Air conditioner wiring diagram showing thermostat, control box, outdoor unit, and heat pump indoor units
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Jonathon Pullen

Technical Services Manager Gulf Coast District

April 13, 2023

Overheated Thermostat

Have you ever run across an overheating thermostat? Where the thermostat displayed 90 and the room temp was in the 70s or it was a runaway system? Did you know why? How did you fix the problem?


I run into this a few times a year. It is fairly easy to trouble shoot and fix. I see this sometimes with an Ecobee thermostat, but it can happen to any brand. The thermostat will be warm to the touch and will throw off the room temperature reading. This results in causing a runaway system in cooling or a possible no heat situation.


We see this problem with 2 stage Infinity/evolution systems wired non communicating. What causes this is the transformer in the outdoor unit is out of phase with the indoor transformer.


Troubleshooting this is easy:

1) Remove the Y1 wire from the outdoor circuit board.

2) Give a call for cooling, take one meter lead and place on Y1 wire and the other to SEC1 on the circuit board.


If the reading is 40VAC and above, the transformers are out of phase and one needs to be reversed. If the reading is 10VDC, the transformers are phased correctly. 40VDC and above will cause the thermostat to overheat.

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