts. The second issue is that we, you or nobody else can guarantee that the pipe your distributor stocks is solid core or foam core. Some distributors stock one or the other based on primary demand and if they did stock both that is no guarantee that they will have solid core every time you order or that what is shipped to you is solid core. There is a very strong likelihood of foam core pipe ending up in many of these applications due to those reasons. The third issue is that PVC is certified and approved for 140°F temperature for "pressure applications". Mod/Con boilers can in fact exceed these temperatures during high fire conditions with Domestic Hot Water demands. The boilers have flue gas sensors that know this is happening and will turn down the input or modulate to protect the pipe. Reducing the boiler input when it needs to recover the heater will lengthen that DHW recovery period. To what extent that happens is a function of boiler sizing and demand? I too have seen a couple of manufacturers that approve CPVC for the first several feet. But the real question is.... How many distributors do you deal with that actually carry 3" and 4" CPVC pipe and fittings in stock? The fourth factor is there is more and more discussion these days about this topic and the trend of the governing agencies uses verbage referencing "Vent Systems". We see this with AL29-4C, Polypropylene and Ipex. They are indeed "vent systems" that have been tested and certified for venting applications. As mentioned, Canada has taken action to discontinue the use of PVC and ABS and that any material used for venting must be a "system" that conforms to their standards. Our stance with our Freedom ™ CM and CHG boilers is to use either AL29-4C or Polypropylene vent systems. These systems are tested and certified for venting purposes and have temperature tolerances far in excess of what you would expect with these boilers. That's my story and I'm sticking to it..... and I will be willing to bet that many other equipment manufacturers will begin to see it that way too as thing progress in
Canada.

Then some recent information from Viessmann President Harald Prell add to the discussion.

Venting of Residential Viessmann Gas-Fired Condensing Heating Boilers

At present, there are industry discussions in regards to venting of gas-fired condensing heating boilers. The following represents the opinion and position of Viessmann Manufacturing in regards to venting Viessmann gas-fired condensing heating boilers, series Vitodens 200 and Vitodens 100.

Since the mid 80's, we manufacture and sell gas-fired condensing heating boilers; now sold in more than 30 countries, including North America. In almost all countries, PPS (polypropylene) is mostly used in coaxial vent-type applications or stainless steel for single wall venting systems. PPS is suitable for a steady flue gas temperature of 250° F (121 ° C) and for short-term exposure up to 280° F (138° C). Stainless steel vent pipe is suitable for 550° F (288° C); typically, an SA240 316 L material is used or the higher grade in North America AL29-4C.

Due to the fact that gas-fired condensing boilers are being vented, not only the temperature of the flue gas becomes an important factor but also, in combination with the extreme high moisture content, the associated acidity (pH level) of the flue gas condensate and the extreme temperature exposure to outdoor conditions need to be considered when selecting materials.

PPS, as well as stainless steel venting systems, have successfully been in use for many years and carry independent certifications for venting these types of heating boilers properly.

CPVC material is certified for 90° C (194° F) and PVC is certified for 65° C (149° F) according to ULC-S636 Standard for Type BH Gas Venting Systems. The Viessmann gas-fired condensing heating boiler Vitodens 200 and 100 series are approved for use with listed CPVC material.

IPEX (the manufacturer of CPVC venting systems) informed that this material is now readily available. Should there be supply issues, please contact your Viessmann sales representative or Viessmann directly- (in Canada at 1-800-387-7373 or in the U.S. at 1-800-288-0667).

My Comment: This next information I find very interesting and certainly does not only apply to Viessmann equipmen only.

The flue gas temperature exiting a gas-fired condensing heating boiler depends on a number of factors; some impact more than others:

1. The maximum allowable supply water temperature rating on the heating boiler or the maximum adjustable aquastat or limit settings. The Vitodens 200 is limited to a max. supply water temperature of 75° C (167° F) and the Viessmann Vitodens 100 series is limited to a max. water supply temperature of 80° C (176° F), plus cut in and cut off differential.

