Arkansas HVACR NewsMagazine November 2018

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liner in a chimney, a flexible metal liner of the appropriate size should be used. Once the vent connector and vent types are decided on, the proper sizes can be read from the IFGC vent 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 used. For the given data, the vent 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 straight forward fashion. However, the size from the table must often be adjusted according to over 30 notes and exceptions explained in the IFGC manual. For example, for every 90 degree elbow beyond the quantity two, the Btuh capacity for a vent size must be reduced by 10%. Similarly, there are maximum horizontal connector length limits. The Btuh capacity of a vent must be reduced 10% for each multiple of a specified horizontal connector length.

There are many more such qualifiers on using the vent 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 multi-story applications. Fortunately, the table look-up values and all the adjustments for special conditions have been computerized. Elite Software has created the only computer program on the market today for this purpose, and it is called GasVent . Sizing vents is fast and easy with the GasVent software. A designer can quickly enter the information and instantly see the minimum and maximum allowable vent sizes, with all the checks and adjustments automatically done. GasVent is very graphic and visually intuitive with detailed help provided for every input item. As can be seen in the sample 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 to perform. While software can't remove all the risks in vent