Furnace won't ignite every time

[mdavis479]

Howdy, I own a Rockwood Windjammer 2809W, bought it last year. I have an unusual problem. Most times but not every time the furnace blower will come on but the furnace will not light, it won't even attempt to ignite....unless I slide the thermostat up to about 90 degrees then it will ignite. Otherwise, the blower will cycle and shut off continuously. We have purged the system, running all the burners on the stove prior to turning on the furnace. But you can typically hear when a furnace is trying to ignite and I cannot hear it even attempt to ignite when it does this. We have awaken early on a cold morning to hear the furnace blowing cold air, then I will slide the thermostat up to 90 and it will ignite? The thermostat is a "Coleman-Mach". I suspect a thermostat problem? But you would think if the thermostat sends a signal to the furnace to turn the blower on the furnace would also take that as a signal to ignite. Why would the blower come on but no attempt to ignite....unless I push the thermostat to 90?

[cadman99]

The furnace is controlled by an internal circuit board. The thermostat is like an off/on switch and merely starts a sequence to light the heater. Once the blower starts it blows air against a "sail" switch which closes letting the circuit board know the fan is running. The fan will run for a short period to clear any propane then the gas valve will open and the igniter will try to light the furnace. After several tries if the furnace does not light everything stops and you start over by switching the furnace to off then back to on.

It seems unlikely (but maybe possible) that moving the thermostat to 90° is what solves the problem since the thermostat just sends a simple "turn on" signal to the heater circuit board. Possible problems ..

Sail switch not working
Not enough dc voltage to actuate the igniter (low battery)
Gas valve not opening
Circuit board is bad
Not enough propane to light the burner

If you do not hear the igniter "clicking" then either not enough battery voltage, sail switch not working or circuit board is bad.

Apparently the Coleman-Mach thermostat is marginal and frequently replaced with a digital one. Here is a link for that replacement.
http://www.klenger.net/RVing/HVAC_Hu...structions.pdf

[Ford Idaho]

Have you checked inside the outside cover for plugged vents or tubes?

Got Mud Daubers ?

[mdavis479]

I will check these things that were mentioned and let you know if I find anything, thanks for the info.

[Mark S]

I recently had a problem where the burner would light for 8 seconds and then go out. Every time. In addition to the above advice I was told to clean the burner, which showed no signs of being clogged or dirty.

So before I gave up, I took the burner out and blew compressed air through it, fully knowing it would not make a difference. Sure enough, that fixed the problem. Go figure.

[TxTravelingRN]

Had the same problem with my Rockwood recently and the problem was the circuit board.

[mdavis479]

Want to thank everyone for the input and provide an update on this issue. The problem was the thermostat, I replaced it and the furnace works like a champ. I think the manual slide on the thermostat was broken, so when it sent a signal to turn the furnace on it quickly lost the signal due to the faulty temperature slide, except up around 90 degrees where the slide would still work.