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]

trailrider wrote:
REPLACE THE MOTOR!!!

Most likely all of your furnace problems will disappear

OK, that is the next step. But I still don't understand how a weak or bad blower motor will cause the furnace to complete the heat cycle, turn off and then immediately start a second cycle including clearing the furnace, ignition, heat, heat off, and finally the fan stopping. Then almost immediately starting the sequence again. Doing this three or four times and then finally tripping the 12 volt circuit breaker.

[trailrider]

REPLACE THE MOTOR!!!

Most likely all of your furnace problems will disappear

[Kayteg1]

delete

[Bird_Freak]

OregonTravelers wrote:
Still having a problem with the furnace. The circuit board was replaced previously and I yesterday, I replaced the thermostat, but the furnace still cycles on and off and then trips the circuit breaker.
Now what?

Don't know what to tell you. You have 2 rv techs here telling you motor but your not listening.

[westend]

Do you have a hand held meter?

[OregonTravelers]

Still having a problem with the furnace. The circuit board was replaced previously and I yesterday, I replaced the thermostat, but the furnace still cycles on and off and then trips the circuit breaker.
Now what?

[rrev]

Had a house thermostat do the cycling thing before. If you already have one, try it. To me I think you have two problems too.

[OregonTravelers]

bid_time wrote:

Chris Bryant wrote:
Y'all can talk about the thermostat as much as you want, but I could replace it with a Morse code key and key the unabridged Oxford English Dictionary and not trip that breaker. This furnace is not an air conditioner. Short cycling cannot and will not cause excessive current draw- certainly not (at 15 amps) nearly double the rated draw (8 amps).
When you have a system that does not work, and you know there is a fault, you fix that fault before anything else. Now I wouldn't do anything before actually measuring the current draw of the motor, but there really is not that much more that it could be.

You know Chris, I respect your opinion a lot. But there is no call to be snarky with the unabridged dictionary thing.

We have a difference of opinion, but I don't think an overdraw by the motor is causing the furnace to cycle. But I can envision the motor overheating by continuous cycling, especially in a furnace with the burner on. After that all types of things could happen. Peace!

Now, Now. No fighting. I appreciate the thoughts and information for each of you. Several months ago, I purchased a new thermostat so plan to install that in the next day or two and we will see what happens after that.
Again, Mahalo for the thoughts and idea's.
Carl

[bid_time]

Chris Bryant wrote:
Y'all can talk about the thermostat as much as you want, but I could replace it with a Morse code key and key the unabridged Oxford English Dictionary and not trip that breaker. This furnace is not an air conditioner. Short cycling cannot and will not cause excessive current draw- certainly not (at 15 amps) nearly double the rated draw (8 amps).
When you have a system that does not work, and you know there is a fault, you fix that fault before anything else. Now I wouldn't do anything before actually measuring the current draw of the motor, but there really is not that much more that it could be.

You know Chris, I respect your opinion a lot. But there is no call to be snarky with the unabridged dictionary thing.

We have a difference of opinion, but I don't think an overdraw by the motor is causing the furnace to cycle. But I can envision the motor overheating by continuous cycling, especially in a furnace with the burner on. After that all types of things could happen. Peace!

[dougrainer]

DFord wrote:
So far, I've seen no indication that anyone has any idea of why the furnace is short cycling when it should be off. I've given a suggested procedure to begin troubleshooting but everyone just wants to swap parts. All the parts changed out so far has just been a waste of money and nothing has been accomplished.

I've studied the wiring diagram and see the thermostat only has two wires going to it. When the thermostat calls for heat, those two wires are connected. When it's not, those wire are disconnected. Until you know why the furnace is short cycling, the reason for the breaker tripping is a secondary symptom of the actual problem. A bad blower motor could not possibly cause the furnace to short cycle.

I would disconnect the thermostat and use a jumper to run the furnace for a couple of minutes. Then remove the jumper and see what happens. The furnace should not reignite again. What we need to know is if the thermostat is causing the furnace to short cycle. The OP stated the furnace reignited when it was short cycling. If that's the case, the thermostat is the cause of the problem. If only the blower is short cycling, the problem is probably the "relay."