2. Venting with a coaxial vent pipe system, where fresh outside air moves around the PPS pipe, preheats the combustion air and cools flue gas temperature further.

3. The heating boiler utilizing one separate vent pipe and a separate fresh air intake pipe.

4. Boiler utilizes standard room air for combustion air or outside air directly.

5. Flue gas velocity within vent system (vent length and restriction).

6. Possible vent restrictions (partial icing or blockage of vent terminal and/or air intake).

7. Excessive wind and pressure impact on terminations.

8. Possible partial heat exchanger flue gas passageway blockage.

9. Cycling frequency pending on control strategy, system water flow, zoning, etc.

After all of the above is considered, is there a safety margin left? After all, reliability and dependability for heating comfort are key factors in our opinion.

The main factor influencing flue gas temperature is however the return water temperature to the heating boiler. This is the primary influence on how high the flue gas temperature will go and exit into the actual flue pipe.

Both models of Viessmann heating boilers are certified to ANSI Z21.13 CSA 4.9 Low Pressure Steam and Hot Water Heating Boiler Standard by CSA. The test procedure within this particular standard calls for a boiler water supply temperature maintained until the limit control functions ± 3° C (± 5° F). When the boiler is tested under this criteria and a very low return temperature is selected (by the manufacturer), it will drive the flue gas temperature extremely low. Typically the flue gas temperature on both Viessmann heating boilers is between 5° C (9° F) and 15° C (27°F) above the return water temperature; therefore, for example, with a low return water temperature selection of 27° C (80° F) into the boiler, a flue gas temperature of 42° C (107° F) would be the net result. This flue gas temperature would not pose a problem in general for any type of PVC or ABS material; however, this test with a very large temperature differential of 55° C (100° F) between supply and return is not realistic. Also, at that temperature differential, the flow rate through the boiler would only be 20% of the actual required flow for a typical 99911 ° C (20° F) hydronic system design temperature differential; again, not realistic in an everyday install.

Example under full input - design condition:

If the boiler water supply temperature would be 82° C (180° F), provided the boiler is certified to that temperature, then one would typically assume a temperature differential of 11 ° C (20° F) and therefore the return water temperature would return back at 71 ° C (160° F) to the heat exchanger. The dew point of natural gas is 57° C (135° F) at sea level.

The boiler would not condense anymore and the stack temperature would certainly be higher than the return water temperature of 71 ° C (160° F). It would probably reach the 85° C (185° F) to 88° C (190° F) mark.

This operating condition now clearly shows flue gas temperatures higher than what the limit is on standard PVC, CPVC and ABS. Even if the heating boiler has a limit at 71 ° C (160° F) set for the boiler water supply temperature and an 11 ° C (20° F) spread to the return water temperature, the return temperature would still be 60° C (140° F). the flue gas temperature could exceed the maximum listed temperature limits.

Especially when heating boilers are utilized to provide domestic hot water through an indirect ¬fired domestic hot water storage tank, return temperatures back to the boiler, when the tank temperature reaches 60° C (140° F), will rarely be less than 60° C (140° F) due to obvious reasons and higher flue gas temperatures will be the result again.

In our evaluation for suitable vent pipe material, we have looked at the following data:

This, in our technical opinion, renders standard PVC, CPVC and ABS unfit for venting a Viessmann Vitodens 200 or Vitodens 100 series gas-fired condensing heating boiler. Also, the Canadian Gas Fired Equipment Installation Code CSA B 149.1 has already prohibited the use of this material. Furthermore to our knowledge, no manufacturer of DWV (drains, waste, vent) PVC had it certified or recommends this material as a vent pipe for gas-fired condensing heating boilers.

