My HVAC Blower fan stays on even when I remove the thermostat face plate (with the fan in the auto position and the thermostat in the off position). I have tested for a short in the thermostat wire and there doesn't seem to be one.

The backstory... We bought the house and everything worked until we turned turned the heat on... Turning on the heat blew the 3 amp circuit in the blower housing. I had an HVAC guy come out and he found 3 issues... There was a short in the wiring between the Thermostat/control board/and the overflow shutoff for the evap pan below the unit. He fixed the short, rewired the thermostat (which he said had no common, but I am looking at the schematic for it now and it looks like it does has a common and is a Honeywell Model #RTH111B1024, which is as basic a thermostat as you can get. I do not know exactly what he did, but he got it working. That was in early January 2021. The HVAC guy was very good and said he had wired a few of these last summer when he could not get regular thermostats and he had to use ones from Home Depot that had no Common, so he already knew the fix...

Fast forward 8 months (August 30, 2021 - almost Labor Day 2021) and the fan runs even with the thermostat in the off position and the fan in the Auto position... it also runs when the thermostat faceplate is removed. This is my wiring as it now stands. It seems if I connect the blue wire to the far left B terminal it might work Here are my collection of images. Any advice would be appreciated.

I believe the relay next to the transformer is might be stuck, but I am a novice at this, so please have patience with me (see lower left quadrant of the wiring diagram shows the relay that I believe is in need of replacement. I am not sure how to test it yet, but will do some searching on this tomorrow. I have my doubts about the wiring of the thermostat... The wiring in the house is very suspect. I had a few outlets wired backwards and a few that had the ground wire wrapped around the pole, but not screwed in. I have checked every one and found 3 such unscrewed ground wires, 2 wall outlets installed backwards (the hot and return switched). I imagine the same guy who left the HVAC with a short above the air handler (see the second pic) did the wiring in the rest of the place?

Allied Serial Number Wiring Above Allied Air Handler (Blower Unit) Control Panel Wiring Wiring Diagram Inside Air Handler - Are these

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    Removal of the thermostat was a good first step. To continue the troubleshooting, remove the thermostat wires from the control board and check whether the fan runs. If it remains off, re-connect the wires one at a time until it comes on - perhaps R first, then G, C, W, Y. We'll figure out whether the fan runs because of an external signal (ie wiring fault) or because of an internal condition on the control board.
    – Greg Hill
    Commented Aug 31, 2021 at 17:02
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    If you undo and tie back the wiring for the fan relay, does the blower motor still run? Commented Sep 1, 2021 at 1:39

1 Answer 1


A HVAC tech diagnosed and corrected the issue by replacing the electronics on the blower motor that had failed, as well as the blower. Thanks for all those who weighed in.

I couldn't figure this out, nor could the first HVAC tech. The second tech knew what it was in a matter of minutes and he had it back to good in a few hours. The new blower is a two stage (the original was a single stage) and seems to be more efficient than the original. I'll see when the bill comes. When the original kicked in (at full speed) it sounded like the house was going to lift off in T-minus -3 2 1... The new one is almost silent when it starts, but seems to have the same top speed and seems to be pushing as much air as the original, if not more.

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    Thanks for coming back to update us!
    – FreeMan
    Commented Dec 13, 2021 at 15:27

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