Furnace burner cycling during normal operation

[4xMike]

Back-story:
My camper has what I assume is the origional ductless Atwood Hydro Flame 7916 16,000 BTU furnace
For reference: http://www.ebay.com/itm/like/131240313321?lpid=82

When we bought the camper, the blower motor squealed like a squirrel being violently beaten for the first 5 minutes of operation. I pulled it apart and lubricated the motor as best as I could but it only worked quietly for a day or 2.

I tested it prior to leaving for the mountains and discovered the DSI circuit board had failed. I replaced the board (Spendy little bugger at $200) and then replaced the motor ($90) as well. At this point I am wishing I had just bought the whole new unit in that eBay listing, but hindsight, 20/20, etc.


Which brings us to:
this last weekend we were in Yellowstone where it was getting into the teens and 20s at night. Not normally an issue as my wife and I likethe cold while sleeping, but my 9-month old son didn't appreciate it as much we do. The furnace worked great each night until about 2 or 3am. The blower was still running but the burner was shut off and had been long enough for the temp in the camper to drop into the 40s. I cycled the power several times and got it to reignite, but as it was heating back up, I could hear the gas valve shut, then re-open after 5 seconds, reignite... then run for sometimes 30 seconds, sometimes 5 or 10 minutes before shutting off and repeating the process for an hour or 3 it eventually shutting off and ceasing further attempts.

With a new blower motor, I doubt it's improper air flow, and with a new circuit board and its otherwise flawless operation, I'm doubting it's the board.... So, I'm thinking it's either the flame sensor, the limit switch or the sail switch at this point.

Before I just start randomly throwing parts on it, does anybody have any experience with this that can enlighten me as to which component to try first?

Please and thank you!

[4xMike]

Thanks for the replies. Sine I've posted this we've been back home and the temp has been in the 80s and 90s, which is very unusual for this time of year. Because of this, I have been unable to test any of your suggestions.

ktmrfs wrote:
couple of things to check:

1) were you running on battery power??? If so, and battery drops to low sail switch won't close, fan continues to run, propane stays off
2) How much propane did you have in the tanks? As the propane level drops tank pressure drops since there is less propane in the tank and as it boils to release gas it's temp drops, which further lowers the gas flow rate. When the flow rate gets to low the furnace can stop
3) was the gas HW heater on as well? If so, again in the middle of the night, if the furnace and water heater try to be on at the same time, and it's cold out, AND your propane tank gets low flow may be to low to support both and they turn off.

It was on battery power, but with a 100% full-charge and a brand new blower motor. Both tanks were full and wide open, and the water heater's pilot light was lit, but the burner was not running at the time this was happening. Though, I suppose the regulator could be the problem by dropping the propane pressure too low, but the stove, fridge and water heater all operate problem-free.

My plan at this point is to wait for some cooler temps to return and then bypass the thermostat, sail switch and limit switch one at a time and monitor to see if one of them turns out to be the problem. If none turn out to be the problem, then I will look into other things such as voltage drops, propane pressure, etc. and report back my findings for other with a similar problem.

Thank you again for the suggestions.

Mike

[ktmrfs]

couple of things to check:

1) were you running on battery power??? If so, and battery drops to low sail switch won't close, fan continues to run, propane stays off
2) How much propane did you have in the tanks? As the propane level drops tank pressure drops since there is less propane in the tank and as it boils to release gas it's temp drops, which further lowers the gas flow rate. When the flow rate gets to low the furnace can stop
3) was the gas HW heater on as well? If so, again in the middle of the night, if the furnace and water heater try to be on at the same time, and it's cold out, AND your propane tank gets low flow may be to low to support both and they turn off.

[Chris_Bryant]

You should be able to test the power to the board while it is running- the thermostat, sail switch and limit switch can all cause this problem- and it can be a combination, as each item can cause a small drop in the supply voltage to the board.
If the voltage at the board stays above 11.5 volts, and the board still cycles, it has to be the flame sensor, which could be a burner/orifice , electrode, LP pressure, or lead problem.

[darsben]

I would check the limit switch and the sail switch.
Carefully jumper the limit switch while watching unit f that is not it then carefully jumper sail switch.
BE VERY CAREFUL as you are removing a piece of safety equipment even if only temporarily.
Safety first