"RELAY" description from the furnace manual wrote:

Simple. IF the motor is defective(IT IS in this case), the motor when it starts to draw too many amps will overheat and slow down. That causes 2 things. 1. The sail switch will probably open and shut the burner down. 2. The slower speed causes the burner case to also overheat and may cause the Limit switch to then trip. You can argue all you want about the parts replaced. In this case ALL the pertinent parts have been replaced and the only part left is the motor which is the ONLY part that will cause the CB to trip without a direct short. As with what Chris stated---Thermostats NEVER and rarely cause operational problems. ALL they do is open and close 2 wires to the furnace, nothing else. Doug

[OregonTravelers]

DFord wrote:
So far, I've seen no indication that anyone has any idea of why the furnace is short cycling when it should be off. I've given a suggested procedure to begin troubleshooting but everyone just wants to swap parts. All the parts changed out so far has just been a waste of money and nothing has been accomplished.

I've studied the wiring diagram and see the thermostat only has two wires going to it. When the thermostat calls for heat, those two wires are connected. When it's not, those wire are disconnected. Until you know why the furnace is short cycling, the reason for the breaker tripping is a secondary symptom of the actual problem. A bad blower motor could not possibly cause the furnace to short cycle.

I would disconnect the thermostat and use a jumper to run the furnace for a couple of minutes. Then remove the jumper and see what happens. The furnace should not reignite again. What we need to know is if the thermostat is causing the furnace to short cycle. The OP stated the furnace reignited when it was short cycling. If that's the case, the thermostat is the cause of the problem. If only the blower is short cycling, the problem is probably the "relay."

"RELAY" description from the furnace manual wrote:

Don. I have been pondering the same thing. I can understand that excessive current draw of blower motor would trip the circuit breaker. But I don't understand why that would cause the furnace to cycle on and off like it does before the breaker is tripped.
I like your suggestion of trying to by-pass the thermostat and see if the problem continues.
Thanks for the insight.
Carl

[DFord]

So far, I've seen no indication that anyone has any idea of why the furnace is short cycling when it should be off. I've given a suggested procedure to begin troubleshooting but everyone just wants to swap parts. All the parts changed out so far has just been a waste of money and nothing has been accomplished.

I've studied the wiring diagram and see the thermostat only has two wires going to it. When the thermostat calls for heat, those two wires are connected. When it's not, those wire are disconnected. Until you know why the furnace is short cycling, the reason for the breaker tripping is a secondary symptom of the actual problem. A bad blower motor could not possibly cause the furnace to short cycle.

I would disconnect the thermostat and use a jumper to run the furnace for a couple of minutes. Then remove the jumper and see what happens. The furnace should not reignite again. What we need to know is if the thermostat is causing the furnace to short cycle. The OP stated the furnace reignited when it was short cycling. If that's the case, the thermostat is the cause of the problem. If only the blower is short cycling, the problem is probably the "relay."

"RELAY" description from the furnace manual wrote:

[OregonTravelers]

Chris Bryant wrote:
Y'all can talk about the thermostat as much as you want, but I could replace it with a Morse code key and key the unabridged Oxford English Dictionary and not trip that breaker. This furnace is not an air conditioner. Short cycling cannot and will not cause excessive current draw- certainly not (at 15 amps) nearly double the rated draw (8 amps).
When you have a system that does not work, and you know there is a fault, you fix that fault before anything else. Now I wouldn't do anything before actually measuring the current draw of the motor, but there really is not that much more that it could be.

Thanks for the insight Chris. My next move is to check the draw of the blower motor and see what that give me.
Carl

[OregonTravelers]

bid_time wrote:

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.

Can't do that with this thermostat. Still feeling that the draw of the motor is causing at least part of the problem. Plan to check that before anything else at this point,

[Chris_Bryant]

Y'all can talk about the thermostat as much as you want, but I could replace it with a Morse code key and key the unabridged Oxford English Dictionary and not trip that breaker. This furnace is not an air conditioner. Short cycling cannot and will not cause excessive current draw- certainly not (at 15 amps) nearly double the rated draw (8 amps).
When you have a system that does not work, and you know there is a fault, you fix that fault before anything else. Now I wouldn't do anything before actually measuring the current draw of the motor, but there really is not that much more that it could be.