DWV approved materials are mostly used in sewer and drainage applications within buildings or below ground. These materials are not subject to this type of temperature (erratic temperature changes) with constant pH levels between 3 and 4, and none of these materials are typically exposed to long periods of ultraviolet light or severe cold outdoor temperatures, when used in vent terminations where all the foregoing conditions occur at the same time. In addition, the expansion factor of the pipes has to be considered, possible change of support, including feasibility of adhesives and terminations.

As high limits and low water cutoffs are safety measures for the pressure vessels, so is venting the safety measure for disposing flue gas.

A venting system will become part of a home. It may get enclosed in a wall or a ceiling and therefore not be easily accessible in the future for inspection.

For the reasons mentioned above, Viessmann does not recommend and is not certified on the Vitodens 200 and Vitodens 100 series with DWV PVC, CPVC or ABS, even though our listed efficiencies are some of the highest in the industry.

PVC, CPVC and ASS may only be used for the combustion air intake side when a separate pipe is used for the combustion air intake system and the separate flue gas pipe is constructed utilizing stainless steel or CPVC ULC-S636.

We recommend, and are approved for, venting and combustion air intake with a coaxial PPS / aluminum venting system for combined venting and combustion air supply.

For separate flue gas venting, both heating boilers are approved with CPVC 90° C (194° F) according to ULC-S636 or AL29-4C stainless steel venting, in combination with DWV PVC, CPVC, ASS or even galvanized sheet metal ducting for the separate air intake for combustion air. Of course, CPVC, ULC-S636 or stainless steel may also be used for the air intake side.

We have opted not to employ a fixed flue gas temperature sensor, with manual reset, to avoid boiler lockout when high flue gas temperatures occur to protect a venting system, in order not to subject our customers to boiler lockouts in the main heating season when higher water temperatures are required to maintain the heating comfort. Instead, we have selected, in our opinion, the correct venting material for the Vitodens 200 and Vitodens 100 series.

For more information, please contact the technical department at Viessmann in Waterloo 1-800-387-7373 or Warwick 1-800-288-0667.

Stay tuned as we look for more information on special venting.

We are now offering our latest manuals Circuitry and Troubleshooting Volume I and Volume II. They are selling for $75.00 for Volume I and $35.00 for Volume II + $10.00 shipping and handling.
We have two OTHER NEW manuals in their second printing. The first one is titled FUNDAMENTALS OF GAS VOLUME I it is priced at $75.00 a copy + $10.00 shipping and handling. In addition we also have FUNDAMENTALS OF GAS Volume II, which covers “Air for Combustion” and “Venting” it is up to date with the latest changes to the Fuel Gas Code Book. It is also being offered at a price of $75.00 + $10.00 shipping and handling.

We also conduct seminars on the following topics and many others:

• Fundamentals of Gas
• Circuitry and Troubleshooting
• Hydronic Controls
• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

Gas Appliance Service Training and Consulting
22 Griffith Drive
Riverside, RI 02915
E-mail gastc@cox.net

Air for Combustion and Ventilation a New Subject- Part One February 2008 issue

by Tim McElwain

After our recent series on Venting and in particular “Special Venting” it is also important to address the need for air for combustion and ventilation. On many of my consulting calls I find that the contractor or technician has overlooked the importance of insuring adequate air for combustion. It is often the case that the homeowner has done some remodeling and has overlooked the fact that what was once a full open basement is now cut in half or even worse the boiler or furnace along with the water heater are now in a small tightly confined space with absolutely no provision for air. Here is an example of what is a full basement and what air requirements there are.

PROBLEM:

RANCH HOUSE 25' X 40' with cellar ceiling 8' high 40 x 25 x 8 = 8,000 cubic feet of space
NATIONAL FUEL GAS CODE: 50 cubic feet of air for every 1,000 btu's

8000
50 = 160 x 1,000 = 160,000 btu's

House Heater = 100,000 Btu's
+ Water Heater = 35,000 Btu's

+ Dryer = 30,000 Btu's

= 165,000 btu’S

For all practical purposes because the Btu's exceed 160,000 this is a CONFINED SPACE and air must be brought from outdoors.

