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type2technician
type2technician, HVAC Technician
Category: HVAC
Satisfied Customers: 251
Experience:  Heating and a/c repairs
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Having issue with Honeywell T8400C - no display, Less than

Customer Question

Having issue with Honeywell T8400C - no display
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: Less than 24 hours
JA: Do you plan on doing the work yourself?
Customer: Depends
JA: Anything else we should know to help you best?
Customer: If I remove the display and cycle power to furnance it will come back and funance will work for a few with no issues
JA: Did you check the furnace's filters?
Customer: Naw - figured it was something electrical since I had no display
Submitted: 7 months ago.
Category: HVAC
Expert:  type2technician replied 7 months ago.

Is this a heat pump system or does your heat use gas heat?

Customer: replied 7 months ago.
Gas Heat
Expert:  type2technician replied 7 months ago.

Remove the thermostat cover and check to see if it has batteries in it.

Customer: replied 7 months ago.
No batteries that I can see - should I remove the (2) screws.
Expert:  type2technician replied 7 months ago.

No. Do you have a voltmeter? Do you see the terminals labeled R W Y G C ?

Customer: replied 7 months ago.
I see the terminals - there is no C. Unfortunately I dont have voltmeter
Expert:  type2technician replied 7 months ago.

One moment please while I look up this model thermostat.

Expert:  type2technician replied 7 months ago.

Accoding to the diagram, there is a C terminal on this thermostat. Are you sure you dont see a C terminal?

Customer: replied 7 months ago.
Just W Y G and R, when removed the thermostat just now it fan came on with heat...When i walked up to it the display was on
Expert:  type2technician replied 7 months ago.

That means that this thermostat has a short in it and is defective or a wire is loose. Check the wiring to make sure they are securely tightend to their terminals. If problem persist, replace the thermostat. Please rate this session if you are satisfied with this answer.

Customer: replied 7 months ago.
Only W G and R are wired. I just reinstalled and the display came back.... this issue appears to be causing intermittent heat operation
Expert:  type2technician replied 7 months ago.

Yes. that is why I recommend you replace the thermostat. This must be a heat only system.

Customer: replied 7 months ago.
that is correct, this is a heat only system
Expert:  type2technician replied 7 months ago.

Do you need any further assistance?

Customer: replied 7 months ago.
It works for about 10 minutes then shuts off... then if I power cycle furnance and remove and re-install termostat it will operate for a few then go back to issue
Expert:  type2technician replied 7 months ago.

Go to the furnace when it shuts off. Look on the furnace control board and tell me what pattern the red light is flashing in. 3 flash pause, 2 flash pause, etc.....

Customer: replied 7 months ago.
I have no idea where that is at, I'm assuming it would be in the attic. I was watching the display just now...heat and fan were on...the display was blank....after a few the display came back on (noticed like a default screen at first) then went normal...after a 3-4 minutes display went blank....heat stayed on.... Few minutes came back on...
Expert:  type2technician replied 7 months ago.

This is an issue with the furnace. This thermostat is powered by the 24 volts that come from the furnace. The furnace 24 volts is dropping in and out due to an isssue with the furnace. I advise you to call a hvac tech out to check the furnace to find out why the power is dropping in and out.

Customer: replied 7 months ago.
Thanks. Just to double check, would it be possible the thermostat is causing the issue if it was defective.
Expert:  type2technician replied 7 months ago.

It is possible. But based on the evidence, I think it is more likely the furnace is the culprit

Customer: replied 7 months ago.
The heat just cut off... display is blank
Expert:  type2technician replied 7 months ago.

I wish you could go to the furnace in the attic to check the status of the furnace control board while it is off.

Customer: replied 7 months ago.
Can you provide an idea of where it is, I can hear it trying to come on and it trying to begin heat stage like it does typically but the blower doesnt come in
Expert:  type2technician replied 7 months ago.

It can be in the attic, or the crawl space or a closet in the house. It varies. I could not tell you exactly where your furnace is located. However you may be on to something when you say the blower does not come on. Your blower motor inside the furnace may be bad. I strongly urge you to call a tech at this point.

Expert:  type2technician replied 7 months ago.

Please rate this session if you are satisfied. If not, I will continue to assist you. Thanks for using Just Answer.

Customer: replied 7 months ago.
I have voltmeter now.
Expert:  type2technician replied 7 months ago.

Remove the thermostat cover. You should see a R G Y W C terminal. Place the voltmeter leads on the R to G then R to Y then R to W and so on and tell me if you get 24 volts on any of them.