Gas/Propane Basics Continued from p. 20 Coleman Co., Inc. Figure 1: Position of the locknut and adjusting screw on a Coleman gas fired furnace is my experience that—like natural gas—it should also be all blue. Combustion air adjustment Figure 1 is just one example of how to adjust the primary air on a burner: 1. Light the pilot burner. 2. Turn up the setting on the room thermostat until the burner comes on. 3. Allow the main burners to run for 10 minutes to warm up. 4. Loosen the locknut on the adjusting screw. 5. Turn adjusting screw in (clockwise) until the yellow tip appears in the flame. 6. Turn adjusting screw out (counterclockwise) until the yellow tip just disappears. 7. Hold the adjusting screw and tighten the locknut. 8. Repeat steps 4–7 on all the other burners. Another example of a different style air adjustment is Figure 2, a Fedders gasfired furnace. A locking screw is located above the primary air opening on each gas burner. The primary air adjustment procedure is as follows: 1. Loosen the locking screw at the base of the burner. 2. Adjust the air shutter opening to a position that gives a slight yellow tip on the end of the flame. 3. Open the air shutter until the yellow tip disappears. 4. Tighten the locking screw. 5. Repeat steps 1–4 for each of the burners. Another example, Figure 3, is on a Carrier furnace. All the burners are adjusted at once when an adjustment is made to the end burner. After the primary air adjustment has been made, check to see that all the burners are level and securely in their brackets, if provided. The burner flames should be uniform and centered in the heat exchanger. The flames should all be blue and quiet. The hot portion of the flame should not be hitting (impinging) on any cooler Carrier Corp. Figure 3: Primary air-shutter assembly on a Carrier furnace surfaces. On most design equipment, the secondary air is controlled by fixed openings built into the equipment. These openings will accomplish bringing in sufficient secondary and excess air as long as all the doors are in place. Before allowing the furnace or boiler to continue operating, you must check to see that it has the proper draft. This can be done by passing a match along the draft-hood opening. If the vent is drawing (pulling) properly, the match flame will be drawn inward (i.e., into the draft hood). If you have a draft gauge or combustion analyzer, measuring actual draft is the better way. This draft should fall in the range of a -0.01" water column (WC) to a maximum of -0.03" WC. If the furnace is not receiving proper draft, the products of combustion escaping the draft hood will extinguish or blow the flame away from the draft hood. The draft must be corrected before operating the equipment. As shown in Figure 4 (opposite page) more than 15 cubic feet of combustion gases per cubic foot of gas actually go up the flue passage in the case of conventional natural gas furnaces and boilers. To the 15 cubic feet of combustion gases, about an equal volume of cooler room air (dilution air) is added. This will cause the flue gases leaving the equipment to slow down and stabilize the combustion process. This helps to accomplish what are called the three “Ts” of combustion: time (how long are the products allowed to stay in the heat exchanger to accomplish good heat transfer), temperature (hot flue gases at a temperature above 300°F and with a stable draft allow the maximum heat transfer to take place) and turbulence of the flue gases to ensure better molecular wiping action for good heat transfer. Fedders Corp. Figure 2: Primary air adjustment on a Fedders gas-fired furnace 24 ICM/March/April 2015
ICM September-October 2014
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