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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8255
Experience:  Furnace, A/C and heat pump specialist.
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I just installed a Nest Thermostat (generation 3) yesterday

Customer Question

I just installed a Nest Thermostat (generation 3) yesterday and I'm unsure if it's wired correctly. When I turn the heat up, it turns orange and gives an estimate, say 20 minutes that it will heat up, but does not actually seem to heat up and is just hovering around 67 or so. It will occasionally blow cold air out of the vents.
Heating system is forced hot air from a new Lennox M195UH gas furnace. It has an external condensate pump plugged into the wall next to it. No air conditioning.
Old thermostat had:
Green wire to G
Red wire to Rh
White wire to W1
Brown wire to O
Orange wire from wall that was not plugged to thermostat (cooling wire?)
I plugged these all in to the Nest. I plugged the Brown to the OB input on the Nest. However, the Nest gave error code E77 and said that it thought it was a heat pump and was missing a wire. So I unplugged the Brown cable and it seems to see it as a basic heating system.
I have power cycled the Furnace several times now as well and the behavior is the same. Do I need to plug the brown wire in elsewhere? Do I need to hook up the orange wire? Or is the nest just much much slower to respond to a heat request?
Thank you
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

Are you sure the brown went to O and not C on old thermostat?O is a reversing valve terminal for a heat pump. Can you look in furnace and verify where each wire is connected there?

Customer: replied 1 year ago.
Hmm, was unable to find the control unit on the furnace earlier, but I will poke around and look at it. It may be that I misread O as C. Would it potentially be harmful to test plugging brown straight into C?
Expert:  airheatman replied 1 year ago.

We want to know for sure before attaching it.The control board will be in blower panel.

Customer: replied 1 year ago.
I opened the blower panel and found the control board. The board has:Red to R
C is unwired
White to W
Yellow to Y
Green to GThe Brown wire is not attached at the furnace side. I do not have the yellow attached on the thermostat side either.
Expert:  airheatman replied 1 year ago.

Ok if you do not have ac, Y is not necessary. Tape that brown off at thermostat, Connect only red to RH, Green to G and white to W1

Customer: replied 1 year ago.
Ok. I have done that.
Expert:  airheatman replied 1 year ago.

Ok so you currently have wired correctly.Are you still getting an error code or just does not seem to be heating?

Customer: replied 1 year ago.
No error code. It seems to not be heating. 30 mins ago it was 72 degrees, I set it to 76 and it said it will be in 30 mins. However it is still 72 degrees after 30 mins and the furnace is not making any noise or anything.
Expert:  airheatman replied 1 year ago.

OK issue may be with furnace. If you turn thermostat to fan on, does blower come on?

Customer: replied 1 year ago.
Yep, I set fan to run for 15 mins and it goes.
Expert:  airheatman replied 1 year ago.

Lets do this. Either at thermostat or on furnace board connect R and W together.If at thermostat, just loosen and put white and red under one screw.If at board jump terminals on board as shown below.

Customer: replied 1 year ago.
I turned off circuit, wired Red to White at the thermostat end, and turned circuit back on. Furnace came on, and the temperature is steadily increasing (measuring with a digital thermometer).
Expert:  airheatman replied 1 year ago.

Ok issue is definitely with thermostat then as we are bypassing by doing that.

And you are sure we have Red to RH, White to W1 and Green to G at thermostat and furnace correct?

Expert:  airheatman replied 1 year ago.

Nothing else but these there

Customer: replied 1 year ago.
just white red and green. Visually confirmed at both ends of the connection. I plugged the Nest back in and set the thermostat to 76. The furnace and fan started running, blowing what felt like cool air, then stopped after a couple minutes. The temperature now has stayed steady at 72 for a few minutes..
Expert:  airheatman replied 1 year ago.

Check the setting screen again and make sure that error is cleared and we don't have any more erros like "Common wire needed" or anything

Customer: replied 1 year ago.
No errors. Fan test is successful, the heat test starts the furnace but it seems to blow cool air (roughly 70 degrees according to digital thermometer).
Expert:  airheatman replied 1 year ago.

When this happens are there any error codes on the furnace? It is strange that heat works fine when called for by jumping terminals yet does not by call from thermostat. Once thermostat signals R and W at furnace, it is up to the furnace as to when to turn blower on which should not happen until burners are lit.

Customer: replied 1 year ago.
Status LED is steady red.
Expert:  airheatman replied 1 year ago.

You may have a bad thermostat out of box.The thermostat is basically a switch.It appears on call for heat it is sending signal from R to G same as in Fan on mode

Customer: replied 1 year ago.
I will contact Nest support and get back to you.. Thanks for the help so far.
Expert:  airheatman replied 1 year ago.

Sounds good. Tell them what we have done.I am a licensed HVAC contractor.We will go from here.Tell them about connecting R and W wires at thermostat and that heat works properly but when connected to thermostat terminals, only blower come son when we call for heat. Look forward to hearing back.

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