Atwood furnace blower no burner

[Jim McGarvie]

I knew something was wrong with our Atwood 8535 furnace when I woke up cold at 3 am and realized the blower was running continuously but I never heard the burner light. We are boondocking in the forest and it got down to 31 outside last night, and 40 inside. We are now running the generator and a small cube heater, but it is a slow process.

I'm vaguely aware of some of the possible issues, many of which I don't think I can fix away from home. Anything come to mind that is relatively simple to correct? We have a full LP tank and the water heater works fine. No outlets are blocked.

As an aside, it's nice these things have the sail switch to prevent the possibly dangerous situation of burner burning but blower not blowing. But why isn't there something to prevent the blower from running down the batteries when the burner won't light?

Thanks.

Jim

 

[PeterSchweizer]

Typically the blower will turn on sail switch se the airflow  turns on igniter and gas. if after 3 try's it will shut off so the blower should shut off also. try turning it off and waiting several minutes and re start.  might be control board as the blower should shut off until you restart it with the thermostat furnace on switch and the temp selection. try moving temp selection 

[Bob Nodine]

19 hours ago, Jim McGarvie said:

I knew something was wrong with our Atwood 8535 furnace when I woke up cold at 3 am and realized the blower was running continuously but I never heard the burner light. We are boondocking in the forest and it got down to 31 outside last night, and 40 inside. We are now running the generator and a small cube heater, but it is a slow process.

I'm vaguely aware of some of the possible issues, many of which I don't think I can fix away from home. Anything come to mind that is relatively simple to correct? We have a full LP tank and the water heater works fine. No outlets are blocked.

As an aside, it's nice these things have the sail switch to prevent the possibly dangerous situation of burner burning but blower not blowing. But why isn't there something to prevent the blower from running down the batteries when the burner won't light?

Thanks.

Jim

 

When dry camping this is often caused by low battery voltage because the blower does not produce enough air flow to complete the sail switch. The furnace will not ignite if the sail switch does not make. Start the generator and see if that helps.

 

[Jim McGarvie]

2 hours ago, Bob Nodine said:

When dry camping this is often caused by low battery voltage because the blower does not produce enough air flow to complete the sail switch. The furnace will not ignite if the sail switch does not make. Start the generator and see if that helps.

 

Thanks Bob. I've experienced that situation once before, and the battery voltage was a little low this time too; right around 12.0V. After a day of solar charging yesterday the furnace worked fine all night, and is still working this morning, with 12.23V. But if battery voltage is the cause, it would be nice if it would continue to work at a little lower SOC.

And I REALLY don't think the blower should continue to run if the burner fails to ignite.

Jim

[Jim McGarvie]

19 hours ago, PeterSchweizer said:

Typically the blower will turn on sail switch se the airflow  turns on igniter and gas. if after 3 try's it will shut off so the blower should shut off also. try turning it off and waiting several minutes and re start.  might be control board as the blower should shut off until you restart it with the thermostat furnace on switch and the temp selection. try moving temp selection 

Thanks Peter. I'm glad to know the blower is not supposed to run when the burner is off. I will see if I can have my control board tested.

[Bob Nodine]

20 hours ago, Jim McGarvie said:

Thanks Bob. I've experienced that situation once before, and the battery voltage was a little low this time too; right around 12.0V. After a day of solar charging yesterday the furnace worked fine all night, and is still working this morning, with 12.23V. But if battery voltage is the cause, it would be nice if it would continue to work at a little lower SOC.

And I REALLY don't think the blower should continue to run if the burner fails to ignite.

Jim

Jim,

The blower continuing to run is a safety feature just in case there is propane present.

 

[millionroy]

Went through this 2 years ago and after some research returned to Camping World for the 3rd time and told them to replace the pressure regulator on the propane tank and it has worked fine ever since.  When it got cold enough, the pressure regulator failed.  Don't claim to understand all that, but it worked in my case.

 

[Jim McGarvie]

2 hours ago, millionroy said:

Went through this 2 years ago and after some research returned to Camping World for the 3rd time and told them to replace the pressure regulator on the propane tank and it has worked fine ever since.  When it got cold enough, the pressure regulator failed.  Don't claim to understand all that, but it worked in my case.

 

Thanks, Roy. That could be the issue. I don't think we have used the furnace in such low temperatures in the past.

Jim

2 hours ago, Bob Nodine said:

Jim,

The blower continuing to run is a safety feature just in case there is propane present.

