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

[bid_time]

Sluggo54 wrote:
Some thermostats have adjustable "range" - i.e., it can be set to raise the temp 1 degree, two degrees, or three. It may be sensitive enough that a slight change in temp could cause problems. It is no cost to open it or take the cover off and find out.

That's a very good suggestion. If the "range" is set to low it could cause the cycling. That furnace should not be instantaneously cycling on and off. That could possibly make something get to hot and make the circuit breaker blow.

[Sluggo54]

Some thermostats have adjustable "range" - i.e., it can be set to raise the temp 1 degree, two degrees, or three. It may be sensitive enough that a slight change in temp could cause problems. It is no cost to open it or take the cover off and find out.

[bid_time]

There is no reason the furnace should cycle on unless the thermostat is calling for more heat. I would say the thermostat is malfunctioning. It should be easy enough to connect some meter test leads to it and check it out.

[OregonTravelers]

At this point I am leaning heavily toward the blower motor. The comments by several of you that the draw of the blower motor might overheat the breaker makes sense. So I am going to see about finding a motor and replacing it.
Again, thanks to everyone for your suggestions and help.
Carl

[dougrainer]

liborko wrote:
Did you check burner? You have to remove valve assembly to do that. Four screws. And replace a firing electrode while it is easy.

I suspect the burner grid is rusted/gone allowing to burn too much gas triggering the limit switch. The motor starting current is very high and repeated starting might overheat the breaker.

Atwood part number for burner:#30268

Atwood part number for firing electrode:#34570

Good luck

He has BYPASSED the limit switch and it still malfunctions. Doug

[Old-Biscuit]

Check the motor amp draw.......then you will know.

Motor is ONLY component that will cause tripping of dc breaker.


Chris and Doug have already covered WHY. Listen up!

[liborko]

Did you check burner? You have to remove valve assembly to do that. Four screws. And replace a firing electrode while it is easy.

I suspect the burner grid is rusted/gone allowing to burn too much gas triggering the limit switch. The motor starting current is very high and repeated starting might overheat the breaker.

Atwood part number for burner:#30268

Atwood part number for firing electrode:#34570

Good luck

[DFord]

I gave you troubleshooting tips that would identify where the problem is. If you don't understand how to troubleshoot the problem, find someone that does. You've already swapped enough parts to have replaced the whole furnace and you're still at step one. Swapping more parts it's not likely to fix your problem.

Good luck, you're going to need it.

[OregonTravelers]

Thanks to everyone for their response and suggestions to my furnace problem. Guess I will see about replace the blower motor and hope that solves the problem.
Many thanks to everyone who offer help and suggestions.
Carl

[hypoxia]

You have the benefit that two of the best professionals on the forum are giving you direction. When they speak, I listen very carefully. The thermostat is simply a switch that sends 12 volts to the furnace.

[DownTheAvenue]

A thermostat is a cheap part to throw at the problem.

[OregonTravelers]

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

[Chris_Bryant]

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.

[dougrainer]

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

[OregonTravelers]

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