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Propane furnace cycling

March 02, 2013, 10:20:50 pm

Propane furnaces normally turn on and off from wall thermostat settings. 

But the furnace's high-plenum-heat safety thermostat will also turn furnace off and when the plenum cools the furnace will re-light, often long before the thermostat setting is reached.

Usually the reason for the early shut-off is not enough furnace duct capacity for the fan to send plenum heat out into the room.  I think this high plenum heat could stress burner, heat exchanger, plenum and other furnace parts.  And besides the room is not getting all the heat it can use.

One way to determine if the propane furnace is cycling on safety thermostat is to notice if furnace turns off before wall thermostat has been satisfied.  We solved this problem with our furnace by keeping all ducts open and adding a 2nd duct on front of the furnace.  We mounted it in the cold air return (see attached) and connected it to an unused plenum access.  We aim the outlet away from the cold air return.  Anything to help remove hot air will go a long way to eliminating the problem.

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Re: Propane furnace cycling

Reply #1 – March 03, 2013, 08:56:58 am

@Barry I love the way you are always analyzing every aspect of your coach.

I've noticed the furnace cycling on and off before the wall thermostat was satisfied but just thought it was normal operation and I suppose it is but your idea does  make it more efficient. It also reminded me that I had closed the adjustable vent on the rear furnace to force more air into the water bay. Since I've yet to see a freezing temperature this year I'm just wasting that heat. Maybe I should get up and go open it! I don't know if I'll add another vent like you but you have me thinking... Thanks!

see ya
ken 

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Re: Propane furnace cycling

Reply #2 – March 03, 2013, 09:10:22 am

We have two furnaces. The rear furnace supplies air to bays through two 2" ducts. I replaced a couple of fully open registers in the bedroom with adjustable registers to allow forcing more air into the bays. I am careful to always leave all three "upstairs" registers partially to fully open to allow adequate air flow.

The instruction/installation manuals for the furnaces specify the minimum number of 4" ducts that should be fully open to allow sufficient air flow. As stated in previous posts, sufficient air flow is required for the furnace to maintain safe operating temperatures.

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Re: Propane furnace cycling

Reply #3 – March 03, 2013, 09:22:23 am

Great job of describing what happens, Barry.
I did only a fair job of explaining what can happen to Mr B2's (Brian) coach since a new door was installed and blocking off the ducting that goes to the bathroom in his GV. Hopefully he will see your post and check his ducting out.
My explanation was just that, if any ducting was blocked the furnace could just plain fail. This per the "guy" at our RV dealership when we bought our coach.

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Re: Propane furnace cycling

Reply #4 – March 07, 2013, 06:19:18 am

This is some very timely info for us.  I've noticed that the furnace is "short-cycling" compared to our 5th-wheel trailer.  I had actually partially closed one of the two vents in the bath under the toilet because it was much warmer there than the rest of the coach.  I was hoping to force the heat out the other ducts. 

Fortunately, the heating season is drawing to a close and I will have some time this summer to work out a fix.  I think I'll try to add an additional duct (or two) in the main living area if possible.  Thanks!

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