 

I guess that makes sense, but it is sure a good way to run the batteries down overnight when boondocking....

Thanks Bob.

[David White]

I have had a regulator to fail in cold weather, I think due to some moisture that naturally accumulates and fails to vent out. Replaced it and all was good.  That would be your least expensive check at this point.

Edited November 6, 2020 by David White

[Jim McGarvie]

6 minutes ago, David White said:

I have had a regulator to fail in cold weather, I think due to some moisture that naturally accumulates and fails to vent out. Replaced it and all was good.  That would be your least expensive check at this point.

Worth a try. Thanks.

 

[walt2137]

On 11/5/2020 at 8:20 AM, Jim McGarvie said:

Thanks Bob. I've experienced that situation once before, and the battery voltage was a little low this time too; right around 12.0V. After a day of solar charging yesterday the furnace worked fine all night, and is still working this morning, with 12.23V. But if battery voltage is the cause, it would be nice if it would continue to work at a little lower SOC.

And I REALLY don't think the blower should continue to run if the burner fails to ignite.

Jim

Jim my furnace will do the same if it fails to ignite and that is normal and I’m sure you are aware that your battery is considered dead at 12 volts and you are really close at 12.3 

Walt

[astgerma]

On 11/4/2020 at 10:55 AM, Jim McGarvie said:

I knew something was wrong with our Atwood 8535 furnace when I woke up cold at 3 am and realized the blower was running continuously but I never heard the burner light. We are boondocking in the forest and it got down to 31 outside last night, and 40 inside. We are now running the generator and a small cube heater, but it is a slow process.

I'm vaguely aware of some of the possible issues, many of which I don't think I can fix away from home. Anything come to mind that is relatively simple to correct? We have a full LP tank and the water heater works fine. No outlets are blocked.

As an aside, it's nice these things have the sail switch to prevent the possibly dangerous situation of burner burning but blower not blowing. But why isn't there something to prevent the blower from running down the batteries when the burner won't light?

Thanks.

Jim

 

I had a problem with my furnace not igniting and the reason was the burner was burnt out .  I am including a photo of burnt out buner.

[Jim McGarvie]

1 hour ago, astgerma said:

I had a problem with my furnace not igniting and the reason was the burner was burnt out .  I am including a photo of burnt out buner.

Yeah, that wold probably do it! Thanks Andy.

[Harvey Babb]

I had this situation and it was caused by low battery and a sticking gas valve. If the furnace will operate normally with the generator running then the gas valve would be the likely cause.

As others have stated, the blower WILL run continuously if it fails to light and this is by design.

The complete sequence (from memory) is:

Thermostat powers the blower relay and sail switch.

On detecting air flow, the sail switch applies power to control board

Control board delays for "purge timeout" (around 15 seconds?)

Control board powers up spark igniter and gas valve.

Control board monitors for flame; if no flame for a time interval, the control board goes back to purge timeout.

If no flame detected after 3 tries of "purge, ignite, retry", control board shuts down and does nothing more. BLOWER KEEPS RUNNING!

If flame is detected, igniter is left "off" and furnace heats until the thermostat shuts it down.

If control board detects loss of flame, it will go back to "purge, ignite, retry" point.

Once thermostat is satisfied, blower relay and control board are turned "off". 

The blower relay is a thermal type, and will continue to run the blower for 30 seconds or so, then stop.

As I said, this is from memory, and it is also possible that different control board versions will vary the sequence somewhat. I can only state that the three I have tested work this way. 

[Jim McGarvie]

Thanks, Harvey, good info.

One question. You wrote "If flame is detected, igniter is left "off" and furnace heats until the thermostat shuts it down." That reminds me of another question I've had about our furnace. Typically the burner will cycle on and off while the blower continues to operate until the thermostat shuts it down. If I could solve that presumed issue it would heat the motorhome more rapidly and reduce battery usage.

 

[Harvey Babb]

To quote Burt Gummer of Tremors fame "I feel I was denied critical need-to-know information"!

You should have mentioned this in your original post as it is symptomatic of another problem with these furnaces.

The flame detection system uses (depending on model) either the igniter electrode or a separate electrode placed in the flame to detect when a flame is present. If anything causes the flame to go out, or to be pushed away from the electrode, the control board will detect this and drop back to the "purge, ignite" stage. As noted by astgerma above, a burnt out burner can cause a "no light" situation as well as the flame detection intermittently failing during operation. 

I would pull the burner and give it a good look. While you have it out also get a flashlight and check in the burner housing for mud dauber nests or other obstruction.

