FYI RV furnace won’t light

[Vibeman]

Since the cold snap I am seeing many posts about furnace problems, I had one myself yesterday, but solved it. I will describe my situation in case it might help someone else. I have a Dometic DFMD 35k btu furnace. LP gas, 12v electric. I am in an RV park in S Texas hooked to shore power. Yesterday my lp tank was below 50% so I flagged down a passing lp gas delivery truck to fill me up. He dragged his hose up to my tank (motor home with mounted lp tank) he immediately shut off the supply valve to the coach, I realized the furnace was running at the time so opened the door and asked wife to turn it off, it probably ran for at least 30 seconds with the valve off. The gas man filled the tank, turned the supply valve back on and left. I went back inside to restart the furnace but it would not ignite. The fan would run for 30 seconds then shut down, no “click” from the gas valve, no “click” from the igniter. I figured there was now air in gas line so I continued to manually restart furnace, still no ignition after dozens of attempts. Kitchen range is right next to the furnace and all burners worked normally. I am not a gas appliance expert but I do know quite a bit about their operation. Things like the sail vane, flame sensor etc, but it just seemed too coincidental for a 1 year old furnace to develop an internal problem at precisely the same time as the tank was filled. After many tries at restarting, I turned off the supply valve again, lit the range burners to bleed the gas line pressure then slowly reopened the gas valve. The range burners re-lit easily but the furnace still would not ignite. So as a last resort I turned off the valve and bled the lines again, but before re-opening the valve, I did some light tap,tap,tapping all over the supply line regulator. I then slowly reopened the valve, tried the furnace again and it lit and ran normally! It would seem that previously the regulator was supplying enough gas pressure for normal 3 burner range operation, but not enough for the furnace to ignite. I hope this might help someone else in a similar situation. I am just glad to have the furnace back in operation, it is supposed to dip down below freezing here tonight!

[16ACE27]

I don't understand how gas/air in the line prevents the ignitor from working or even the gas valve from opening.

[Vibeman]

Like I said, I’m no expert, my thought was that maybe there is a minimum gas pressure sensor that needed to be satisfied before the valve would open.