topic Re: Nest Thermostat Error N72 in Nest Thermostats

Nest Thermostat Error N72

FrankieJ — Tue, 12 Oct 2021 21:08:59 GMT

Error N72: "Power wire Rh detected. No connection to equipment." 24VAC confirmed.

I recently installed a Gen 3 Nest Thermostat. We have completely separate heat and Air Conditioning systems.

From our standard, ducted air conditioning system we have four wires:

From our very basic, boiler-and-radiator based oil heat system we have 2 wires:

This has worked for years with a battery-powered, programmable thermostat. In the winter, we cut the circuit breaker from the air conditioning system, so all we have is heat, using W and Rh.

Installed the G3 nest yesterday and it reports: "Power wire Rh detected. No connection to equipment. Learn more at nest.com/n72".

A few things to note:

The Tech Info diagram on the unit itself shows "power" greyed-out on Rh.
Testing with a digital multimeter between Rh and W1 shows over 24VAC consistent.

Thoughts? Thanks for any help you can provide!

Re: Nest Thermostat Error N72

Nestoldum — Fri, 22 Oct 2021 02:03:29 GMT

This happened after 3 years of proper functioning. Rh greyed out. For months I've been trying with no luck. Very very disappointing that this big company cannot offer a solution.

Re: Nest Thermostat Error N72

Galaxy2020 — Wed, 27 Oct 2021 05:07:05 GMT

Any luck with solution? I have same error but with traditional thermostat both heating and cooling works fine.

Re: Nest Thermostat Error N72

CaptainJeff — Fri, 29 Oct 2021 15:49:18 GMT

I have the same exact problem. Installed Nest Learning on 1 April. Ran heat through May, then AC and Heat into June until we went to straight AC for summer. Multizone heating system (forced hot water) with two Nest E's on other zones. Testing heating system from Nest Learning on 6 Oct. Taco box just kept clicking. You will see other posts about this across the web. Call heating company for PM b/c I assumed the problem was with boiler. First thing tech said when he arrived was "you have a Nest thermostat, correct?". He performed the same troubleshoot you did (W to Rh wire - triggered heat). Notified Google and then sent me new baseplate (arrived last night and installed - no fix. Same exact issues). Contacted them again this morning and just went through 2 hours of painful chat troubleshooting with totally useless reps (Madison and then Enzo). If you get them, stop the chat, pull the Nest out of your house and get another brand. I'm done with having a teenager in their basement ask me if my house is powered and calling it support/troubleshooting.

Re: Nest Thermostat Error N72

CaptainJeff — Fri, 29 Oct 2021 15:53:37 GMT

I have the same exact problem happening right now. Installed Nest Learning on 1 April. Ran heat through May, then AC and Heat into June until we went to straight AC for summer. Multizone heating system (forced hot water) with two Nest E's on other zones. Testing heating system from Nest Learning on 6 Oct. Taco box just kept clicking. You will see other posts about this across the web. Call heating company for PM b/c I assumed the problem was with boiler. First thing tech said when he arrived was "you have a Nest thermostat, correct?". He performed the same troubleshoot you did (W to Rh wire - triggered heat). Notified Google and then sent me new baseplate (arrived last night and installed - no fix. Same exact issues). Contacted them again this morning and just went through 2 hours of painful chat troubleshooting with totally useless reps (Madison and then Enzo). If you get them, stop the chat, pull the Nest out of your house and get another brand. I'm done with having a teenager in their basement ask me if my house is powered and calling it support/troubleshooting.

Re: Nest Thermostat Error N72

CaptainJeff — Fri, 29 Oct 2021 15:56:06 GMT

Posted below. I need to get different unit tonight or spend night 23 cold. Totally useless support from Madison, Enzo and Caitlin. I wonder what help desk scrap heap they found them.

