Furnace cycling on and off

[K&Gs3400RL]

Took the 5er to the dealer and,The thermostat was replaced Not that day because they could not duplicate the problem. Got it home and same thing was happening, So they sent someone out to replace it. But it is still cycling on and off every 4 to 5 minutes.
Example
On Offf
6:03 6:07
6:10 6:14
6:17 6:22
6:26 6:31
6:34 6:38

:confused::confused::confused:

[Ron]

Is it the fan or burner or both cycling on and off?......Ron

[chuck&gail]

Make sure none of the vents are blowing directly on the thermostat.

Also might be worth putting a thermometer near thermostat. Is it cold enough that it SHOULD cycle that often?

Do these modern fancy thermometers have an anticipator adjust like the older ones, to adjust temp swing? I don't know, but maybe thermostat mfgr does?

[K&Gs3400RL]

Yes the fan comes on then about 30 seconds later the Burner kicks in. Then you can hear the burner shut off followed by the fan.

Two vents close but I dont think either are blowing on it. Doesn't matter on out side temp 32* or 38* it cycles on and off the same the same. Anticipator Mmmm never heard of one, something to read up on tho.

[Ron]

Are you getting enough air flow through the duct work, possibility not enough flow (collapsed duct) and the high limit swith is shutting it down. Also the sail switch may be malfunctioning when getting warm. Curcuit Board? Good Luck Happy Campin........................Ron

[K&Gs3400RL]

Thanks for the info, Between here and the Montana site I have quite a few things to check. I better get busy.

[Ron]

Good luck, and let us know what you find. Happy Campin................Ron

[hankaye]

Howdy All;

Checking in late here...
In my older (95) TT I had to swap out the thermostate. The new one was didital found out fast that the temp drop was only 2 degrees. The older Analog thermostat had a temp. differential of 5 or so degrees. So, ...
I got used to a more even temp. but alot more cycles of the furnace....
As the Dealer replaced one and then that one as well .........
I have to ask, was the older thermostate analog or digital???
Also, will the difference between the 2 types effect the furnace's "brain"
(circutboards, chips whatever?)?

hank

[SteveC7010]

I noticed that this is all happening in 30-40 degree temps.

That got me to thinking about the general weather hardiness of RV's compared to a regular house.

* RV walls are probably no better than R-10.
* Ceilings maybe a bit more, but not much.
* Floors probably less.
* Windows are single pane and not 100% air tight.
* Exterior doors are foam filled, but have aluminum frames, and not particularly air tight gasketing.
* Compartment doors probably have very little R factor and their gaskets are not 100% either.
* Roof vents are a single layer of plastic: zero R value.
* There is no insulation in my trailer between the basement and the inside cabinetry of the kitchen. I suspect most are similar.

So what we have here is not the best structure for cold weather. I suspect that heat is dissipating much more quickly than normal and there are likely "cold" spots here and there. If you're using a digital thermostat, the hysterisis program is probably +/- 1 degree on the set point. The Duotherm analog thermostat would be wider, by a degree or two.

The furnace comes on, runs for 4 or 5 minutes and brings the temp back up +1 degree or so above the set point and shuts off. In the three minutes that follow, the warm air in the area of the thermostat cools a degree or two below the set point and the cycle starts again.

RV furnace systems are really designed to take the chill off of a cool day or evening, not provide single source heat in cold weather. There's not enough fan to really move air properly, the ducting is undersized, and the outlets and cold air returns are not at all well placed.

If the thermostat is located on an outside wall or other place where it might cool more rapidly than you might want, consider relocating it or finding a way to insulate that wall. Conduction through the wall may be cooling the thermostat more than ambient air.

If there is a heat duct really close to the thermostat, it may be warming it too quickly forcing a more rapid cycle.

There are a lot of tricks to keeping an RV warm in cold weather. Heat shrink window film on most or all of the windows is a good start. There are commercial insulation things for the 14x14 ceiling vents or you could craft up your own.

That enclosed underbelly and polar package may not be as functional as you hope. I found open flaps on my underbelly and a lot of gaps where air can move in and out.

I have seen people live in RV's up here in northern NY state year 'round. They have to do a lot of inventive things to get through the winters like skirting the entire trailer and then warming the entire under area.

[campingcpl]

My parents had the same problem. They had it back to the dealer and even back to the factory for this problem an after changing out the thermostat a couple of times and replacing the furnace it was still doing it. My dad finally blocked off the vent near the thermostat and relocated the thermostat away from the air flow of the vents and then put a clear cover over it that he can remove to adjust the thermostat. After all that they finally got it to where it doesn't keep cycling on/off. It's still not perfect but better then listening to it turning on and off every few minutes. I replaced our thermostat with a Hunter thermostat that you can get at Wal-Mart and it has made a big difference in how much the furnace runs.

[mhs4771]

Check the hole in the wall where the wire for the thermostat comes out. Depending exactly where mounted some have had prolems with heat from the belly traveling up through the wall and hitting the backside of the thermostat. Putting some insulation in the hole and covering with duct tape seemed to do the trick for many with you problem on the MOC forum.

[K&Gs3400RL]

Its Fixed :bdance:
They came out two weeks ago and brought a Brand New Thermostat and control board that are compatible with each other. The first time they just changed the thermostat, and it did not fix the problem. Then they sent out a New control board, that did not fix it either. So after talking to Atwood and Dometic they Ordered a New Thermostat and Control board that they say is compatible. And it is fixed. If anyone needs more info let me know.

[Johnnyfry]

Lots of good suggestions here.

Do you have a digital stat? If so, check to see what the hysteresis (temp drop before calling for heat) is. Mine (Dometic) can be set for either 1 or 2 degrees drop. It will definitely short cycle on the 1 degree setting

All in all, the marginal insulation in these things ("Polar package" notwithstanding!) does cause rapid heat loss when it is cold and windy, so short cycling is to be expected under those conditions.

You might also use a thermometer to check outlet temp at the heat register (or grill or whatever you call it) to see what the temp rise is. You may also have duct leakage in the "basement" area.

John