Re: Furnace won't ignite without help
John Kalmbach posted a couple of long explanations of possible heater problems many years ago on the Yahoo List (before this forum was born). The following are a couple of his responses to furnace questions. I hope one will give you some answers:
Question: My Suburban furnace only works when it's 30 degrees F or over. Below 30, it runs the fan, fires the spark but won't run. I just had the tank filled and ran through the purging procedure--bled it for 20 minutes. Nothing but LP smell came out of the bleeder valve--seemed like a strong LP smell. Should a visible vapor, or liquid, come out of the bleeder?
Kalmbach: Since the igniter is working, the problem is most likely a lack of LP Gas to the burner. Several things can cause this. Confirm the main LP Gas valve is "on". Confirm that the house battery voltage is adequate to spin the furnace fan fast enough to engage the sail switch [the sail switch is in the heater duct and insures that the fan is running adequately before the propane is released]. Easiest way would be to start the van and see if the furnace lites when the van engine is running (provided the alternator is charging the house battery).
If the furnace doesn't operate when the van engine is running, restart the furnace and check that you get an LP Gas odor at the exterior furnace vent. You should be able to smell LP Gas once you hear the igniter at the exterior vent. If you don't smell LP Gas at the exterior vent, check for any type of blockage for the interior air return. Sometimes blankets or sleeping bags cover the air return and prevent the furnace sail switch from making contact, which will not allow the LP Gas solenoid valve on the furnace to open (I know that the igniter shouldn't operate if the sail switch makes contact, but sometimes it does, maybe just not as long). I've seen an accumulation of dust in the exterior vents cause problems too.
If you do smell LP Gas at the exterior furnace vent, then it's possible that the burner needs to be cleaned. It tends to get rusty and can prevent LP Gas from getting close enough to the igniter.
Question: We have a 2004 Sportsmobile with the propane fueled hot air furnace. I filled the tank in May, just before our 3-week trip to Utah and Colorado. When filled, the gauge registered about 3/4 full. We used it quite a bit on our trip, as it was a cold spring in the Rockies. It got down to 1/2 full according to the gauge. Last night we were in the Sierras and it got down to 30 degrees. I turned the thermostat to 80 degrees to warm the van up. When warm, I turned it off. In the night I reached over and turned it back up to 80. The fan came on but I never heard the burner kick on. After a bit, the fan turned off. I waited a bit and tried it again. Same thing. This morning I checked the gauge and it still registered 1/2 full. Do you think I have a bad gauge or a problem with the igniter? I will fill the tank to see if it works, but wondering if anyone has run into this.
Kalmbach: The usual culprit for the furnace not operating at high elevations is air in the system. If the furnace performs well at lower elevations, then you can almost bet you have air in the tank that needs to be purged.
The opening of the vapor tube is high in the tank. Since propane is heavier than air, the propane settles below any air remaining in the tank, and the air, which expands as you increase in elevation, is what the furnace is trying to ignite. To add confusion to this condition, you'll find that the propane stove and propane water heater usually operate okay at the higher elevations. They are not as temperamental as the furnace, which has many safety features preventing it from operating unless everything is as it should be.
To purge the air out of the LP Gas cylinder, make sure you are outdoors and away from any flame or spark-producing item. Open the "Purge Valve" which is a small (about 5/8" diameter) brass valve above the main "on-off" valve. If there is air in the cylinder, the propane odor will not be as strong as if it were pure propane. I've seen it take up to 30 minutes to purge most of the air out of the tank (this was at 250' elevation, if you perform the purge at higher elevation it will purge quicker). You can also have a qualified LP Gas filling station purge your cylinder. You will have to run the furnace through several cycles before it will start after purging the air.
While this worked for almost every customer who reported this condition, there are a few other issues that occurred. Confirm that there is nothing blocking the return air grill or path. The sail switch inside the furnace will not allow the gas solenoid valve to open if there is insufficient airflow. One way to determine if the gas solenoid valve is opening during start-up is to smell at the exterior vent after turning thermostat "on". If you don't have any LP Gas odor, you probably have an air flow issue. If you do have an LP Gas odor, you could have an igniter problem (no manual override), or it could still be an issue with air in the system (did I mention purging ALL the air out of the system!).
'01 Ford EB50p Quigley 4WD