Furnace problem

Furnace fan comes on...purges...main flame lights run five minutes then flame goes out and fan continues to run...then flame lights repeats

Blocked/obstructed discharge and/or return....lack of proper airflow thru heat exchanger causes 'high limit switch' to open (shuts down gas valve).
Fan cools heat exchanger high limit switch closes and circuit board opens gas valve ....called 'limiting'

Check vents/registers, return air and anything up against furnace that would block return air

Mine does that if parked for a while, and gets damp,at the igniter, just starting a few times clears it

No obstruction intake or out put RV tech has gone through it, cleaned it check for obstruction. Spent $ 300 in repairs and parts and still not working right.

Did they measure how hot discharge was getting right at furnace and how fast it got hot?
High temp limit switch
Trips when temp gets to hot in heat exchanger

Obstructions/blockages could be within the ducts, at registers and the path of return airflow...not just at furnace combustion intake/exhaust


Might check LP System Pressure....too high and furnace overfires
S/B 11" WC (0.4 psi) measured with manometer and at least 50% of propane in service

I’ll check for obstructions and then talk to service tech tomorrow

okiejoe wrote:
No obstruction intake or out put RV tech has gone through it, cleaned it check for obstruction. Spent $ 300 in repairs and parts and still not working right.

Oh wow. I'm sorry to hear this.

At $300 in, a new furnace is a better deal than diagnosing it with an RV tech.

Trouble shooting by replacing parts isn't an economical solution but as Old-Biscuit says, test the high temp limit switch or they are about $10 apiece, just replace it.

High temp limit switch is just an on off switch. Connect a meter or test light from the load side to ground and turn the sucker on. If the the voltage drops to zero and the furnace stops running, wala problem solved.

Well the tech checked things out and it looked like a sail switch problem so he replaced that and it worked good for about 2 days then started acting up again so he looked at the circuit board because he saw some corrosion on it from what looked like it got wet so he replaced that for just parts cost only and it seems to be working as it should but I am beginning to think it may very well be a limit switch because furnace burns well for about 6 to 8 minutes the burner shuts down but blower continues as it should clearing the gases but before blower shuts off the burner ignites and burns again for 6 to 8 minutes then repeats so it could be that limit switch shutting it down.

Well after some good suggestions I went out and removed some panels inside and definitely noted that there was one air duct hose that had a tight “S” curve in it then connected to an air duct in the floor that ran over to the other side to heat hallway and bathroom so I cut a little over 2 feet out and reconnected the duct and WALA problem solved. Thanks so much for all the input. It looks like there was just to much of a curve in the Duck work, it was crammed under the kitchen cabinet & sink. Apparently the restrictions were causing excessive heat to build up and the the limit switch would shut it down.
Thanks again

okiejoe wrote:
Well after some good suggestions I went out and removed some panels inside and definitely noted that there was one air duct hose that had a tight “S” curve in it then connected to an air duct in the floor that ran over to the other side to heat hallway and bathroom so I cut a little over 2 feet out and reconnected the duct and WALA problem solved. Thanks so much for all the input. It looks like there was just to much of a curve in the Duck work, it was crammed under the kitchen cabinet & sink. Apparently the restrictions were causing excessive heat to build up and the the limit switch would shut it down.
Thanks again

Wow … thanks for the follow-through on letting us know what the fix was!

We love our RV to be warm so we're sensitive to furnace problems, and we had an expensive-to-fix one recently. What we had was a leaking kitchen faucet over the years slowly permitting water drip down onto the furnace's control board and then eventually shorting out the board. We thought that the faucet's small leak was only onto the kitchen counter's surface and I let it go because the faucet was difficult to get at from underneath.

Thanks to your post above, I'll also check some of the bends in our furnace's duct tubes that run in behind the counter and under the bed area.