topic Re: E79 Code on Nest Thermostat (10 slot) in Nest Thermostats

E79 Code on Nest Thermostat (10 slot)

davidmia — Thu, 06 Oct 2022 11:38:02 GMT

I re-Installed my Nest Thermostat this summer because I installed a new AC system. The Nest was working fine during the summer months and the unit was charging as required. The AC has a 4 wire connection and it was connected to the Y1 (Cool), G (fan), Rc (Power) & the C (Common). The heating system is a separate hot water system and is a 2 wire configuration. W1 (heat) & Rh (power). When I switched to the heat on the Nest thermostat an Error code came up (E79) that there is connection on the C (common) wire and the Nest wont charge and it wont let the heat work. Also the Nest wont let me switch to the AC until I fix the wiring. Please let me know how to rewire this correctly?

Re: E79 Code on Nest Thermostat (10 slot)

Houptee — Thu, 06 Oct 2022 14:48:39 GMT

When they installed the new AC system did they connect the C common at the AC air handler unit?

Go to the AC unit and check what wires are connected and take a pic of them.

Your + power RC and your C common needs to come from the Air Conditioner transformer to keep the Nest charged up and running.

Re: E79 Code on Nest Thermostat (10 slot)

aatienza — Sun, 09 Oct 2022 18:37:23 GMT

Hey there,

Checking in — we hope you've got the answer you're looking for. Let us know if you have more questions in mind.

I appreciate the help, Houptee.

Thanks,

Archie

Re: E79 Code on Nest Thermostat (10 slot)

davidmia — Tue, 11 Oct 2022 12:03:33 GMT

I dismantled the wires from the AC system (Y1, G, Rc) and left the W1 C & Rh wires. and it was charging the unit but when now I turn it on an Error comes up which is E102. I do not know what's the problem now.

Re: E79 Code on Nest Thermostat (10 slot)

davidmia — Tue, 11 Oct 2022 12:12:06 GMT

Yes the C wire was installed on the air handler.

Re: E79 Code on Nest Thermostat (10 slot)

Houptee — Tue, 11 Oct 2022 13:55:04 GMT

Check if the C wire is in the same cable as the RC wire. Which means they should both come from the A/C unit transformer.

You cannot have the RH red wire from the heating system, with only the C wire from the A/C unit because the heat has separate transformer power supply.

You need RC and C from same source transformer.

Re: E79 Code on Nest Thermostat (10 slot)

davidmia — Tue, 11 Oct 2022 14:11:04 GMT

I understand. So I will need a C wire from the Heating system i assume? Is this correct?

Re: E79 Code on Nest Thermostat (10 slot)

davidmia — Tue, 11 Oct 2022 14:12:15 GMT

I have 4 wires from the AC and only 2 wires from the heating system.

Re: E79 Code on Nest Thermostat (10 slot)

Houptee — Tue, 11 Oct 2022 14:30:31 GMT

No you do not need a C from the Heating system.

You need the RC and C from the A/C system to provide the power to keep Nest charged.

Then the RH and W are all you need to connect from the heating system cable.

Re: E79 Code on Nest Thermostat (10 slot)

Houptee — Tue, 11 Oct 2022 14:33:06 GMT

OK good.

Connect the 4 wires from the A/C to RC , C, Y1, and G

Connect 2 wires from heating to RH and W1

Re: E79 Code on Nest Thermostat (10 slot)

davidmia — Tue, 11 Oct 2022 15:21:07 GMT

I did that and the AC works perfectly when in the cool mode, but when I switched to the heat mode it gives me the error code of E79 and the boiler does not kick on.

Re: E79 Code on Nest Thermostat (10 slot)

Jake — Wed, 19 Oct 2022 20:10:24 GMT

Hey davidmia,

I am sorry to hear about the trouble you are having when switched into heating. I wanted to check in with you, and provide you with an article that has more details on the error you are seeing. Please let me know if that helps, or if you have any questions from here.

Best regards,

Jake

Re: E79 Code on Nest Thermostat (10 slot)

EdwardT — Sat, 22 Oct 2022 20:17:12 GMT

Hi there,

I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

Thanks,

Edward

Re: E79 Code on Nest Thermostat (10 slot)

EmersonB — Sun, 23 Oct 2022 22:26:56 GMT

Hi everyone,

@davidmia we haven’t heard any updates from you. I’ll go ahead and lock this thread in 24 hours. If you're still in need of assistance, feel free to start a new thread and we'll be happy to help.

@Houptee, @Jake, @Edward and Archie I appreciate your help.

Regards,
Emerson