Question about Honeywell Electronic Programmable Heat/Cool Thermostat - CT3300A100 Heater

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Honeywell thermostat t8400c intermittent blank screen

I noticed my honeywell thermostat screen was blank the other day and so I powered down the heating unit and powered everything back up. It seemed to do the trick for a while, by my display screen is blank again. Not sure if I just need to replace the thermostat or if this is a symptom of something wrong on my HVAC. When I detach and reattach the screen on the thermostat comes on for a few seconds, and then goes blank again.

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  • 407 Answers

It actually sounds like the batteries are going dead if the 24v from the furnace are not providing the power to the thermostat.

Posted on Jan 23, 2010

  • Joel Gilbert Dec 09, 2011

    What batteries? There are no batteries with this thermostat. Joel

  • Gary Wood Apr 06, 2013

    there are batteries inside these stats, a CR2032 which is not rechargeable, when the battery dies, so does the control unit

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Thermostat is not coming on we are in san antonio is there help out there for us


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I HAVE A HONEYWELL MANUAL THERMOSTAT NO BATTERIES ALL IT HAS ON THE BACK IS T8400C 1099 (2) 0710 1B HELP, IT DOES NOT SHOW ANY THING THE SCREEN IS BLANK


This model of Honeywell thermostat uses the 24v from the unit to power it. One of three things must have happened.

1. You have lost power to the furnace, therefore no power to thermostat.
2. You only have 4 wires hooked up on the thermostat and not 5. A lot of older homes had just 4 wire running to the thermostat, which will not work for this thermostat. You should have a wire connected to the (C) or (B) terminal on the thermostat, which ever one they have, I can't remember off hand. These are the Common terminals. They have to have the common from the furnace 24v power to operate.
3. Faulty thermostat, which will obviously have to be replaced.

Hope this helps and gets you to cooler days!

Jun 28, 2012 | Honeywell Heating & Cooling

1 Answer

My honeywell t8400c also went blank. I can not find a battery anywhere and if plugs into the wall to get electricity. Everything works but the digital read out. Any suggestions


it should take a battery and it has wires coming from the unit for power as well the stat splits open from the wall leaving some of it mounted on the wall the batteries are in the part that pulls off the wall and the wires are on the part that stays on the wall the power wire is red on r terminal

Sep 23, 2010 | Honeywell Electronic Programmable...

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Honeywell T8400c thermostat went blank. central heating is not working although warm water is alright. any idea?


I would check the batteries in the thermostat. If the batteries are okay then you might have a blown fuse in the furnace or the furnace lost power check the breaker.

Jan 21, 2010 | Heating & Cooling

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Sounds like you are loosing the low voltage when this happens either from a loose connection or a possible short. Make sure all connections are tight

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i would have the thermostat receiving 24 volts all the time by running the c terminal from the air handler to the thermostat and go from there

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T8411r1028 honeywell thermostat blank screen where would fuse be?


That thermostat should get power from 24V side of a transformer in the air handler. There should be a fuse in there on the PCB, but just as likely, there may be a 2-wire sensor in your drain trap that will shut it down if it senses water backing up in the drain piping. Clear that line from the outside of your house with a shop vac, cycle the breaker for the air handler if needed. You may also be low on freon, though, so a service call might be a good idea anyway.

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