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Honeywell T8000C Thermostat Problem

Came back from a vacation and found the downstair thermostat's display blank. I tried all - reset breaker, reseat the unit. The upstair system is OK. The downstair system is not operational. Took the upstair thermostat downstair, and it also went blank. Replaced it back to the upstair wall mount and it is OK. There is a heater/fan in the attic that I have not checked. TIA for some pointers...

Posted by ucanfixit on

  • 5 more comments 
  • ucanfixit Jan 04, 2008

    Mr. Therinnaiguy,

    It has got a bit cold lately in San Diego, so I decided to follow your advice and give it a try before calling the expert..,

    As you pointed out, a blown 3A-32V spade fuse was found!!!

    Got new one from Autozone to replace the blown one, all the voltages come back. Heater is now working again.

    You are an angel!!!! Thanks very much.

  • tosha001 Apr 22, 2008

    I have the Honeywell square thermostat and I cam home yesterday from work and noticed that the display screen was blank. The a/c unit was already off so not I cannot turn the unit back on. I have tried resetting the breakers and all, but still nothing. Please help

  • wholphin May 01, 2008

    yesterday we lost power and upon power restore, the lcd was blank on the downstairs Thermostat but the upstairs one was ok, brought the upstairs one down and it worked downstairs.

    What could be wrong with the unit in this case? Should i try to replace the fuse on the bad Thermostat? or is purchase a new one?


  • monique765 Jun 07, 2008

    I have a similar problem. My power was out for a few days and when it came on, our screen on the thermostat was blank. Is the fuse you are talking about directly on the thermostat or is it in the hvac unit? I am unsure.

  • ucanfixit Jun 20, 2008

    Sorry, I have not checked back for a while.

    The fuse is in the hvac unit. Mine is in the attic. The problem is not with the control on the wall. The panel will need to be removed in order to see the ckt board and the blown fuse.

    Good luck!

  • Perplexed411 Oct 05, 2008

    I have a Honeywell T8000c Thermostat that controls a Comfortmaker NTG3 model furnace. The Honeywell display does not work since there is no 24 volt power to the wall plate or at the circuit board of the hvac unit. The fuse is OK and the blower motor operates continuously if the furnace unit switch is in the on position. According to the manual wiring diagram, the blower operates directly off the house 115 volt power while the electronic gas ignition and the thermostat use 24 volt power from the unit transformer. Is there anything else that I can check before calling a service technician since I can't operate the furnace.

  • avery_b Dec 20, 2008

    i have a honeywell T8000C thermostat with a blank display. circuit breaker is fine and the fan is still running. what should i do?



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Chandler Ede

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If you have a volt meter, remove the thermostat off from the mounting plate and check for 24 volts AC across red (R) and White (W). Then check across Red and Yellow (Y). If you do not find any voltage, follow wire as much as you can all the way back to the air handler that is connected to it. Look inside and see if there is a small Auto (car) type of fuse in the wiring harness. Test and replace if necessary. Jump out the Red and Green terminals on the air handler and replace the door. See if the blower turns on. If so, you have narrowed it to the wiring to the thermostat. If not, you have a power problem and will need a technician.

Posted on Sep 28, 2007



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In my case the blank thermostat was indicative of a problem at the furnace. A blown limit switch to be exact due to a neglected filter.

Posted on Feb 22, 2010


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