Furnace keeps blowing out

TOM7292 · March 28, 2009, 07:34 PM

Sent: 4/22/2003

THE PILOT STAYS ON BUT WHEN FURNACE KICKS IN IT ONLY STAYS LITE FOR A FEW MOMENTS THEN IT BLOWS OUT. ANYBODY HAVE ANY IDEAS TO KEEP IT GOING.

denisondc · March 28, 2009, 07:34 PM

Im not much help - but check whether the pilot flame is big enough, that the pilot orifice is clean - use a microscope to look? and that your supply of gas is adequate - that the furnace supply valve in line with the furnace is all the way open, that your regulator isn't set too low. Turn on a stove burner, and cycle the furnace on while that range burner is going - see if there is a noticeable drop in the height of the stove flame - would indicated inadequate supply flow or pressure. And with the furnace shut off - make sure you don't smell propane AT ALL in the area around the furnace and its supply lines.

MSN Member · March 28, 2009, 07:35 PM

From: Harold

HAL HERE. JUST A NOTE FOR 2 MORE ITEMS TO CHECK. 1. CHECK THE EXHAUST FOR BLOCKAGE. 2. CHECK POSITION OF THERMOCOUPLE, IT HAS TO BE IN THE MIDDLE OF THE PILOT FLAME. WHEN FURNACE FIRES UP IT COULD BE PULLING THE PILOT FLAME AWAY FROM THE THERMOCOUPLE WHICH WOULD CAUSE THE GAS VALVE TO CLOSE.
IF THIS FURNACE WAS A HOUSE FURNACE I WOULD HAVE YOU CLEAN THE FLAME SENSOR .

MSN Member · March 28, 2009, 07:35 PM

From: nvdesertrat0775

Actually, it sounds more like a bad or blocked sail switch. Tom, you didn't say what model it is, but you should be able to open up the front of it and (after disconnecting some wires -which of course you'll keep written track of so you can reconnect them correctly) slide the workings out.
Near the back, on the left usually, as you face it, there should be a switch with one rather long arm. That's the sail switch and it's job is to make sure there's sufficient air for the furnace, and to shut it off if there isn't, which sounds like what's happening to you.
Make sure there's a clear path from that switch to the outside of the RV...no bird's nest or accumulated junk. Make sure the switch arm is free to move. (Don't bend it!) Make sure all the wires are connected, on both sides of the furnace.
There's another possibility I don't really want to mention, but...
Again, I don't know the year or model you have, but on some units there's a single wire that goes from the main valve down into the burner area...and it's a fragile thermocouple. If it's faulty, it will shut down the main burner or not allow it to open. It might also have been moved away from the pilot flame (inside). A very small distance could make a big difference, so the next time the pilot is lit, make sure the flame actually hits that thermocouple.

DaveVA78Chieftain · March 28, 2009, 07:35 PM

Here is the basic operation for a furnace with a pilot light. Hope it helps

1. The thermostat is turned on and calls for heat.
2. Electrical current is sent from the thermostat to the time delay relay. The relay closes after a delay of 10 to 20 seconds and allows current to flow to the circuit breaker.
3. The circuit breaker is placed in line to monitor the draw of the motor. Excessive amperage draw due to a faulty motor will trip the circuit breaker at it's preset limit. The current flows through the circuit breaker and on to the blower motor.
4. The blower motor must come up to 75% of it's rated speed to continue the ignition sequence. If it doesn't - the motor is faulty, voltage is too low, the return inlet air is blocked, or there is a blockage in the exhaust.
5. Once the motor comes up to speed, the sail switch is physically moved by the air flow and closes a micro-switch to initiate a separate circuit. The sail switch must have a certain force to close it and if the motor is not running fast enough, or there is a blockage of some kind, or a "sticky" switch, then the sail switch is not blown closed and the circuit is interrupted.

Note: This circuit is totally independent of the thermostat or motor circuit and only comes into play when the fan blows "hard enough" to close the sail switch.

