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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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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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Related Questions:


I have a honeywell smart thermostat and it stopped working and I have no air conditioner

When did it last work? The first thing I would do is turn off the breaker that powers the A/C system. If it's a central system there are probably two 2-pole breakers (outside unit and air handler) turn them off, wait a few minutes and turn them back on. Let the system re-set (compressor time-out is at least 5 minutes after turning it back on. After the system is reset, see if your thermostat is working correctly and if not, try this link for an overview of Honeywell troubleshooting. Honeywell Thermostat Troubleshooting Six Step Guide

The off switch on my Honeywell T8000C stat will not turn off my A/C unit. Is there a way to reset correct this function? Is this stat still made? If not what is the suggested replacement?

page 13 . Look at home depot Lowe's for similar models as this is not made anymore (you can buy used from eBay.
Model # RTH221B is $28.00. According to your model number it is a standard 24 VAC thermostat. You can pick from several models and prices, but the I showed is the closest to yours


Honeywell t8000c display is blank, unit doesnt run

I would say that you have one of two issues. Either your thermostat is bad or you do not have 24v coming from the heating unit. In this case the problem would be in the low voltage circuit of the heater. At the heater you should have 24v AC when you place an electric meter across the R terminal and the C or W terminal. If you have no vaoltage across the R to either the C or W then you have a bad transformer or a blown fuse in the furnace. If you do have voltage ther and also at the t-stat then the t-stat has gone bad, replace it.

Honeywell T8011r does not have anything on display - does not appear circuit breaker is thrown - could it be wire disconnected at the plate or is there a fuse inside the unit?

Check to see if you have power at the furnace. You thermostat is powered by 24vac coming from the furnace. You should have power on the "R" terminal.

A/C not turning on - Digital screen is blank

Your honeywell should have battery back up but the primary power for this is a 24 volt transformer located in the air handler probably in your attic. All these controls run on 24 volts

T8000c honeywell thermostat is blank

This unit doesn't require a battery. It runs off the 24v from the unit. Check that your transformer is still working. Check the control board for 24v pwr. and main pwr. Check your breaker to the unit. It could be as simple as a bad stat too. You can check by using a jumper wire and jumping across from r term. to y term.

Central honeywell air conditioner

Sounds like it may have lost power. Check the breaker and the fuse at the furnace. If they are all in the on position and working then it sounds like you may need a technician. If the display is blank on the thermostat you may also want to replace the batteries.
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