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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 863
Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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I decided not to install the WiFi thermostat because I only

Customer Question

I decided not to install the WiFi thermostat because I only have 4 wires on the existing unit.... when I put the old Honeywell back on, the display screen will no longer light up.... how do I get the furnace and thermostat to work again?
Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.

Hi I'm Brian and I'm here to help!

Can I get the model number of your old Honeywell thermostat please? It would also be helpful to know what color wires go to what terminals on your old Honeywell too.

Thanks,

Brian

Customer: replied 1 year ago.
9904
Customer: replied 1 year ago.
Green to G
White to W
Red to R
Blue to Y
Expert:  Brian HVAC Guy replied 1 year ago.

I am not familiar with that model number for Honeywell. Honeywell's model numbers typically begin with T, TH or RTH then at least 2 numbers but usually four. Is your old thermostat a digital one or older analog mercury style?

Customer: replied 1 year ago.
I sent photos
Customer: replied 1 year ago.
did you receive photos of outside and inside of thermostat?
Customer: replied 1 year ago.
the face comes off and the back half is still on the wall..... the face includes a LCD window and all the control buttons
Expert:  Brian HVAC Guy replied 1 year ago.

I'm not sure why but no photos are posted on this end. Regardless, your wiring is a simple as it gets and sounds right. If by chance there are two R terminals (R, Rc and/or Rh), make sure the two of them are jumpered together in the case that the jumper may have fallen out. Also, many electronic thermostats that don't have a C terminal (24volt common), use only batteries to power them. If that's the case, you should check the batteries are new and the battery contacts are shiny and clean.

Customer: replied 1 year ago.
I have not altered or removed any wires.... I just need to wake up the thermostat after replacing the front cover
Expert:  Brian HVAC Guy replied 1 year ago.

So that tells me you have an electronic thermostat. The front and the base connect with pins on the front that plug into a socket on the back and it's important that the base is mounted flat, the pins on the front are straight and clean and the two mate together solidly. The lit lcd display on the front part of the stat should <WORK> even if it's not on the base as long as the batteries are in, good and making good contact.

Customer: replied 1 year ago.
There are no batteries in this unit
Customer: replied 1 year ago.
I am stumped
Expert:  Brian HVAC Guy replied 1 year ago.

That would mean it is what's called a 'power stealing' model. Honeywell made many of those. My best guess is that for some reason there is not 24 volt power on the wiring at the thermostat. We can check that though. Remove the face of the thermostat to reveal the wiring terminals and jumper R to G with a small piece of wire or a bent paperclip or the like. Even two small screwdrivers criss/crossed and touched to the two terminals at the same time while contacting one another will work. Your indoor blower should start when you short R to G. Let me know what happens...

Customer: replied 1 year ago.
When I touched them with a paper clip I heard the fan go on and then off when I removed the clip
Expert:  Brian HVAC Guy replied 1 year ago.

Ok, now try to jump R to W and your heat should come on.

Customer: replied 1 year ago.
Blower now stays on but air flow feels cool
Expert:  Brian HVAC Guy replied 1 year ago.

Hmm there may be a separate issue with your furnace. I would try power recycling the furnace again. It would appear there is more going on than just the thermostat.s

Customer: replied 1 year ago.
Blower off again... Cannot find a door to locate pilot light
Expert:  Brian HVAC Guy replied 1 year ago.

With the thermostat in the off position, you do have at the very least some power going to the thermostat, it should light up. I don't really understand why it's not. Do you by chance have a volt meter to read 24 volts ac?

Customer: replied 1 year ago.
My volt meter is 110 and higher Shucks
Customer: replied 1 year ago.
I will have to get hvac people here tomorrow I guess... Luckily we have another furnace on the other side of the house to stay warm tonight
Customer: replied 1 year ago.
I appreciate your efforts in trying to help
Expert:  Brian HVAC Guy replied 1 year ago.

Well, I'm afraid I'm at a bit of a loss too. It sounds like the issue may be in the furnace itself. Perhaps a safety tripped during all this or something similar. The fact that the blower ran at least once when you jumped R to G tells me you have 24 volt ac at least in some places (maybe not all that there should be). It tells me power to the furnace is ok. It's hard to trouble shoot from here without a meter. I'm sorry I couldn't be more help!

Customer: replied 1 year ago.
Thanks again Brian
Expert:  Brian HVAC Guy replied 1 year ago.

You're welcome. Best of luck with the tech tomorrow.

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