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jpeyster58 09-22-2012 08:55 AM

Thermostat Blank Screen
I have been troubleshooting an intermittant issue with my Thermostat display going blank and my Control board loosing power. Following is the model numbers of the equiptment I am dealing with

Carrier model 58STA070-12 AC unit
Honeywell T8000C Thermostat

6 days ago we had heavy rains and since that time I have been experiencing this issue intermittently. When the AC is working it works great and when it stops working it seems like there is no power at all in the thermostat or at the control board of my Carrier model 58STA070-12. Both my upstairs and downstairs carrier units are running on the same circuit breaker so I would think the circuit breaker is OK (The downstairs unit is not affested). The thermostat does not require batteries so I know that is not an issue. I thought is may be an issue with the transformer so I swapped out the transformer from my downstairs unit and that did not make any difference. It appears there is a short somewhere in the low voltage side that is causing this and I have been trying to go through everything to find and fix this short.This problem is intermittent and there is a trend of the problem happening mainly in the overnight hours when the outside temperature is in the 50's or 60's. Mostly during the daytime hours, the system works fine.

Could any if the following be causing my problem?
  • The relay in the outdoor compressor unit could be hanging up causing a short (more during the cooler times of day)
  • Defective Circuit Breaker (One breaker is running both air handles and each outside Compressor is on its own circuit)
  • The twist cap wire connectors that connect the low voltage to my relay switch on the compressor
  • The capacitor in the compressor unit (does not appear to be swelling)
  • The wiring on my Thermostat (nothing has changed here)
  • Loose power connector on my control board
Tonight when it goes out I may take a heat gun out to the relay and warm it up a bit so see if the temperature increase makes any difference.

Looking for any advice on this issue.


hvactech126 09-22-2012 11:37 AM

when thermostat is blank you need to measure Voltage between R and C at the furnace control board.
24V = problem in wiring between furnace and thermostat.
0V = problem in furnace or no power into transformer.

Another thing that it may be (since this is intermittent) is if you have a condensate pump or a drain pan with a float switch is that it may be cutting out power to the thermostat.

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