Atwood 8500-IV furnace problem

One thing I forgot to mention. The problem with the furnace cycling one and off and then tripping the 12 volt circuit breaker happens more when the thermostat is set at 65 degrees or more. It used to not happen below 65 or above 70 degrees. But now the problem occurs at all temperatures about 60 degrees.
That is why I was thinking at perhaps the problem was in the thermostat.
Carl

OregonTravelers wrote:

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

The breaker is tripping. There is nothing in a furnace short cycling which will cause an increase in current draw- it is not like an air conditioner, thus, something is wrong with either the blower motor or breaker.
The only other problem could be a short, which should be pretty obvious, as it would be right there in the furnace.

Have you checked the Hi limit switch? That is the switch that continues the fan running when the burner shuts off. IF that Limit is weak, it may open and then close. When it closes it will start a new cycle, which should NOT happen as long as the tstat is not calling for heat. WHY this continued operation would kick the CB, is unusual. Usually the only thing that will kick a CB is a defective fan motor. A Direct short would happen all the time. I would replace the motor. Doug

DFord wrote:
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.

I am pretty sure all of that has been checked and found to be OK. The furnace run thru the cycle fine and shut off when temperature is reached. Then immediately starts over again with the complete cycle. It will do this three or four times and then kick the 12 volt circuit breaker. The furnace seems to work fine and runs thru the heat cycle but then starts all over again.
I think the only thing that has not been replaced is the thermostat and am wondering if that might be my problem?
Carl

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

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.

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