Furnace runs for a while then goes into shutdown

MCTalley

Well-known member
RESOLVED: Furnace runs for a while then goes into shutdown

I'm going to call Dometic tomorrow, but thought I'd throw this on the board to see if anyone has had a similar issue. This started this spring right at the end of the heating season, so we didn't do anything about it. Sadly, it didn't magically fix itself over the summer, so here we are.

Turn on furnace, fan runs, flame starts, heats the trailer as expected. Eventually, before the trailer gets up to temperature, you'll hear a click in the air conditioner (commanding the furnace to shutdown). The furnace will go through a cool-down cycle, shut off and will refuse to restart until you drop 12V power (or wait a long enough period of time).

We mentioned it to the two techs that were running around at the Urbanna rally and they suggested a bad limit switch (not the sail switch).

Anyone else had a similar issue?
 
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danemayer

Well-known member
Hi Malcolm and Val,

It sounds like a safety lockout - normally reset by cycling the thermostat off of HEAT and back again. But a safety lockout is usually triggered by sail switch failure or flame sense failure, which doesn't seem to be the case based on your description.

If you're not hearing a click in the A/C unit when the thermostat calls for heat the 2nd time, I'd check the relay in the gray box behind the A/C intake in the ceiling. Look for loose wires. Also check the thermostat wire connections.

One other thought - when you're expecting the furnace to restart, try increasing the set point temperature 10 degrees higher to see if it starts.
 

MCTalley

Well-known member
Hi Malcolm and Val,

It sounds like a safety lockout - normally reset by cycling the thermostat off of HEAT and back again. But a safety lockout is usually triggered by sail switch failure or flame sense failure, which doesn't seem to be the case based on your description.

If you're not hearing a click in the A/C unit when the thermostat calls for heat the 2nd time, I'd check the relay in the gray box behind the A/C intake in the ceiling. Look for loose wires. Also check the thermostat wire connections.

One other thought - when you're expecting the furnace to restart, try increasing the set point temperature 10 degrees higher to see if it starts.

Increasing temperature setting, turning thermostat off and back to heat, etc. do nothing. You literally have to drop power to it (or wait some long period of time that I haven't investigated the length of yet) before it will start up again.

It will run a fairly long time, so I'm with you on it probably not being the flame sensor or sail switch.
 

danemayer

Well-known member
Ok,

if you hear the relay click (up in the A/C unit) when the thermostat calls for heat, then my money's on the furnace module board. If you don't hear the click when the thermostat calls for heat, my money's on the thermostat or wiring between thermostat and relay.
 

Rickhansen

Well-known member
Low air flow causing overheating. Make sure all vents are fully open, unobstructed, and try again.
 

MCTalley

Well-known member
I'll have to go through all of the motions again as soon as it is cool enough to actually need heat (probably tonight) and see what happens. Unless something underneath has blocked off one of the ducts, I don't think we have a low flow situation, though it is possible that it was marginal up until this spring and then something happened.
 

TravelTiger

Founding Texas-West Chapter Leaders-Retired
Malcolm, you didn't accidentally forget to cut a floor vent out when you did your flooring?!


Sent from my iPhone using Tapatalk
 

MCTalley

Well-known member
Looking ahead to the sub-freezing temperatures forecast next week in Nashville, I decided to do some more scientific troubleshooting this morning. We have a change in operation:

1. Verified indoor temperature according to thermostat is 65 degrees.
2. Cycled control to Furnace position.
3. Quickly increased desired temperature setting from previous 68 to 72.
4. About 2 seconds later a click is heard in the overhead A/C unit.
5. About 2 seconds after the A/C click is heard, I hear a louder click from the furnace.
6. Then... Nothing. Complete silence. No fan, no attempt to start the burner (which makes sense).

I changed the thermostat to Off, waited half a minute and tried the above again (raising the temp the second time to 80 degrees). Same result.

To to eliminate gas flow as a potential problem, I lit all three burners on our stove and ran the sucessfully and simultaneously on high.

I went outside and turned 12V power off at the main switch, waited 10 seconds and turned it back on.

returning inside, the only noted change was the first time I turned the furnace back on, I heard something beep for about one second when the furnace relay clicked. Subsequent attempts did not beep, only the A/C click followed by the furnace click.

Thinking logic board at this point. Calling Suburban tech support this afternoon.
 

MCTalley

Well-known member
Update - Resolved.

Mobile tech came by this afternoon and had it running in 30 minutes after swapping out the logic board in the furnace. We are now sitting in a warm and cozy trailer.
 
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