So as we will see in these articles more air may be needed.

The changes to the 2002 National Fuel Gas Code reflected the need for more careful calculations for Combustion Air. The procedures remained pretty much the same in the 2006 code. The revision provides five different procedures for supplying combustion air:

1. 100 percent indoor air

2. 100 percent outdoor air

3. A combination of indoor and outdoor air

4. An engineered system

5. A mechanical air supply.

The objective it seems is to provide technically based methods for calculating required indoor air volume for all structures, including those with very low air infiltration.

In Figure 1 from AGA the different methods are shown.

Figure 1 Methods of determining Air for Combustion

In order to make the code more flexible there are now two ways to calculate the required indoors air volume. This is especially important when a building is designed for low air infiltration.

The method that has been in the code for many years is retained and is called the Standard Method (50 cubic feet of volume per 1,000 Btu). This method can be used for buildings that have a 0.40 air change per hour (ACH) or higher air infiltration rate. The new method, Known Air Infiltration Rate Method (KAIR), can be used when the structure's air infiltration rate is blown or estimated, or when the structure has less than 0.40 ACH.

An important feature of KAIR is that it recognizes the difference in combustion air volume requirements between the two appliance types, fan-assisted appliances and other than fan-assisted appliances, which is "all other appliances." The amount of required indoor air for each equipment type is different due to the different amounts of dilution air that each requires. If installations include both equipment types, required air volume is calculated separately for fan-assisted and for other than fan-assisted appliances.

Prior to the 1984 edition, the code requirements for indoor combustion air did not provide guidance for "unusually tight construction." In the 1984 edition, a requirement was added stating that if indoor combustion air was used and if the construction was unusually tight, outside combustion air had to be provided. This requirement addressed valid concerns that buildings were being built to minimize air infiltration since the energy crisis of the 1970s. Although this addition provided no guidance on what unusually tight construction is, the addition provided a positive step toward addressing the meaning of the term.

In the 1988 edition, a definition of unusually tight construction was added, which included three items:

1. Walls and ceilings exposed to the outside atmosphere have a continuous water vapor retarder with a rating of 1 perm or less with openings gasketed or sealed.
2. Weather stripping has been added on open able windows and doors.
3. Caulking or sealants are applied to areas such as joints around window and doorframes, between sole plates and floors, between wall-ceiling joints, between wall panels, at penetrations for plumbing, electrical, and gas lines, and at other openings.

A further revision to the definition was added in the 1999 edition, adding a fourth item to the definition of unusually tight construction

4. The building has an average air infiltration rate of less than 0.35 air changes per hour.

The fourth item provided additional guidance and changed the definition to not automatically include all new construction but to base the need for outdoor combustion air on the actual air infiltration rate.

The 2002 edition of the code goes further by eliminating the definitions of unusually tight construction, confined space, and unconfined space and providing two methods to determine the amount of indoor volume needed for buildings with varying air infiltration rates. The code is also now clearer as to what conditions are needed to mandate the use of additional outside combustion air, which many jurisdictions had not applied uniformly. Some jurisdictions considered all new structures built to a model or state energy code to be inherently unusually tight.

The terms confined space and unconfined space were eliminated because they no longer denote one single value (50 cubic feet per 1,000 Btu) but varying volumes, depending on the method used and the amount of air infiltration. A new term, required volume, is used to better describe the revised code's intent. Required volume does not require a definition.

Other changes include the following:

The combination outdoor/indoor combustion air provisions have been simplified and revised to allow the use of a single high outdoor opening. The section was simplified to improve its applicability. The former coverage was nearly impossible to follow and explain. The revised coverage allows the use of a one single high mounted outdoor opening when the combination outdoor/indoor combustion air openings method is used. The previous code only allowed the use of two outdoor openings, one high and one low, which severely restricted the usefulness of this method. Because previous GRI research demonstrated the viability of the single high outdoor opening (for all air from the outdoors), it is logical to extend its feasibility to the combination of outdoor/indoor coverage adopted by the code in 1999.