 

 

[Jim McGarvie]

7 minutes ago, Harvey Babb said:

To quote Burt Gummer of Tremors fame "I feel I was denied critical need-to-know information"!

You should have mentioned this in your original post as it is symptomatic of another problem with these furnaces.

The flame detection system uses (depending on model) either the igniter electrode or a separate electrode placed in the flame to detect when a flame is present. If anything causes the flame to go out, or to be pushed away from the electrode, the control board will detect this and drop back to the "purge, ignite" stage. As noted by astgerma above, a burnt out burner can cause a "no light" situation as well as the flame detection intermittently failing during operation. 

I would pull the burner and give it a good look. While you have it out also get a flashlight and check in the burner housing for mud dauber nests or other obstruction.

 

 

I agree, I should have mentioned it originally. I had forgotten all about it. So that is not normal operation to have the burner cycle on and off (every few minutes) while the blower continues to run until the thermostat shuts off the furnace? Good to know. I guess I need to get deeper into this (removing the burner) than I thought.

Thanks Harvey.

[Harvey Babb]

The only time I would consider it "normal" for it to cycle that way would be on initial startup of a cold room or a large change in thermostat setting. In that case, the thermostat will sometimes "short cycle" faster than the blower relay delay. 

After the room coming up to temperature you should never see short cycling happen.

[pulsarjab]

The electrical connections are very simple and easy to get to. When I had issues, I found the connectors had corrosion. Cleaned connectors and back in business.

[Bob Nodine]

12 hours ago, Harvey Babb said:

The only time I would consider it "normal" for it to cycle that way would be on initial startup of a cold room or a large change in thermostat setting. In that case, the thermostat will sometimes "short cycle" faster than the blower relay delay. 

After the room coming up to temperature you should never see short cycling happen.

One other thing that will cause that is if the furnace output is blowing on or near the thermostat. I once purchased a new 5th wheel and took it back to the dealer several times because the furnace would continually short cycle. They replaced the thermostat and that did not fix it and they said there was nothing else they could do. One day I was sitting in the trailer and looked at the floor register sitting right under the thermostat and it finally hit me what the problem was. Purchased one of those floor registers that could be shut off and problem fixed. Needless to say I wrote the manufacture a letter informing them they need to pay attention to the location of the thermostat.

 

[Jim McGarvie]

3 hours ago, Bob Nodine said:

One other thing that will cause that is if the furnace output is blowing on or near the thermostat. I once purchased a new 5th wheel and took it back to the dealer several times because the furnace would continually short cycle. They replaced the thermostat and that did not fix it and they said there was nothing else they could do. One day I was sitting in the trailer and looked at the floor register sitting right under the thermostat and it finally hit me what the problem was. Purchased one of those floor registers that could be shut off and problem fixed. Needless to say I wrote the manufacture a letter informing them they need to pay attention to the location of the thermostat.

 

Thanks for that reminder, Bob. I discovered that a few years ago and bent the fins of the register away from the thermostat. I think it may have helped. But we had our flooring replaced this summer and they replaced our register with a plastic one whose fins can't be bent. I will replace the register and try that again.

Jim

4 hours ago, pulsarjab said:

The electrical connections are very simple and easy to get to. When I had issues, I found the connectors had corrosion. Cleaned connectors and back in business.

Thanks, I'll do that too.

[Jim McGarvie]

Follow up from the OP:

1. I replaced the floor outlet below the thermostat with a metal one, and bent the louvers to all point away from the thermostat. No change in performance.

2. I replaced the control board with a Dinosaur. No change.

3. I replaced the LP regulator. No change.

4. I bought a manometer and checked the LP pressure with the new regulator. Ours is 12" with furnace and water heater operating. Should be plenty; our local RV shop owner says that is not too high.

5. Ordered a "tune-up kit" from PDX RV, which includes Burner Assembly, Electrode, Electrode Gasket, Limit Switch 190°, and Sail Switch.

6. In preparation for replacing the above parts, I removed the furnace from the coach.

7. After removing the furnace I discovered the installation was only using 3, 4" duct openings. According to the service manual for our model, it requires a minimum of 4, 4" ducts. I believe inadequate ducting may be the reason for our short cycling.

8. I plan to add a fourth 4" duct. The only feasible way of doing that is to simply route it to the side of the cabinet containing the furnace, below the sink, and to an outlet behind the couch.

I will give another update when the project is complete. In the meantime, any and all comments are welcome.

Jim