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Part One

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 that 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 required. As we head toward efficiencies of 90+ venting requirements drastically change.

Warnings like this in Installation and Operation manuals are commonplace.



· 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.


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. Right up front in many cases it is obvious they did not read the 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. What I am going to list 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.

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 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 a later article.

As was mentioned 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 combustion 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 appliance with an input over 50,000 Btu/hr shall be at least a 12 inches vent termination clearance. The bottom 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 accomplished 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 2) we will address other requirements for venting in particular vertical venting. 

In a later article we are going to address the venting categories mentioned in this article.

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