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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8768
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Honeywell thermostat 8000. I set it to cool and it

Customer Question

I have a Honeywell thermostat 8000. I set it to cool and it keeps changing to heat by itself. It worked fine when we first installed it.
JA: Did you double-check the thermostat's settings? And what about the filters?
Customer: yes. We have an identical one in another room and it works fine. What kind of setting, or lack thereof that would cause this happen? Thanks,
JA: Do you plan on doing the work yourself?
Customer: Yes
JA: Anything else we should know to help you best?
Customer: Nothing comes to mind
Submitted: 3 months ago.
Category: HVAC
Expert:  Phil replied 3 months ago.

Hello, the heat and cool temperatures in the programming are likely set too close together... click here for the programming manual.

Set them all 2 to 4 degrees apart, for instance set cooling at 72F and heating at 68F.

Let me know how it goes.


I am a retired mechanical engineer and HVAC/ electrical/ refrigeration and boiler contractor ... with 56 years in this business. The last 20 of those years consulting for major firms nationally.


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If you can agree to that, rate now, lets continue by all means.

Customer: replied 2 months ago.
Sorry. I tried all sorts of combinations keeping the heating and cooling temperatures apart. Nothing worked. Thanks
Expert:  Phil replied 2 months ago.

Can you tell me specifially and very pointedly exactly what you did? What differential settings did you try (DIFF settings)

It is possible that the thermostat has failed, but this is not a common failure mode.

We can go from there without any time limits