A draft hood is a device built into an appliance, or made a part of the vent connector from an appliance, that is designed to: 1. Provide for the ready escape of the flue gases from an appliance in the event of no draft, backdraft or stoppage beyond the draft hood; 2. Prevent a back draft from entering the appliance; and 3. Neutralize the effect of stack action of the chimney or gas vent upon operation of the appliance. Crossover burner Most modern gas-fired furnaces consist of several individual heat exchanger sections with a gas burner inserted in each section. In this arrangement, all burners must fire almost simultaneously or there will be danger of unburned gas flowing into the Trane Co. combustion chamber, which could cause a delayed ignition. It is the inherent nature of many warm air systems to have flame roll out on ignition. Many times, a step-opening gas valve is used; this type of gas valve will come on at a lower fire. This is accomplished by coming on initially at a lower gas pressure of 0.7–0.9" WC, then after about 10 seconds going up to full pressure, 3.5"–4" WC. Simultaneous ignition can be provided by installing a crossover burner (sometimes called a lighter tube or runner) across the top of the main burners near the port area. The illustrations in Figure 5 and Figure 6 (next page) show the types of crossovers used by Trane gas-fired furnaces. Atmospheric gas boilers are less likely to have these lightoff Figure 5: Crossover located perpendicular to and across the top of main burners near the port area problems. The open configuration of the boilers helps with safe light-off of all the burners. Figure 4: Illustration of the required air for every cubic foot of natural gas that is every 1,000 BTUs ICM/March/April 2015 25
ICM September-October 2014
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