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

[DFord]

I said a bad motor would not cause the short cycling. Not having your exact furnace model number, I was working from the wrong wiring diagram in the manual I downloaded so that's why I thought there were only two wires to the thermostat and of the five wires you found only two were for the furnace. My method of troubleshooting would have negated the need to replace the thermostat.

In my second post detailed the operation of the time delay relay that controls the fan. Have you been able to locate the relay and do any tests on it?

"RELAY" description from the furnace manual wrote:
This component is commonly referred to as a time delay relay. The same relay is used on the 7900, 8500 and 8900 series furnaces.

FUNCTION - The relay has one primary function.
-to purge the plenum of heat and the chamber of any unburnt gases after each heating cycle.

OPERATION - The motor voltage path of relay is normally open. There should always be voltage from the circuit breaker to the relay terminal of the circuit breaker. There should always be continuity between the thermostat terminal connection and ground terminal connection or the relay.

Only when the thermostat contacts are closed is voltage supplied to the thermostat terminal of the relay. This voltage heats a coil in the relay body. In approximately 20 seconds this heated coil causes a bimetal disc to close. Voltage now passes through the relay and on to the motor, which in turn should allow the furnace to ignite and start a heating cycle.

When a heating cycle is complete, the contacts of the thermostat open and voltage ceases to the heater coil of the relay. In approximately 45 - 90 seconds, the heater coil cools down, the bi-metal disc opens and voltage ceases to the motor as well.

[OregonTravelers]

DFord wrote:
OregonTravelers, I given you several suggestions to test and troubleshoot and never heard you cared to follow any of them. Instead you chose to throw more parts at it. You're on your own.

Don.
On 3/2/16 you suggested the thermostat had two wires going to it. My thermostat has five wires going to it. You suggested using a jumper to run the furnace.
Rather than using a jumper I chose to replace the thermostat entirely with a new one that I already had. That did not solve the problem. The furnace still cycles on and off.

[DFord]

OregonTravelers, I given you several suggestions to test and troubleshoot and never heard you cared to follow any of them. Instead you chose to throw more parts at it. You're on your own.

[Chris_Bryant]

OregonTravelers wrote:


Chris
You may be correct about the weak breaker, but I will have to challenge you about soldering never being done at the factory. I purchased this coach new in 2005, The only soldering that has been done on the circuit breaker was done to repair a wire solder that had worked loose because of a poor soldering job at the factory. In addition there are approximately 25 circuit breakers ranging from 7 amps to 15 amps on the panel in a compartment in the coach. ALL of the wires are soldered to the various circuit breakers.

I'm not talking about the coach- I'm talking about the furnace. I've never been talking about the coach. The furnace has a circuit breaker in it.THAT is what I have always been talking about. That breaker should never be soldered, if it was bad, it should be replaced.
If you are saying the coach breaker is tripping, you have a short in the coach wiring.

[OregonTravelers]

Because the breaker is defective, the voltage is cut going through it- it is a high resistance circuit. Once you have less than optimal voltage feeding the system, all kinds of things happen- like the time delay relay acting funky.
Now it could be the time delay relay itself- why I suggest replacing the circuit breaker with a fuse.
However, if the breaker is soldered in the system, it has been heavily modified, as this was never done at the factory. In this case, all bets are off.

Chris
You may be correct about the weak breaker, but I will have to challenge you about soldering never being done at the factory. I purchased this coach new in 2005, The only soldering that has been done on the circuit breaker was done to repair a wire solder that had worked loose because of a poor soldering job at the factory. In addition there are approximately 25 circuit breakers ranging from 7 amps to 15 amps on the panel in a compartment in the coach. ALL of the wires are soldered to the various circuit breakers.

[Chris_Bryant]

OregonTravelers wrote:

Chris Bryant wrote:

OregonTravelers wrote:

OK Chris. The dead horse has been whipped. Finally had the blower motor checked. The motor draws 7 amps. Exactly what it is rated to draw. Furnace still cycling on and off.

OK- follow along. If it is drawing 7 amps, and the 15 amp breaker is tripping, the breaker is bad- likely cutting voltage through it. Replace it.
This is why you test things, rather than guess and throw parts.

On edit- you don't even have to buy a part to check this- just pull the leads off the breaker and hook a 15 amp blade fuse in place- the connectors will fit fine.

OK, but please explain to me how the circuit breaker causes the furnace to cycle on and off? The furnace starts, heats to the temperature on the thermostat, then furnace shut of and the fan continues to run for a short while, then everything stops. After a few seconds the fan starts, then the furnace starts and runs for a few seconds. Then the furnace shuts down, the fan runs for a few more seconds then everything turns off. Then the cycle starts all over again. This cycling happens three or four times and then the circuit breaker trips.
Don't understand how the circuit breaker would cause this.
Explain please.
The other issue is that the wires are soldered on to the circuit breaker.