Locate the two wires that come and go to the sail switch - with your test light, probe each one - you should have power at both wires, with the furnace fan operating. If not, replace the sail switch.
6. The sail switch closes and sends the power to the limit switch. The limit switch is normally closed, but will open at a preset temperature to prevent furnace overheating. With the fan running, probe each terminal of the limit switch - you should have power at each terminal. If not replace the limit switch.
7. The limit switch then sends power to the gas valve.
8. The gas valve will send gas to the main burner, if the pilot light is lit. If the pilot light goes out no gas will go to the main burner for obvious safety reasons!
9. The main burner is ignited by the pilot flame and will continue to burn until the thermostat is satisfied. Once the room temperature comes up to the thermostat setting, the thermostat contacts open and cut off power to the gas valve, thus shutting off the gas supply to the main burner. The pilot flame continues to burn.
10. If the ignition sequence is interrupted at any point, there will be no ignition. Safety is utmost!

Rick Shaw · March 28, 2009, 07:36 PM

I don't know if you answered this one or not. I remember someone saying to me that there should not have been any propane in my coach. I had a problem with my furnace not shutting off one day on my way home from a trip. I had to pull the fuse because I tried turning the thermostat off but it would not cooperate. It just kept running. I pulled the fuse and the gas valve was still on. In the morning the coach was full of propane. Now I can run the furnace but it will still not shut off. I pull the fuse and shut the gas valve off. It seems to me that the furnace runs very hot...hotter than normal.

denisondc · March 28, 2009, 07:36 PM

I would say your all-purpose safety valve/pilot source/solenoid assembly is gone bad. I don't think they can be fixed. I would remove it, price a new one, and while recovering from the sticker-shock, I would see if there was any way to see into that valve assembly and clean it out - it must have a piece of crud stuck in the valve somehow. Even if I got a moth to fall out of it, I would suspect it evermore. The other alternative is not to use the furnace - period.

denisondc · March 28, 2009, 07:37 PM

ANY noticeable smell of propane inside the Winnebago is a reason to shut the tanks Off, let it air out, and not turn the propane back on till you eliminate the leak. I like using -low to moderate- air pressure into the gas lines - instead of propane,- and a sudsy solution and paintbrush.

DaveVA78Chieftain · March 28, 2009, 07:37 PM

I do not have the reference book I have here with me at work but agree with denison that the gas valve may have bit the dust. Will look at my book at home tonight. Here however, is some words I found that are related to a pilot style water heater gas valve (I suspect they apply to any pilot style valve)

The gas valve is a regulator, thermostat and safety device all in one unit. It is not servicable and any problem with the gas valve requires replacement. This is a safety feature built into the valve - the thinking is that if it fails once, it will most likely fail again. No attempt should be made to repair one of these valves.
I do recall the rule, do not stick anything into the burner orifice or it may be damaged. It is ok to air blow it clean though.

What brand/model heater is this?

Rick Shaw · March 28, 2009, 07:37 PM

From: rshaw500

I want to state that I have not had a problem with the Coleman furnace since then. As I stated I pull the fuse and turn off the gas valve at the furnace. When I woke up that morning the furnace was blowing but it was blowing cold air and you could smell the propane. I believe that my furnace is like everyone else's ....runs on 12 volt or 110 current. I think that the battery got too low but did not stop the blower from running. God knows why the propane kept going. The furnace works great if I pull the fuse to stop the blower.
Rick

denisondc · March 28, 2009, 07:38 PM

Your furnace is very very different from mine in one respect. When I shut mine off it eventually stops blowing, after a couple of minutes, and there is no smell of propane. None.
By the way, it runs on 12v only, blowers and all.

Rick Shaw · March 28, 2009, 07:38 PM

I believe that all of the furnaces keep running a little after they reach the required temp or when you shut them down. I believe this is to cool off and to expell any hot air left in the heating chamber. I think I had a thermostat malfunction that night and the furnace just kept running and blowing. As I said before, I have had no problem with it since then.....just need to replace the thermostat.

DaveVA78Chieftain · March 28, 2009, 07:39 PM

As far as the blower staying on:

The Time Delay relay performs two separate jobs - one to handle the relatively high current needed to run the blower motor - and two, to allow the blower to run for 45 to 90 seconds after the thermostat is satisfied. This allows excess heat in the chamber to dissipate before the blower stops.

The relay is normally open and should always have power from the circuit breaker. Only when power from the thermostat is present does the relay close after a 20 second delay. Power then flows to the blower motor.