I've installed a new Goodman furnace (96% efficient). I'm trying to test fire it with a 20 lb propane tank. The gas valve has had the propane conversion springs put into both the high and low fire pressure settings. The orifices have also been changed to the propane orifices. I have a two stage regulator coming from the propane tank into the furnace. The propane tank is full. The furnace goes through it's startup cycle with the vent fan and the hot plate igniter working properly. I hear the gas valve turn on but no gas seems to get to the burners. I've checked the input side of the gas valve with a manometer and it's showing 12.8" WC. I've checked the output of the gas valve and when the valve makes a noise, the manometer only goes up to .20" WC. This is a gas valve and furnace that's never been used and but has been sitting for ~2 years. Everything seems to be pointing to the gas valve. Is there anything else I can check before ordering a new gas valve?
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Does the input pressure remain at 12.8" WC when the furnace valve opens, or does the input pressure fall to near-zero also? Is there another know-working appliance you could test fire using this same propane tank and regulator?– Greg HillCommented Nov 1, 2021 at 17:07
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Good suggestion. I tried it and the input pressure remained the same as the output only dropped .2"WC. The input was a little high at 14.2" WC. The two stage regulator I"m using on the 20 lb tank isn't adjustable.– patCommented Nov 2, 2021 at 0:02
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Another good suggestion regarding the "excess flow" device built in to the regulator. I will check that as well. I did check the input pressure when the gas valve turned on previously and it dropped only .2" WC (and maintained 12+" WC). Would an "excess flow" device result in the input pressure dropping to zero when the gas valve turned on?– patCommented Nov 3, 2021 at 14:22
2 Answers
The propane conversion instructions said that after replacing the natural gas springs in the gas valve with the propane springs that you should turn the low fire adjusting screw 8 turns. The startup sequence is
- draft inducer fan starts
- hot plate is energized
- gas valve turns on for ~4 seconds at low fire setting
- if flame is detected, turn on gas valve at low fire setting.
My problem was that the adjustment called out in the propane conversion instructions for the low fire setting wasn't enough pressure. I was able to increase the low fire setting (to 6" WC) and the furnace started properly.
Your propane regulator probably has an "excess flow" device built-in. This is a safety device that is supposed to stop the propane flow if the flow exceeds the design rate. This can help prevent a disaster if the propane hose is cut, burned through or becomes disconnected.
To confirm this conjecture, take the tank and regulator outdoors to a safe area, and with the pressure-regulated supply hose open to the air, very slowly open the tank valve just a crack to see if propane is flowing. If it is, open the tank valve more. If the flow suddenly stops, you have an excess flow device and the furnace needs more propane flow than your device will allow.
