Proper Gas Vent Design
If a gas vent is not properly designed, acidic condensate from the vent gas can easily form on the inside of the vent pipe and cause rusting and pitting. As the corrosion process continues, the vent pipe becomes riddled with holes and ultimately fails. When this happens, vent gas containing carbon monoxide and all the other products of combustion can leak into occupied areas of the building.
Low-efficiency gas appliances such as older 78 percent AFUE furnaces rarely have such a condensate problem because the flue gas exits with a relatively high temperature, making condensation difficult to occur. However, many furnaces and heaters have an efficiency just below 83 percent and the flue gas exits much cooler. These are known as Category I type gas appliances, and they are much more prone to developing unwanted condensate in vents and connectors. Because of the many dangers related to acidic condensate, the sizing of gas vents is extremely critical for Category I-type gas appliances.
Furnaces with efficiencies 83 percent or greater are specially designed to handle condensation. These appliances, typically classified as type II, III, and IV, also require proper vent sizing, but they are much more forgiving than Category I appliances. Condensation is expected to occur in some of these appliances, and safeguards are in place to handle it.
The document governing proper gas vent design is published by the National Fire Protection Agency as NFPA 54: National Fuel Gas Code [NFGC] 2006 edition. This NFGC document relies heavily on venting tables developed by the Gas Research Institute. The corresponding international version of this document is IFGC-06: International Fuel Gas Code 2006 Edition, published by the International Code Council.
Many factors affect the required vent and connector sizes for gas appliances. The appliance input Btuh rating, outlet diameter of the appliance, number of elbows, single or multistory application, vent gas temperature, ambient air temperature, vent wall conductivity, vent connector type, vent thermal mass, vent pressurization (fan-assisted or natural draft), vent height, and vent lateral length all affect vent design. The NFGC manual provides a voluminous set of venting tables that take all these factors into account so that both minimum and maximum allowable vent and connector sizes can be specified for any given situation.
DESIGNING THE PROPER SYSTEMThe first step in designing a vent system entails selecting the vent and connector material types. The vent connector is the pipe that connects the appliance to the main vent pipe. A vent connector can be either single-wall metal pipe or Type B, which is double-wall metal pipe with an insulating effect. The main vent can never be made of single-wall metal pipe. It must be either Type B pipe, tile lined masonry chimney, or a flexible metal liner.
Selection of single-wall metal or Type B vent connectors is mainly governed by cost and applicable restrictions. Single-wall metal vent connectors are lower in cost than Type B connectors, but they operate at much higher surface temperatures than do Type B connectors. Consequently, there are many restrictions on the use of single-wall metal connectors.
For example, single-wall metal connectors cannot be used in attics due to the fire hazard, and they must have greater clearances on all structural components than Type B connectors. A good strategy for maximum safety and minimum chance of violating building code requirements is to use only Type B double-wall metal vent connectors.
If a chimney is not being used for the vent, NFGC specifies that only Type B pipe can be used for the vent. If a chimney is involved, it must have an appropriately sized tile liner or a flexible metal liner. Some chimneys appear to have a tile liner, but upon close inspection, the liner may only exist at the top of the chimney.
Venting combustion gases through a chimney without a proper liner often results in the collapse of the chimney as condensate can dissolve the mortar between bricks. Even if a tile liner is in place along the entire interior of the chimney, it is also possible that it may be too large for the appliances being vented through it. If there is any doubt as to the presence, size, and quality of a tile liner in a chimney, a flexible metal liner of the correct size should be used.
Once the vent connector and vent types are decided on, the proper sizes can be read from the NFGC venting tables. In the simplest case with only a single appliance, the information needed to size the connector and vent includes whether the appliance venting is fan-assisted or natural draft, the appliance input Btuh rating, appliance outlet diameter, lateral distance from the appliance to the vent, height from the appliance to the top of the vent, and the number of 90-degree elbows. For the given data, the venting tables list the minimum- and maximum-size vent and connector diameters that can be used.
Sizes from the vent tables can be easily read in a straightforward fashion. However, the size from the table must often be adjusted according to over 30 notes and exceptions explained in the manuals. For example, for every 90-degree elbow beyond the quantity two, the Btuh capacity for a vent size must be reduced by 10 percent. Similarly, there are maximum horizontal connector length limits. The Btuh capacity of a vent must be reduced 10 percent for each multiple of a specified horizontal connector length.
There are many such qualifiers to the venting tables and making sure all of them are honored is the main difficulty of vent sizing. It is not uncommon for a vent design to need three or more adjusting factors. This is particularly true for multiple appliance and multistory applications.
AVAILABLE COMPUTER PROGRAMSUnlike for standard HVAC calculations, there are not many computer programs available for venting design. In fact, at this time there are only two automated solutions available to do all the table look-up values and all the adjustments for special conditions. Exhausto has a Web-based application program called FanCalc2005 that can perform many of these calculations. To access the software, the user must contact a local Exhausto representative. More details are available at www.fancalc.com.
The other automated solution is a stand-alone Windows program from Elite Software called GasVent. A designer can quickly enter the information into GasVent and instantly see the minimum and maximum allowable vent sizes with all the checks and adjustments automatically done. See details on GasVent at www.elitesoft.com.
Both GasVent and FanCalc 2005 are very graphic and visually intuitive with detailed help provided for every input item. On the GasVent screen, all of the required input dimensions are clearly labeled so it is easy to know what to enter. Calculation results are instantly displayed on the same screen.
There is no question that gas vent sizing can be quite technical and tedious. While software can’t remove all the risks in vent design and installation, it can definitely help you create more accurate designs in much less time.
Publication date: 08/06/2007