Three new combustion air tables were added to provide users with the calculated values without the need to perform calculations for the required air volumes under the Standard and KAIR methods.

The importance of combustion, dilution, and ventilation air, as shown in Figure 1 cannot be overemphasized. Typical gas-fired natural draft (draft hood) furnaces require approximately 21cubic feet of air (i.e., combustion, vent dilution, and ventilation) for every cubic foot of gas burned. Although modern fan-assisted combustion system furnaces do not need dilution air, they still require approximately 15 cubic feet of air for each cubic foot of gas burned. This amount of air can range between 1500 cubic feet per hour and 2100 cubic feet per hour for each 100,000 Btu/hr of gas input. This air is needed to support the combustion and venting process of the appliance and to provide ventilation cooling for the casing and internal controls. This instruction will help the reader determine the proper method for meeting these requirements. (See Figure 3)

It is important to note that due to all of the changes in gas equipment in the last few years that air for combustion requirements must change to meet these new requirements. Certainly increased efficiencies and the use of fan assisted combustion systems prompted these changes in order to meet the demands of this new equipment. A training session conducted by the Gas Research Institute at the New England Gas Association Operations School conducted at Bentley College in Massachusetts in 1994. At that session they introduced changes that resulted from research they conducted and reported in 1994. The report noted that a single opening to the outdoors could be used if certain qualifications were met. We will discuss this in detail later in this instruction. The 1996 code implemented this one opening procedure as part of the code. Then the (1999) added some language to the code that allows the reduction of the two openings to outdoors. This can be accomplished if the indoor space in which the equipment is located can contribute part of the needed air.

Air for combustion, ventilation, and dilution of flue gases for gas equipment installed in buildings is to be obtained by applying one of the methods covered in Figure 1. When indoor air is not sufficient then one of the other methods must be applied.

There are some exceptions to these requirements, they are that they cannot be applied to direct vent appliances. The other exception is Type I (residential) dryers that are provided with make up air. There will be a special section concerning make up air for dryers and information on air requirements and the affect of dryers on combustion air.

All equipment other than Category I and natural draft appliances must have combustion, ventilation, and dilution air. The procedures for providing that air will be covered in the manufacturers directions. The determination as to what appliances are being referred to are those with power vents, either a part of the equipment or added on to the equipment, clothes dryers, and high efficiency furnaces (above 95%).

All equipment must be installed so as to not interfere with proper circulation of air.

If draft hoods or barometric are used they must be in the same room as the equipment. This is to prevent the possibility of different pressures.

A provision should be made for make up air to compensate for exhaust fans, attic fans, kitchen ventilation systems, clothes dryers and fireplaces, The operation of these systems can create a difference in pressure in the building. The result of that pressure difference is that combustion gases may spill into the dwelling from the draft hood or barometric creating a hazardous condition. This in conjunction with insufficient air for combustion and ventilation can cause incomplete combustion. The result being flame rollout and the possibility of high levels of Carbon Monoxide.

This instruction contains provisions intended to provide adequate combustion and ventilation air for gas appliances based on an air infiltration rate of one-half air changes per hour. Because of increased interest in energy conservation, consumers are attempting to reduce heat losses by adding weather-stripping, caulking, and insulation to their residences. Although these measures are effective in reducing energy consumption, they also can reduce the air infiltration rate of the building to levels below those used to determine proper combustion air. In these situations, additional air must be provided for proper operation of the equipment. In these tight buildings, the operation of exhaust fans can create negative pressure in buildings. Air will enter the building through any opening available, and the vent of a gas appliance may provide the easiest available opening. An example of this is shown in Figure 4.

