Goodman furnace won't shut off

[thalij]

Hey, I've got a Goodman GMT115 gas furnace that, if turned on, will not stop heating the house. This started two nights ago, and it may be a fault in the fan/blower relay switch or something. I've tried turning off the thermostat, but the only way to regulate it now is to turn off power to the furnace. The manufacturer website doesn't give any help. Matching up the manual's wiring diagram with the furnace control panel and wiring doesn't quite mesh. Any help in diagnosing the problem, or identifying where the fan/blower relay is?

[Dr. Hardcrab]

wrote

I'm sorry.....

[Smarty]

Somewhere on the furnace you should find a terminal strip where the wires from your thermostat connect. Temporarily disconnect the red wire and see if the furnace then shuts. The burner, if ignited, should quit immediately, and the blower may take a while to turn off since it is most likely on a delayed relay / switch. The key thing is to isolate the problem to either the thermostat versus the furnace, which this simple test will achieve.

If the thermostat is stuck in the continuous on condition, then removing the red wire will reveal that the thermostat and not the furnace is at fault.

If, on the other hand, the furnace continues to run despite the red wire being disconnected, then the furnace itself is the problem.

There should be a wiring diagram someplace inside the furnace or in the owner's manual if you want to try to troubleshoot the furnace with our help. If not, call a competent repair person.

[Speedy Jim]

If it continues to heat the house (burners on) after the thermostat is satisfied, then it's not a fan/blower relay problem.

If you're confident working on the wiring, disconnect the thermostat wires (Red and White) at the furnace. If the gas burners now shut off, the thermostat is at fault or the wiring to the stat is shorted. (Shut the power off before working...)

If you're not confident or lack the experience, wait for the tech.

[Stormin Mormon]

Sounds electrical. I had a furnace I worked on, years ago. They'd replaced the stat three times before I learned of the trouble. Problem turned out to be a sticky relay in the control box.

[thalij]

Brilliant, men! I had read some about the red and white wires, but your discussion was a little clearer in guidance. I tracked down the red wire, disconnected it and tried the furnace. It didn't start, so I reconnected the red and voila, it seemed to "reboot" things and it is now cycling regularly and maintaining a consistent temp.

[RBM]

That doesn't sound like your problem is solved. I have a feeling that the problem will reoccur, and possibly has to do with the type of thermostat you're using. It may have a current robbing, charging system that's pulling to much current for the relay it's controlling, causing it to stick in the closed position. Do post back if it happens again

[thalij]

Good foresight on the problem not truly being solved. I had a feeling, but was hoping for dumb luck. There are two thermostat controls (Honeywell T8112) in the house. Does it sound like I need a new blower relay switch?

[Speedy Jim]

Not the relay.

We're betting that the thermostat is at fault.

If opening the Red (or White) wire shuts the burner off, then the furnace is working normally.

Jim

[Stormin Mormon]

I'd think with excess draw, the relay would not close at all. My vote is sticky relay. Not releasing.

[Smarty]

The furnace should only be connected to and operated by one thermostat, not two.

Are you aware of how the 2 thermostats are wired??

Smarty

[Bob F]

With two thermostats, there could be enough current draw to prevent the relay from dropping out after it closes.

Bob