Furnace works but only 'manual'

[sportsdad60]

Hey all,
My furnace developed a bad habit now that we're in colder climates.
When in HEAT mode on the Airexcel thermostat, the heat comes on and heats to desired temp.
Then it shuts off.
The problem is, it doesn't come back on when the temp drops below the pre-set temp.
I must turn the thermostat to "off", wait for the click, then cycle back through to heat again for it to work again, once.

I replaced the Airexcel thermostat yesterday but the issue is not resolved.
Any clue of what is causing this?

A/C works fine, comes on when temp gets above temp setting. It's only the heat that won't cycle on automatically.

[howson]

sportsdad60

Since doing a reset allows the furnace to work and you've already eliminated the thermostat as a possible cause, all the usual suspects (sail switch, etc) are likely not the cause of the fault. I suspect the main board is bad, but it would take some very specific troubleshooting to verify.

What I mean by "very specific troubleshooting" is getting to the main board in the furnace (sometimes that's not easy) and measuring the thermostat's input at the main board. If it can be confirmed that the thermostat is calling for heat (when below the set point) and the signal is making it to the input of the board without it responding...the board is most likely the failure point.

Why I think it's the main board (without doing any checking) is because a power cycle "reboots" the board. But that's an educated guess on my part.

Consider calling Dinosaur. While I've never talked to them myself, I've heard nothing but good things from those that have spoken to the company. If you do try a new board, the quality of their replacement far exceeds the Suburban OEM item.

More info here: https://gdrvowners.com/forum/operati...s-2020-catalog

Howard

[sportsdad60]

Thanks Howard.
I was afraid this might be the case. The board was replaced 14 months ago under warranty for the same issue. Looks like I have another bad board.
I guess I'll have to dig out my warranty for the furnace to see if it's still covered (2021)

[howson]

Thanks Howard.
I was afraid this might be the case. The board was replaced 14 months ago under warranty for the same issue. Looks like I have another bad board.
I guess I'll have to dig out my warranty for the furnace to see if it's still covered (2021)

In the 310GK the furnace is underneath the shower. If you do the work yourself, look closely for any sign of a water leak (no matter how insignificant). If you have someone else do the work, have them look for this issue, too. What I'm suspecting (since this is the second board) is that a small water leak may be getting to the board.

Hopefully I'm wrong, but something to check while doing the work.

Howard

[Scott'n'Wendy]

Try unplugging and reconnecting all the connectors on the board. It sometimes reestablishes a poor connection.

[sportsdad60]

Well, last night it decided to work as it should. Thankfully, because we'll be in colder climates tonight in CO Springs, then Sheridan, WY Friday, then home (Flathead lake) on Sat.

[Scott'n'Wendy]

Gotta love those intermittent problems. Safe travels!

[howson]

sportsdad60

Information from a good source (outside of this forum) you might find interesting. The failure mode we were discussing sounds exactly the same as your symptoms. Below is a direct quote.

I had one doing something very similar. Finally caught it in the act. The thermal overload switch was slow to turn back on when the heat exchanger overheated. The switch opens when it should,190°, but failed to turn back on at 120°. It eventually came back on but by then the control board was in lockout. I managed to catch it before the board locked out and found no 12v at the sail switch from the thermal overload. Swapped the thermal overload switch and it went back to operating normally.

Swapped the Airxcel thermostat and Fan50 control board twice before catching the thermal overload switch failure in the act.

[howson]

sportsdad60 -- another testimonial along the same lines as post 9.

I had two furnaces this winter with the identical problem. They were both Suburban SF-35Q furnaces from around 2018 vintage.

Symptom was that the fan would come for the 30 seconds or so, then shut down. No spark and no gas valve. The sail switches were working. On both furnaces I installed a Dinosaur Fan 50+ board and both furnaces were completely dead: no blower, no LED, no nothing. Replaced the original board and back to the original symptom behavior.

Turned out the limit switch was the problem and replacing them (with an upgraded part number) solved the problem. I tested the limit switches in the furnace they read open circuit. After I removed them and tested again they showed closed circuit. It looks like the switch didn’t reset automatically until I bumped it around. It looks to me like there might be a bad batch of limit switches out there so I bought some spares to keep on hand.

Later another source mentioned part numbers:

The new (limit switch) part number is 525006.
This replaces 232504 and 233532.