Figure 4

• Fundamentals of Gas
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• Electric Ignition Systems
• Advanced Electric Ignition Systems
• Powerpile Systems
• NEW COMBUSTION TESTING DESIGN GAS EQUIPMENT

If you are interested in information call 401-437-0557 or write to:

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Riverside, RI 02915
E-mail gastc@cox.net

Air for Combustion and Ventilation a New Subject - Part Two March 2008 issue

by Tim McElwain

Indoor Combustion Air

The required volume of indoor air shall be determined in accordance with the Standard Method or the Known Air Infiltration Rate Method except that where the air infiltration rate is known to be less than ACH; the KAIR method shall be used. The total required volume shall be the sum of the required volume calculated for all appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Indoor Opening Size and Location are considered a part of the required volume.

The Standard Method has been in the code since the 1950s and remains unchanged. It is limited to buildings with an air infiltration rate of 0.4 air changes per hour (ACH) or more. A new method has been added to the 2002 code, KAIR Method, which can be used to calculate the required indoor volume for all structures, including those built with very low air infiltration rates.

The code does not require the determination of the structure's air infiltration. The Standard Method can be used for many installations. However, if the structure's ACH is known to be less than 0.40 ACH, then the Standard Method for obtaining combustion air cannot be used, and the KAIR Method must be used.
It may be difficult to determine air infiltration in many cases. The National Fuel Gas Handbook has a procedure in Supplement 5, which is patterned after the ASHRAE Method. Another alternative based on building blue prints could be used. There could also be a pressurization test conducted, it is commonly called “a blower-door test.” ASHRAE standard 62 recommends that buildings have a 0.35 air change with at least 15 CFM per person.

IS THERE ENOUGH AIR FOR COMBUSTION?

This is the real question and sadly the need for air in the equipment room is sadly overlooked. It is assumed that infiltration air will take care of the requirements of the equipment. The area in which the equipment is located must be -measured and calculations made to determine if there is enough air for combustion.

The immediate need is to determine if there enough air for combustion. A simple in house test can be conducted to determine adequate air for equipment operation. It is a step-by-step process as follows:

1. In as much as is practical close all doors, windows etc. into the space in which the equipment is located.
2. Bring on all the equipment in the house, which has a fan operation such as clothes dryer, range hoods, exhaust fans, bathroom fans, attic fans to operate at maximum speed.
3. Close the fireplace damper.
4. Bring on the boiler or furnace in question. The equipment should be operating at peak efficiency no floating flames or flashback. If the equipment is a two-stage gas furnace or boiler bring it on to high fire.
5. Test for spillage of flue products at the draft hood of the equipment. This can be done with a draft gauge, flame from a match or candle, or smoke from a cigarette, cigar or pipe.
6. Bring on all the other fuel gas burning equipment within the area of the boiler or furnace to full input and recheck at draft hoods of all the equipment for spillage.
7. Return all equipment, doors windows etc. to normal position. If you have a question as to whether sufficient air is being supplied this test should assist you in determining if the requirements for air for the equipment installed are being met. When an appliance is located in a confined space Figure 3 illustrates some of the things that must be considered.

STANDARD METHOD

UNCONFINED SPACES

Unconfined spaces will normally have enough natural infiltration of air. If the construction is unusually tight, air from outdoors will be needed. The minimum dimension of rectangular air ducts shall not be less than 3 inches. The screening to cover the openings to the outside shall not be less than 1/4 inch mesh and shall not be capable of closure. Insect screening and louvers reduce effective area. In calculating the free area of air openings, consideration must be given to the blocking effect of louvers and grilles. If the free area through a design of louver or grille is known, it should be used in calculating the size opening required to provide the free area specified. If the design and free area is not known, it may be assumed that wood louvers will have a 20 to 25 percent free area and metal louvers and grilles will have a 60 to 75 percent free area. The louvers and grilles must be fixed in the open position or interlocked with the equipment so that they are opened automatically during equipment operation. The return air, circulating for reheating by the furnace should be isolated from the air supply coming from outside thru these ducts. In other words you cannot get your air for combustion and return air for the furnace from the same room. This will be covered in detail later. Let us look at an example of how calculations may be applied.

