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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8768
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Thermostat changes temperature settings by itself. Why?

Customer Question

Why does my thermostat change temperature settings daily? I set my house temperature on 73 and it changes by itself every day. If I don't watch it, it goes down to 57.

Submitted: 5 years ago.
Category: HVAC
Expert:  Phil replied 5 years ago.

Phil :


You may have programmed it for 57...and it is simply reverting to that programming after you manually adjust it to 73.

Its either that, or the thermostat is going bad.

I would try re-programming the thermostat from scratch.... let me know how that goes..

JACUSTOMER-75lceis9- :

How do I program it? I have my grand kids and I'm constantly having to watch the thermostat.

Phil :

Describe what you see on the face of the thermostat. All the levers and buttons.. and if it has a display screen on it, or just levers you can set.

Next I will need the model number of the thermostat, that should be behind the front cover

We can go from there

JACUSTOMER-75lceis9- :

Ok, it's a Honeywell, with a temperature display. It has up and down buttons on the right; on the left hand side there are buttons that say Fan, Auto and On. On the right under System it says Cool, Off and Heat. On the temperature display it says Set on the left side and Room on the right side

Do I just take the back off to get the serial number? The top part comes off but the bottom part seems like I might have to snatch it off. Will I break it if I do this?

Phil :

Use the up and down buttons to the desired temperature, 73 F in this case.. then press and hold the set button for 10 seconds.

Let's see if that holds for now.

The serial number is not needed... but the model number is needed.

That should be behind the front cover that generally just pops off... but do look for any tiny screws on the sides of the thermostat that may be holding the cover in place first.

JACUSTOMER-75lceis9- :

Ok, hold on please

T8400C 1024 (2) below these numbers are 0312 2A

Phil :


I will look it up

JACUSTOMER-75lceis9- :

Oh, no when I put the unit back on the wall it turned off. No readout, it's a blank screen

Phil :

Its not programmable

JACUSTOMER-75lceis9- :

what do I need to do

Phil :

Look this manual over for how it operates..

We can discuss it from there

JACUSTOMER-75lceis9- :

Thanks so much

I need someone to come to the house, cause I don't know anything about this. Thanks so much for your help.
Expert:  Phil replied 5 years ago.

You are welcome