Carl

Because the breaker is defective, the voltage is cut going through it- it is a high resistance circuit. Once you have less than optimal voltage feeding the system, all kinds of things happen- like the time delay relay acting funky.
Now it could be the time delay relay itself- why I suggest replacing the circuit breaker with a fuse.
However, if the breaker is soldered in the system, it has been heavily modified, as this was never done at the factory. In this case, all bets are off.

[OregonTravelers]

Chris Bryant wrote:

OregonTravelers wrote:

OK Chris. The dead horse has been whipped. Finally had the blower motor checked. The motor draws 7 amps. Exactly what it is rated to draw. Furnace still cycling on and off.

OK- follow along. If it is drawing 7 amps, and the 15 amp breaker is tripping, the breaker is bad- likely cutting voltage through it. Replace it.
This is why you test things, rather than guess and throw parts.

On edit- you don't even have to buy a part to check this- just pull the leads off the breaker and hook a 15 amp blade fuse in place- the connectors will fit fine.

OK, but please explain to me how the circuit breaker causes the furnace to cycle on and off? The furnace starts, heats to the temperature on the thermostat, then furnace shut of and the fan continues to run for a short while, then everything stops. After a few seconds the fan starts, then the furnace starts and runs for a few seconds. Then the furnace shuts down, the fan runs for a few more seconds then everything turns off. Then the cycle starts all over again. This cycling happens three or four times and then the circuit breaker trips.
Don't understand how the circuit breaker would cause this.
Explain please.
The other issue is that the wires are soldered on to the circuit breaker.

Carl

[Chris_Bryant]

OregonTravelers wrote:

OK Chris. The dead horse has been whipped. Finally had the blower motor checked. The motor draws 7 amps. Exactly what it is rated to draw. Furnace still cycling on and off.

OK- follow along. If it is drawing 7 amps, and the 15 amp breaker is tripping, the breaker is bad- likely cutting voltage through it. Replace it.
This is why you test things, rather than guess and throw parts.

On edit- you don't even have to buy a part to check this- just pull the leads off the breaker and hook a 15 amp blade fuse in place- the connectors will fit fine.

[OregonTravelers]

Good Morning All.
Several folks have suggested that the blower motor might be the cause of our furnace problem. Had the motor checked and the motor draws 7 amps. Exactly what it is rated at.
So now the high limit switch has been checked, the board has been replaced, the thermostat has been replaced and the motor has been checked. All items deemed to be working as they should. The furnace still cycles on and off.
Suggestions?
Carl

[OregonTravelers]

Chris Bryant wrote:
At the risk of....


There are many ways a bad motor can cause cycling- in addition to what Doug pointed out, a 15 amp draw will pull the voltage down, and can easily pull it down far enough that the time delay relay will disengage, as well as the electronics dropping out.
We know that the motor has problems- a quick check of the current draw would verify that. Why throw parts at a furnace before you fix the main problem?

OK Chris. The dead horse has been whipped. Finally had the blower motor checked. The motor draws 7 amps. Exactly what it is rated to draw. Furnace still cycling on and off.

[OregonTravelers]

OK Will check the draw on the motor as soon as I can get to a shop and purchase one. I have one, that someone gave me, but it is a cheapie and I wouldn't trust it.
Mahalo
Carl

[Chris_Bryant]

At the risk of....


There are many ways a bad motor can cause cycling- in addition to what Doug pointed out, a 15 amp draw will pull the voltage down, and can easily pull it down far enough that the time delay relay will disengage, as well as the electronics dropping out.
We know that the motor has problems- a quick check of the current draw would verify that. Why throw parts at a furnace before you fix the main problem?

[liborko]

You may have a defective motor but that does not cause cycling. The thermostat can not control furnace that does not work properly.
If your burner does not work properly the firing/flame sensing electrode will not keep flame going. So I suggest again remove valve assembly and check condition of burner and firing electrode. Replace the electrode with the style that has wire coming out instead of plug-in terminal that has a tendency to corrode & make poor connection. You already have Atwood part number.

[westend]

Doug G, in a previous post, outlined a scenario why the motor could be the cause of your furnace problems, including the short cycling.

If it was me, I'd want to diagnose the condition of the motor, even knowing it was the cause of the problem. You may also have a worn circuit breaker or an additional issue with another part of the furnace. A meter is the tool to use for accomplishing that.