PROBLEM:

RANCH HOUSE 25' X 40' with cellar ceiling 8' high 40 x 25 x 8 = 8,000 cubic feet of space

NATIONAL FUEL GAS CODE: 50 cubic feet of air for every 1,000 btu's

8000
50 = 160 x 1,000 = 160,000 btu's

House Heater = 100,000 Btu's

Water Heater = 35,000 Btu's

Dryer = 30,000 Btu's

165,000 btu’S

For all practical purposes because the Btu's exceed 160,000 this is a CONFINED SPACE and air must be brought from outdoors.

EQUIPMENT LOCATED IN UNCONFINED SPACES

Equipment located in buildings of unusually tight construction shall be provided air for combustion, ventilation, and dilution of flue gases using the methods described. An unconfined space, which is shown in Figure 5, is defined as follows:

Space, Unconfined. For purposes of the code, a space whose volume is not less than 50 cubic feet per 1,000 Btu/hr of the aggregate (sum total) input rating of all appliances installed in that space. Rooms communicating directly with the space, in which the appliances are installed, through openings not furnished with doors, are considered a part of the unconfined space.

THIS Requirement IS SOMETIMES REFEREED TO AS THE 1/20 RULE, BECAUSE 1000 BTU/HR DIVIDED BY 50 CUBIC FEET OR, FOR EACH CUBIC FOOT OF ROOM VOLUME, YOU CAN INSTALL 20 BTU/HR.

The requirement for additional air for combustion, ventilation, and dilution recognizes that the definition of confined space came from research on buildings of normal construction tightness. When unusually tight construction is used, additional air is usually required, and the code provides guidance to local code authorities for determining when outside combustion and ventilation air is necessary. In Figure 5 with 8,000 cubic feet of space this would allow up to 160,000 Btu's before the space would be considered confined. Therefore, there is no requirement for outside air. It is important to remember to check for negative pressure caused by mechanical exhausting, fans, fireplaces, clothes dryers, bathroom fans, kitchen range or oven hoods, attic fans and any other fans in the building etc. If you have negative pressure then additional air will be required.

AIR FOR COMBUSTION

ROOM SIZE:

LENGTH _____ WIDTH _____ HEIGHT _____

TOTAL _____ CUBIC FEET

Mechanical Code is 40 cubic feet per 1,000 Btu's

National Fuel Gas Code is 50 cubic feet per 1,000 Btu's

CUBIC FEET = X 1,000
40

CUBIC FEET = X 1,000
50

MAXIMUM BTU PER SPACE _____

IF THE AMOUNT OF BTU'S PER SPACE IS GREATER THAN THIS AMOUNT THEN IT IS A CONFINED SPACE

TABLE 1

WORKSHEET 1

In unconfined spaces in buildings of other than unusually tight construction infiltration can be adequate to provide air for combustion, ventilation and dilution of flue gases.

It was however necessary to add item (4) to the 1999 code (4) the building has an average air infiltration of less than 0.35 air changes per hour. The concept of providing additional air to an unusually tight building was first introduced in 1974. In 1988 a definition was added. Since that time it can be considered normal in most new residential construction. It has become apparent in recent years that problems exist with combustion air obtained through relying on infiltration air. It was important therefore to add the air change per hour criteria to provide guidance when it is determined that a building contains the first three construction details. There may however still be enough air through infiltration to provide combustion air. This recognizes that there may be enough combustion air available due to the features of a buildings design and construction. The amount of air may be affected by the design of windows and doors in the building and how well the building is constructed. We know that air infiltration of a building occurs mainly as a result of pressure differences caused by winds on the building or on the building interior being at a lower pressure than the atmosphere. The temperature inside the building also affects it. The air change rate of 0.35 was selected to correspond to the definition of an unconfined space (50 cubic feet per 1000 Btu/hr), which is known to provide sufficient air for combustion and ventilation. This is illustrated in Figure 6 air infiltration and construction type.