Hot Topics: Nest Thermostat C Wire

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Original Post: Nest Thermostat and C Wire

bshughes657 Member

I'm trying to get the nest thermostat to function with the Honeywell Aquastat L8148e. There is a stand alone air handler in the attic. My wires are Rh and W1 from the Aquastat and Rc and G and Y1 from the air handler.

I get 24 volts at the aquastat between the Rh wire and W. Zero volts between the white wire T at the Aquastat and at the thermostat. 24 volts between Rh and C at the thermostat. When I connect the C wire to the nest, it tells me it recognizes that a wire is there, but there is no power.

Without the C wire I get an E30 code saying there is no power to the Y1 terminal. The air conditioner will not work. Is there something wrong with the Nest or do I need to make some other adjustments?

PJmax Group Moderator

Welcome to the forums.

"When I connect the C wire to the nest..."

What C wire? You don't have one in your description.

The C wire must come from the air handler.

Rh and W1 are strictly for the heat.

You need a fourth wire to the air handler.

If you wanted to go with a poor mans C wire, you change wiring at both ends.

At the nest, Rc is Rc - Y1 is Y1 - G wire becomes C.

At the air handler, Rc is Rc - Y1 is Y1 & G and G wire becomes C.

In other words, you lose the FAN ON position on your thermostat. The G wire becomes your C wire.

bshughes657 Thread Starter

I'm sorry I didn't completely explain. I ran an extra wire from the thermostat to the Aquastat on the boiler and attached it to the W terminal on the Aquastat. Are you saying that I must get the C wire from the air handler and not the Boiler?

bshughes657 Thread Starter

The air handler is in the attic and the wire connections are difficult to see. I did notice there was a block where the wires connected that was numbered 1-5 or 6. Is there any way to tell which connection would be for the C wire? Extra wires are already there if I just knew where to connect one.

Geochurchi Member

Hi, can you post a pic of the control board at the AH so we can see what you have?

bshughes657 Thread Starter

Here are three pictures of the terminal. I don't know what the red and white wire on #7 is. I have Blue, Green and Red at the thermostat with the White and Yellow free. The second picture is the condensate pump.

thermostat wiring casing near insulation

thermostat wiring box

thermostat wiring

bshughes657 Thread Starter

Hopefully you can see the situation and advise me. I have a free yellow and white wire, if I just knew where to connect the wire at the air handler I think I would be in business.

Geochurchi Member

Hi, it appears that #s 6&7 go to the condenser unit outside, there also appears to be a transformer there with a Black and Blue wire coming off of it, where does the Black wire connect?

Do you have a volt meter? If measure between the Black &Blue, you should get 24 VAC, if that is the case you can connect one of the spare conductors to the Black and the other end to C on the Tstat.

PJmax Group Moderator

That looks like an old system. That will more than likely have the C line grounded. Connect your white wire to any screw on the metal frame. Give that a try, it can't hurt anything.

bshughes657 Thread Starter

If I get the C wire properly installed, where should I check for 24 volts? At the red wire or the blue wire? The blue wire is Y1 in the thermostat and the red wire is Rc.

Geochurchi Member

Between the Red and C, C will be which ever color wire you connected to the common at at the AH.

bshughes657 Thread Starter

Thanks for the replies. I really appreciate your help. This system belongs to my daughter. I’ll have to go to her house and give it a try. I’ll be sure to let you know how everything worked out. Thanks again.

bshughes657 Thread Starter

Had 24 volts between the blue and black wires. I connected a wire to the black wire and connected it to the C terminal on the nest. All is working well right now. Thanks for all the help. You guys are awesome

PJmax Group Moderator

Good job. Thanks for letting us know how you made out.

Geochurchi Member

Ya, put yourself in for a raise.