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T8411 thermostat screen is blank? Only happens at night.

T8411 thermostat screen is blank? Only happens at night.
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Answered in 3 minutes by:
9/30/2013
heatncoolrepair
heatncoolrepair, HVAC Technician
Category: HVAC
Satisfied Customers: 75
Experience: 13 Years of Residential Furnace & A/C Repair Experience.
Verified

Neal :

Hello, welcome to Just Answer! My name is XXXXX XXXXX I will be assisting you. Please give me a moment to review the information you have provided.

Neal :

Hi there, I looked over the manual for this and it looks like this thermostat is powered by the furnace transformer and has no batteries in the thermostat, correct?

Neal :

I see you have left the chat, but basically you need to make sure you have power going to the thermostat, 24 volts from R to C. If you have power there, then you should have a display at all times. If not, you are either losing power to it at night somehow, or the more likely cause is a bad thermostat. I have replaced out several of these thermostats, and had one at my home that was bad as well. It looks like you have already checked voltage to the thermostat, so I would replace it. Any more questions? Otherwise, please don't forget to rate me on my service tonight!

Customer:

no batteries correct

Neal :

Yea, I found a picture of it and remembered this one does not have batteries.

Customer:

why would it be intermittany

Neal :

There is a capacitor in the thermostat that stores power for memory and display, like a computer that gets weak.

Customer:

why would it be intermittent and i switched the tjermostat from my othrer unit amd it works on it

Neal :

and that thermostat works fine, correct?

Customer:

neither works upstairs both work downstairs.

Neal :

Are you telling me you have a zone system, with 2 thermostats then?

Neal :

If so, then it could be a bad zone control board.

Customer:

i dont know. i have a unit upstairs and unit downstairs.

Neal :

2 separate systems then? each with their own individual thermostat?

Customer:

yes.

Neal :

and you can switch the thermostats and the same thing happens regardless of what thermostat you are using?

Customer:

yep :(

Customer:

and in the morning it will be fine

Neal :

Then you have to be losing 24 volts to the thermostat somehow. What do you do to get it to turn on when the screen is blank?

Customer:

nothing we suffer, lol.ceiling fans only.

Customer:

my little volt meter says 12v

Neal :

The only way to find out where you are losing power is to trace it with a multimeter when the thermostat screen goes blank. It could be the furnace door switch, the circuit board, a loose or bad connection. It hard to say with out being there to trace the circuit. Are you good at reading wiring diagrams and using a multimeter?

Customer:

maybe but i dont have a multimeter

Neal :

12 Volts......that is the problem.......not enough power for the thermostat. Start by checking to see how much power is coming out of the transformer......if it is 12 volts, you need to replace it, but if you get 24 volts, you will have to continue down the circuit to find out what is causing the resistance.....a bad wire, connection, or safety switch, etc.

Customer:

transformer? the breaker box?

Neal :

Ah no, there is a transformer installed in your furnace or air handler, whatever you have, that supplies 24 volts to R and C on the circuit board. Put one probe on R and the other on C, with the furnace door switch pushed in at the same time to see how much voltage you are getting from the transformer. What brand, model, and serial number is XXXXX furnace or air handler?

Customer:

lennox cb28uh 042 230 pq

Customer:

01 not pq

Neal :

air handler?

Customer:

sn 5894e24762

Customer:

sure

Neal :

Looking at the wiring diagram for this, it shows you should have 24 volts to the thermostat. If you only have 12 volts, the transformer will need to be checked like I stated above, and if it is not giving you 24 volts, then you will need to trace the circuit to find where you are losing voltage.

Neal :

If the transformer is only giving you 12 volts, then you would need to replace the transformer.

Customer:

i will have to borrow my dad's multimeter tomorrow. randomly it is working. i popped it off and they still read 12v?

Neal :

Yea, start with making sure the meter is working correctly. Maybe verify it with the other system you have. Then do what I said.....this will be a more complicated task, and you will need a multimeter to figure out your problem. Once you can supply the thermostat 24 volts, it will not go blank anymore.

Neal :

Any more questions? Otherwise, please don't forget to rate me on my service tonight! Thanks, Neal

Customer:

thanks Neal. giving you a 5 : D

heatncoolrepair
heatncoolrepair, HVAC Technician
Category: HVAC
Satisfied Customers: 75
Experience: 13 Years of Residential Furnace & A/C Repair Experience.
Verified
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heatncoolrepair
heatncoolrepair, HVAC Technician
Category: HVAC
Satisfied Customers: 75
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Experience: 13 Years of Residential Furnace & A/C Repair Experience.

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