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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8505
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a goodman gmt070-3b furnace. When I run my ac the

Customer Question

I have a goodman gmt070-3b furnace. When I run my ac the furnace kicks on as well. I just replaced the board after checking the wires from the thermostat and replacing the thermostat. What do I try next?
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

1. Give me model on thermostat

2.If you can attach a clear pic of thermostat wires and wires at furnace will help.

If you have a paper clip icon on your page you can attach using that link.Or

You can upload the pictures using the tiny pic site .

Here is a link to the site.

Click on Browse , select the file from your folder.then click UPLOAD NOW.

They will give you a quote to enter in the box to verify you are a human.Enter the quote and click UPLOAD NOW

You will see your picture on right side of screen.

Then copy the link URL for email & IM

Paste that link here on the site.

Customer: replied 1 year ago.
The thermostat model is Honeywell RTH230B
Expert:  airheatman replied 1 year ago.

It is a bit hard for me t tell at furnace wiring.This is how it should be .PLease verify:

Red from furnace to red at thermostat

Green from furnace to Green at thermostat

White from furnace to white at thermostat

Blue from furnace to red outside and

Blue from thermostat straight to white outside

Customer: replied 1 year ago.
The wiring is fine. The problem randomly started after it worked fine for years It has to be a part that went bad. I already tried a different thermostat and replaced the board in the furnace. What's another part that could cause this?
Expert:  airheatman replied 1 year ago.

My first suspicion is the board,but you have changed that I understand.

Lets do this. Remove the wires from wirenuts,red,green ,white and the and red that goes outside.

Turn power on and verify nothing comes on.

Next touch RED to GREEn and see if blower ONLY comes on

Next RED to White and see if HEAT ONLY comes on

Next touch RED to the red going outside and make sure just outdoor unit comes on.

Customer: replied 1 year ago.
Ok. Just did that. The only thing that kicked on was the outside ac. No blower or heat. I did this with thermostat set to ac.
Expert:  airheatman replied 1 year ago.

OK now wires were all disconnected right? If so thermostat should not be controlling anything.That is what we were doing was eliminating thermostat or wiring outside of furnace.

And are you saying when you connected red and green from furnace blower did not come on and when you connected red and whie from furnace heat did not come on?

Customer: replied 1 year ago.
Oh ok, I wasn't thinking right. Just did it again. And everything worked. I didn't keep the wires touching for enough time the first time.
Expert:  airheatman replied 1 year ago.

OK issue has to be with board or how it is wired.Can you upload me a clear pic showing board and wires?

Customer: replied 1 year ago.
Try these. Hope you can decipher them. So the thermostat is definitely not the issue?
Expert:  airheatman replied 1 year ago.

Thermostat is definitely not the issue as we bypassed it and everything worked. Something in that board has to be sending signal to the induced draft motor and gas valve on call for cool.I am sure you do not want to hear that it is the board since you have changed board and still have same problem.

So let me open this to other experts who may have other ideas.I will opt out to see if anyone else has ideas. Please hold rating until we see if we can get you an answer.Thanks for letting me work with you to this point.

Customer: replied 1 year ago.
Ok. Sounds good thanks!
Expert:  Phil replied 1 year ago.


Lets do a few slightly different tests.

Follow the wire that leaves the W terminal on the thermostat and connects inside the furnace... disconnect it and put tape on the end so that it does not short out on anything. Then turn the thermostat on so that system goes into cooling mode.

If it goes into cooling mode, without the furnace burners coming on as before, then the the problem is in the wiring between the furnace and the thermostat...

Since you replaced the thermostat, I suspect a staple used to secure the thermostat wire to the wall studs during construction has cut through the insulation and has shorted the white wire to the green or yellow wire.

Do that test, let me know how it goes.

Let me know if there are any spare thermostat wires that you can see in the thermostat wire bundle.

We can go from there.

Customer: replied 1 year ago.
I did that at the beginning of summer and that worked. I just have a hard time thinking the wire went bad.
It did seem to go bad after a thunderstorm so maybe if there was a surge that melted the wires.
My house is a ranch so I doubt it would be difficult to tie new wire to the old and pull it though
Expert:  Phil replied 1 year ago.

Hello again,

Thermostat cable comes in 4, 5, 6 and 8 strand, If your system uses only 5 strand for instance, the installer would have pulled 6 or 8 strand cable and left the remaining wires wrapped around the end of each end of the cable.

One of those can be used as a spare to replace the white that case you will not have to pull new wire.

I can coach you how do it the easiest way to pull new wire if you have an attic. (drill through the wall plate just above the thermostat in the attic, poke the new thermostat wire through the hole and down into the wall until it hits the fire break) then remove the thermostat and use a coat hanger wire to fish the new cable through to the thermostat)

Before we do that however, do the test, remove the white wire and tell me if the heat comes on when you set the thermostat for cooling.

We can go from there.

Customer: replied 1 year ago.
I disconnected the white wire at the beginning of summer so we could have ac. That got the burners to not kick on. So I'll try the new wires. I have a ranch so wouldn't I be able to tie the new wires onto the old wires and pull them through the floor? Or is the hole too tight for the knot?
Expert:  Phil replied 1 year ago.

Hello again, you can try that, but it is about 80% likely that the thermostat wire was installed before the sheet rock, and is stapled to the studs.... another issue is that the fire break that goes between the studs is generally about 4 feet off the floor, and will have a hard time most likely pulling anything through it.

but it would not hurt to try, we might get lucky. If that fails you will have to pull it down from the attic.


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