Problem with Furnace

[PeterAndMargaret]

Hello,

 

We have a Suburban furnace in our Fleetwood Flair. It was working fine until a few days ago. When I start the furnace now it is blowing out air but it is cold air. Any ideas on where I should start looking for the problem?

 

Peter

 

[ronpurdum]

Try the sail switch. It is just a small flapper type thing that must blow outwards to allow the gas to flow. It will blow outward but the switch may be bad. Simply use a jumper wire to jump the two contacts. This is a very common problem and is easy to change.

[Big Greg]

http://bryantrv.com/docs.html

 

There's a link to Bryant's service manuals, following the fridges you'll find several furnace manuals including Suburban, look for your model. Greg

[Kirk W]

There are several possible things that could cause this to happen. If the sail switch does not close you would get the symptom that you have, but it isn't the only possibility. The sail switch is actuated by a vane that is located in the air path of the blower that moves air through the furnace into the RV. That same motor also has another blower on the other end of the motor which provides the air to the combustion chamber for burning the propane. If that motor does not move enough air the sail switch will not close and that prevents the gas from turning on as a safety measure. If you are not connected to shore power the problem could be low battery voltage since a DC motor slows down as voltage falls and slowing down means less air moved. Once voltage drops to 11V or less most furnaces will no longer light. If you are not on shore power, try running the generator to see if the furnace will light then.

 

An obstruction is the air path can also do the same thing, like a crushed heat duct or some obstruction in the combustion air path. If the furnace worked yesterday but don't today, this isn't likely to be the problem.

 

Anything that prevents the lighting of the propane will also cause that same symptom. If you turn off the furnace for a minute or two and then listen very carefully at the furnace while someone turns it back on, you should be able to hear the snapping of the ignition probe attempting to spark and light the flame. It will try several times, then stop trying and go into lock-out. If you turn it off it will then try again. It would also do this if you were out of propane, but I suspect that you would know that since the stove top would also stop working.

[oldjohnt]

Good answers above, indeed several issues could cause your same problem. Its been my experience its so often due to low DC voltage and as few as tenths of a voltage on the low side can make a difference. Once I had tech support on the line as I was troubleshooting and told them I was armed with a 12 volt test light to which they replied NOOOOOOOOOOOo you need a quality DC voltmeter. As mentioned try it when plugged into shore power or have your genset up and running as maybe your house batteries are below par??? Other fixes I have used are to remove the small flat ribbon cable on your ignitor circuit board and gently use a lead pencil eraser to lightly clean and brighten up the small flat board contact strips (or contact cleaner etc). On mine if you turn up the T Stat you hear it click lightly,,,,,,,,,,then theres a delay,,,,,,,,,then the fan starts running,,,,,,,,,,,then if alls well (sufficient air flow and sail switch closes) the ignitor starts firing its click clik clik HV sparks and the gas valve opens and she fires.

 

Check voltage !!!!!!!!!

 

John T (live somewhere in south central Florida at an antique tractor show, Florida Flywheelers)

[PeterAndMargaret]

Many thanks for the replies and advice. I have tried the furnace when we are connected to shore power and with the generator but still the same result. I have tried to listen for the sound of the ignitor as it tries to ignite the propane but cannot hear it. The problem is not with the propane as the hot water heater and stove top are working.

[Wrknrvr]

Is there outside access to your furnace. If there is then most of the possible causes are fixable from the out side.

Does your furnace stop on its own if there is no combustion or does it keep running till you turn it off.

 

Vern

[oldjohnt]

Hard to say what the problem is now since it doesn't appear its a low voltage problem. Those ignitor circuit boards can go bad but I would still try removing the flat ribbon cable and clean/buff/shine up the flat copper looking contact strips on the circuit board. If she's not firing the HV sparks which ignites the gas (you can see and hear those sparks on some units) the HV ignitor assembly (sparker) can have hairline cracks or carbon shorting traces on the white ceramic/porcelain looking insulation. RV dealers may have circuit board testers and if the contacts are clean, no low voltage problems, no cracks or carbon shorts on the ignitor and the sail switch closes but she don't ignite, I suspect a bad ignitor circuit board. I usually buy replacements from Dinosaur Electronics. Again the T stat closes and clicks, delay, fan starts, sail switch closes, HV sparks, gas valve opens, FIRE............

 

John T

[Kirk W]

I have tried the furnace when we are connected to shore power and with the generator but still the same result. I have tried to listen for the sound of the ignitor as it tries to ignite the propane but cannot hear it. The problem is not with the propane as the hot water heater and stove top are working.

The next part is, how much are you willing to do and do you have a meter and tools to work on the furnace? Also is your furnace a model that has outside access or does it just have the small exhaust port below on the outside?

If this is what you have you would need to remove the furnace to work on it.

[PeterAndMargaret]

It turns out that the problem was simply a dirty connection.