Atwood 8500-IV furnace problem

[OregonTravelers]

Need a little help. We have a 2005 National Sea Breeze LX with an Atwood Hydro Flame 8500-IV series furnace. The furnace has been causing us problems for some time and hoping someone can offer some help in finding the problem.
When the furnace is turned on, the fan runs and then a short time later the furnace fires and begins to heat. The furnace works fine and heats to the temperature set on the thermostat. Now comes the issue.
Once the selected temperature is reached, the furnace shuts off and the fan continues to run for a short while. Just like it is supposed to. Then as soon as the fan shuts down, the entire furnace start sequence starts over again. The fan starts, the furnace fires and begins to heat. After a few seconds, the furnace shuts down and the fan continues to run for a short while and then shuts down. Then the entire process starts over again. Fan, furnace on, furnace off, fan, shut down. The sequence happens a few times and then the 12 volt circuit breaker trips. If I let the room cool enough that the room temperature is below that set on the thermostat the and reset the circuit breaker the furnace will run thru the heat press fine until the set temperature is reached then the start and stop sequence begins all over again until the circuit breaker again fails.
1. The sail switch has been replaced.
2. The propane pressure has been checked.
3. Heat ducts checked, cleaned and moved to remove possible restrictions.
4. Air return checked and cleaned.
5. The control circuit board has been replaced
6. The connections on the 12 volt circuit breaker has been re-soldered.

I would appreciated any suggestions on what may be causing the furnace to recycle problem and what I can do to fix the issue.
Thank you

[OregonTravelers]

Chris Bryant wrote:
Check the current draw of the motor, then replace the breaker. Original was often a 7 amp, replacement is a 10 amp.

Don't guess that is the problem. The main furnace, the one with the problem has a 15 amp circuit breaker and the second furnace in the bedroom as a 10 amp breaker. Any other suggestions?
Carl

[DFord]

OregonTravelers, Try to figure out if the "call for heat" signal coming from the thermostat is the problem - is it continuous or intermittent.

Atwood manual for furnaces with wiring diagrams. See if you can use this manual to help troubleshoot the problem.

You could put a 12v test light in parallel with the furnace control wires and watch it to see if the on/off is being caused by the thermostat or if the thermostat call for heat is on all the time and the cycling is being done by the furnace itself.

From the wiring diagram:
There's terminal block where the 12v power enters the furnace. Terminal #1 has a red wire (12v pos), right next to that is terminal #4 has a yellow wire for 12v neg (ground). The next terminal down is #2 and it feeds the thermostat 12v pos. The thermostat signal comes back on the next terminal down (#5) on a blue wire. The diagram shows both wires for the thermostat are blue - #2 comes from the circuit breaker and #5 goes to the connection common to the sail switch and the ignition control board. The two other terminals are #3 & #6 and are not used - from the top the terminals numbers are 1-4-2-5-3-6 (confusing).

Put your test light between terminal #4 for 12v neg and terminal #5 to see when the thermostat is calling. The furnace should run continuously as long as the test light is lit. If it doesn't, then look at the sail switch and limit switch again.

In the connector on the ignition control:
The white wire on the ignition control comes from the limit switch and sail switch circuit. The blue wire is 12v pos from the thermostat. The red wire goes to the gas valve and the black wire is 12v neg. There are two red wires separately on the ignition control - one to the blower motor and one to the circuit breaker.

If the circuit breaker was killing the circuit, everything would stop at once. The blower would not finish its normal run. Look elsewhere.

[Chris_Bryant]

Check the current draw of the motor, then replace the breaker. Original was often a 7 amp, replacement is a 10 amp.