Atwood Heater 8500-IV

[our-3400rl]

We have a 2011 Montana 5th wheel and have started full timing. Since its cold in the Pacific NW Now, and we are using the Heater It cycles on and off. It stays on for 3 to 5 minutes and then shuts off for 3 to 5 minutes, The Burner lights and we have heat But with it coming on and off so often it never really heats the trailer that good.

[Dave78Chief]

Is the thermostat positioned such that a heated air vent is blowing across it resulting in the thermostat shutting down before the rig is properly heated?

Dave

[our-3400rl]

It is in the normal place for Montana's right side going up the stairs to the bed room. The kitchen vent is the closest but even directing it away from the thermostat doesn't help and we can close the door to the bedroom and still nothing. The thermostat has been replaced along with the limit switch. And I assume they don't know what to do next because its been a week tomorrow they were replaced.

[Dave78Chief]

It sounds more like an air flow or thermostat issue rather than a issue with the furnace itself. If it was furnace itself then I believe it would be going into lock out mode rather than cycling. Did they use the Thermosat supplied with the furnace or are you using a combination AC/Furnace Thermostat such as the Duo Therm 5 button control? If Duo Therm, then they may have instllaed a remote temperature probe which maybe incorrectly positioned. No matter what Thermostat they used, you need to determine if the thermostat is the source of the frequent cycling by monitoring the control signal to the furnace with either a multimeter or test light. If the thermostat is commanding the On/Off cycling then it is an air flow problem. You may be able to adjust that by opening/closing distribution registers which from pictures appear to be in the floor.

Dave

[our-3400rl]

Looks like Atwood is sending us a New Controll Box. Keeping fingers crossed.

[Gary RVRoamer]

The most likely reason for this is that it is cycling on the high limit cut-out switch rather than the thermostat. That happens if the air flow through the heat exchanger is inadequate, e.g. blocked air inlet or restrictions in the ducts going out to the registers. If you don't have enough air flowing, the heat builds up inside the furnace until it shuts the burner down. The fan continues to run and it cools it down, so the burner ignites once again. This is a fairly common problem, resulting from crushed ducts, closed off heat outlets, or stuff stored in front of the furnace air inlets. It can also happne if there is excessive tubing in the duct work. National RV removed 7 feet of excess tubing from a 2002 Dolphin we once had, increasing the heat flow to all registers and stopping the short cycle problem.

[our-3400rl]

Well its still not fixed. The limit switch, thermostat and the control board have all been replaced. I have put insulation behind the thermostat. Straightened and shortened the ducting behind the wall in the basement. So Now we wait to see whats next.

[toadswife]

We are having the same problem with our heater as well. We are on our shakedown trip in our Evergreen 5th wheel and the only way the heater will come on is if we turn the switch off let it reset and then turn it back on. Once it reaches temperature, it will not come back on again until we turn it off and back on...very irritating. I can hear it clicking to ignite and there is the lp is full, yet it will not light. Dealer sent a new thermostat but in the meantime we are using a space heater. Will take it back to the dealer when we get home to figure it out. I think it has something to do with the temperature of the lp...we had similar problems with lp when we lived in Wyoming. Anyway, the good thing is being mainly southern campers, we rarely use our heat so it's not that big of a deal now. But when we begin full timing in a couple of years it will be much different. Let us know what you find out...I'd like some heat!

[our-3400rl]

Lets see Thermostat has been changed, Limit switch, Control board. Checked for obstructions in the ducting, Put insulation behind the thermostat in the wall. All soft ducting checked for holes crushed or sharp bends sending it in a different direction. Voltage checked.
We have a temporary fix As of now, we have an old style 1970's (slider) thermostat installed directly to the heater bypassing the control board And it is working. They have ordered a New Thermostat and control board that is suppose to be compatible. we will see if they are when they get here after the first of the year.

["007"]

Welcome to irv2.
Here are threads on your ATwood heater, have you checked them out?
Heater has a LED that flashes to indicate what the problem may
be.

[Gary RVRoamer]

If the fan runs and you hear the igniter clicking, the thermostat and controller are ok. Either the furnace fails to ignite or the gas valve has failed to open and provide fuel. Since it seems it ignites ok the first time, I wonder if some component is failing after it heats up?

Once it fails to ignite, the furnace goes into lock-out and won't attempt to relight until it is reset.

[Dave78Chief]

Our-3400rl,
"Bypass Control Board"? I doubt they did that at least not the one inside the unit. If it is working with that old thermostat then in reality, you have your answer as to what the problem is. Unless it is something like a Duo-Therm 4 or 5 button CCC, then a thermostat is nothing more than a temperature contrlled On/Off switch. It was cycling On and Off to frequently.

toadswife,
You have a different issue from Our-3400rl. Yours sounds more like a gas problem or the board is not sensing the flame.

Dave

[our-3400rl]

They replaced both the Thermostat and the control Board at the same time. I was told they had to make sure they were compatible. The first time they changed the Thermostat then another time they changed the Control board. But it wasn't fixed till they changed both at the same time.

[Dave78Chief]

Glad to hear they got it worked out. Not the usual sort of problem. Thanks for the feedback.

Dave