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The display on my thermostat T8400C 1024 isn't working. Everything else works but the heat and the flame is not igniting either. Please help.

I have 4 wires. The white to the W, the blue to Y, the green to G and the red to R. The fan is running, but I can't get heat and there is still no display on the thermostat. Why would the fan be running even when it did not receive any signal from the thermostat?

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Sounds like a manual limit needs to be reset (thermal disc). Generally the manual limits are around the exhaust - backdraft, or burner assembly - flame rollout.

Posted on Feb 21, 2014

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SOURCE: changed honeywell T8400C 1024 to

On the t-stat and the furnace make sure the wires are hooked like this...

Red to R
White to W
Green to G
Yellow to Y
Blue to C

then it should work..

Thermostat Wiring and Terminal Designations


Posted on Jun 12, 2011

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Changed honeywell T8400C 1024 to Hunter 44760, I have five wires white, red, yelliow, green, blue. Hunter Thermostat reads "NO A/C Power.

On the t-stat and the furnace make sure the wires are hooked like this...

Red to R
White to W
Green to G
Yellow to Y
Blue to C

then it should work..

Thermostat Wiring and Terminal Designations


Jun 12, 2011 | Heating & Cooling

1 Answer

I'm still having the problem with the heat Some times the dryer heats for a minute and sometimes for 3 seconds, i replaced the coils and the hi-limit thermostat .I hear this clicking so i know the ignitor...

The previous problem is addressed by replacing the high-limit thermostat according to your last post. The igniter is definitely working since the gas is being ignited.

Yes there's a flame sensor along with the igniter on the burner cone and it must be not working properly. The cycling thermostat is good as per previous troubleshooting, the high-limit thermostat has been replaced, and the igniter is good. These findings and observations suggest that the flame sensor is not working properly. It is either dislodged from its proper position and cannot sense properly the heat from the flame and the igniter or it is faulty and needs to be replaced.

Check that the flame sensor is aligned properly with the slot on the burner cone. Replace the sensor if it is aligned properly. The sensor looks like this.

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I have a Honeywell T8400C 1024 thermostat. the last two nights, our heat continues to run without shutting off at the pre-set temperature. We have woken up to 81 degrees in our house. What is the...

Try taking the batteries out for 5 min,or disconnect power for 5min.
To clear it out.
Then re input all your times days and temps,

Is t stat in a cold location near a door or window?this could also override your settings,most progrmamble stats will go into override at 58.f im guessing this is not your problem.

If it continues after you reset,i would consider a new thermostat.

Hope this helps.

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