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Tim H.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3971
Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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Thermostat display goes blank and AC shuts off periodically

Customer Question

Periodically, the thermostat display goes blank, AC shuts off. Comes back on by itself sometimes in minutes, sometimes in hours.  I tried unplugging thermostat from wall mount, then plugging it back in. Sometimes it comes back on, sometimes not.

Submitted: 6 years ago.
Category: HVAC
Expert:  Tim H. replied 6 years ago.
This points to a bad thermostat.

Try this: take the batteries out of the thermostat and turn the power to the furnace off for 30 minutes. Now replace batteries with fresh alkaline batteries and turn furnace power back on. Try again.
If this does not correct the problem, I recommend replacement of the thermostat.

Customer: replied 6 years ago.

The thermostat has no batteries. It is not programmable. It is your basic unit, the one the contractor installed when the house was built about 5 years ago. My next door neighbor has the exact same thermostat, but a different make of heat pump.

Doesn't the thermostat get it's power from the heat pump/condensor outside? Is there a circuit board out there that may be going bad?

Expert:  Tim H. replied 6 years ago.

Sorry, I didn't have the thermostat make and model from you so I had to base my answer on general information.

The thermostat gets power from the furnace or, in the case of a heat pump, air handler.

Customer: replied 6 years ago.

No problem. This is what we tried last night - swapped thermostats with my neighbor. My unit worked when plugged into the neighbors wall mount, and his worked when plugged into mine. There was about a 3-4 minute delay before the AC kicked on with each unit - I assume that is normal. Put my unit back in my house, and the neighbors in his. After a few minutes the AC kicked on and ran, blowing cold air, then about 10 minutes later, the AC shut off and the thermostat was off, no display, no nothing, like it had no power. I unplugged the unit from the wall mount, checked that all the wire connections were tight and plugged it back in. Nothing happened, thermostat display remained dead, and I went to bed. I think it may have kicked back on once during the night but shut down again - all by itself.

The one thing I didn't think to do was leave my neighbors unit plugged into my wall mount long enough to see if it would go dead too. What do you think?

Expert:  Tim H. replied 6 years ago.
I still would suspect the thermostat first. What is the make and model of the stat?
Customer: replied 6 years ago.


Here's a photo. No visable manufacturers name. A sticker on the back has this number - T8411R 1028 (2). Under that number is ***** 1A. "Assem. In Mexico".

These stats are very basic, as I mentioned, installed by contractors when our houses were built about 5-6 years ago.

Expert:  Tim H. replied 6 years ago.
This is a Honeywell heat pump thermostat. It gets power from the system transformer. See here for more information.
Customer: replied 6 years ago.

Ok, got the PDF file.

Do you think the system transformer is bad?

Expert:  Tim H. replied 6 years ago.
Check for a constant 24 volts at R and C at the air handler or furnace low voltage terminal strip.
Tim H. and 2 other HVAC Specialists are ready to help you
Customer: replied 6 years ago.
Ok - I'm catching on - there should be 24 volts AC powering the stat continuously across R and C, from the system transformer. Am I correct in assumming L1 and L2 are 120 VAC input that powers the transformer? If so, I think I've got the necessary info to isolate the problem.
Customer: replied 6 years ago.
Thanks for the assistance and the PDF file. If I can't find anything obviously wrong, I'll call a repair guy.
Expert:  Tim H. replied 6 years ago.
Yes, L1 and neutral provide 120 volts to the board and transformer and transformer steps it down to 24 volts.