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Tim H.
Tim H., HVAC Professional
Category: HVAC
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Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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Periodically, the thermostat display goes blank, AC shuts off.

Customer Question

Periodically, the thermostat display goes blank, AC shuts off. Comes back on by itself sometimes in minutes, sometimes in hours.
Submitted: 3 years ago.
Category: HVAC
Expert:  Tim H. replied 3 years ago.

Tim H :

Thanks for giving me the opportunity to help! Are you there?

Expert:  Tim H. replied 3 years ago.
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Expert:  Tim H. replied 3 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 3 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 3 years ago.
Sorry, I didn't have the thermostat make and model (you didn't provide), 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 3 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 3 years ago.
I still would suspect the thermostat first. What is the make and model of the stat?
Customer: replied 3 years ago.

 

graphic

 

Here's a photo. No visable manufacturers name. A sticker on the back has this number - T8411R 1028 (2). Under that number is XXXXX 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 3 years ago.
This is a Honeywell heat pump thermostat. It gets power from the system transformer. See here for more information.
Customer: replied 3 years ago.

Ok, got the PDF file.

 

Do you think the system transformer is bad?

 

 

Expert:  Tim H. replied 3 years ago.
Check for a constant 24 volts at R and C at the air handler or furnace low voltage terminal strip.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3216
Experience: Experienced in all aspects of HVAC, both residential and commercial.
Tim H. and 8 other HVAC Specialists are ready to help you
Customer: replied 3 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 3 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 3 years ago.
Yes, L1 and neutral provide 120 volts to the board and transformer and transformer steps it down to 24 volts.
Customer: replied 3 years ago.
OK, Mr. Expert - the problem had nothing to do with the thermostat. It was the condensation overflow switch. The drain line was plugged up, thus the condensation water had backed up and lifted the little float which tripped the switch that shut the whole thing down. The CLASSIC symptom, which ANY technician worth their salt would know, is the thermostat does dead then periodically comes back to life, with the system functioning normally otherwise. It took a local repair man TWO SECONDS to diagnose the problem. You're some expert - surly not worth the $15 it cost me. Rest assured I will pass on my less than impressive experience with you to any and all.
Expert:  Tim H. replied 3 years ago.
Nowhere in your posts did you indicate you were working with the A/C. Since we're kind of in between seasons here (spring), most of my customers are still working with no heat issues. In any case, we we were doing some troubleshooting steps to determine the problem when you decided to call in a repair guy.

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Tim H.
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