Re: Nest Thermostat Error N72

EricPizz — Fri, 29 Oct 2021 18:44:11 GMT

I have the same exact error and problem, same exact wiring setup as the original person who posted with the separate heat and cooling systems. The thermostat has been working well for the past 2 years. Within the past day, my Nest 3rd Gen started reporting E102 and N72 errors. I had the heating repair company here and he confirmed that the boiler, heating system and control are working by jumping the Rh and W1 wires. He proved that the issue is the Nest thermostat. Strange that the original post for this issue was 2 weeks ago, and all replies are within the past week. Was there a Google/Nest update that caused this error to appear? I am going to call Nest Support to find out, reference this thread and seek a replacement Nest thermostat. I do like the Nest thermostats and they have been extremely useful to monitor and control heat remotely, effective in helping reduce energy use and generally reliable until now.

Re: Nest Thermostat Error N72

CaptainJeff — Fri, 29 Oct 2021 18:55:29 GMT

My Nest E's are great. None of them are controlling an AC and Heating System. It's only the Nest Learning. Be prepared for a frustration and disappointment if you contact Support. They will have you running around your house doing all manner of reindeer games before blaming you for the problem. Extremely unfortunate and very disappointing.

Re: Nest Thermostat Error N72

Nestoldum — Fri, 29 Oct 2021 19:26:27 GMT

I agree Nest/Google support fails on this issue. I think it is whether software or hardware, related to the Rh port on the base plate. At least on mine. If I connect Rh to Rc and W1 to Y1, heating still is triggered (although system thinks I'm cooling the house). So it is not my boiler definitely. I was considering getting a new Nest just for the baseplate but hearing new baseplate didn't do the trick for @CaptainJeff changed my mind. And it also supports the argument that all this is software related.

Re: Nest Thermostat Error N72

CaptainJeff — Fri, 29 Oct 2021 19:36:56 GMT

I agree. The Tech wants me to switch wires ( not home at the time of chat this a.m.). He wouldn't tell me what he wanted me to do - said I needed to be at the thermostat to disclose this "top secret" info. Now I know. The AC works, heat doesn't. He'll want me to run it in the AC mode to heat. It has to be software. It worked when I was heating the house from 1 April through mid June (I sent him the June Nest Report showing I was heating). Now it doesn't. They have absolutely no clue. I tried to reinstall my Nest E to this location, but the rewiring done to support the Nest Learning won't let it operate. It knows there is an AC unit in the loop.

Re: Nest Thermostat Error N72

Fatima — Sat, 30 Oct 2021 00:28:56 GMT

Facing the exact same issue as @CaptainJeff. Have been on the phone twice with support and no luck. I went out and purchased a new learning Nest thermostat and issue continues, clearly it’s not the device and it can’t be that all of have them air conditioning working and all of a sudden the heat isn’t. I’ve had this thermostat for 5 yeahs, no issues. Google is giving me the run around to get an HVAC vendor to look at the Rh wire as it is not receiving enough voltage. Beyond frustrated.

Re: Nest Thermostat Error N72

CaptainJeff — Sat, 30 Oct 2021 01:00:31 GMT

I just finished a 1 hour + session (phone) with a second tier support person named Michael. He was excellent. He was referencing manuals for my boiler and the zone box where all the thermostats “talk” to the boiler. I need to add an isolation relay coil so the Nest Learning Thermostat can get enough power. This was consistent with what my Heating System Tech said. So, it’s not the baseplate or the thermostat. We were able to disconnect the ac and get the thermostat to turn on the heat.

Re: Nest Thermostat Error N72

Fatima — Sat, 30 Oct 2021 01:07:21 GMT

oddly I have an isolation relay coil for this zone and that is what makes the clicking sound when attempting to turn on the heat. It just clicks away and nothing.

you have me thinking if I should replace it. I called an HVAC tech for tomorrow morning, I’ll try before then to attempt coil replacement myself.

keep you posted!

Re: Nest Thermostat Error N72

capecoddah — Sun, 31 Oct 2021 02:33:18 GMT

I have exactly this problem. My variation is in a northern climate, with a generator, we power our heating system not our air conditioner on a power fail. This keeps our pipes from freezing. But not with a fancy nest thermostat I guess. Because it seems to (does anyone know the real answer here) get it's power from the air conditioner not the heating system which is 2 wire, and even when I added a C wire to the heater side and removed the C wire from the AC side, it still gives me errors. I have two, one in the box, the other on the wall. Looks like they are both going back to Google. Come on guys, this isn't hard. Can someone at Google tell me how it works, what the algorithm is for deciding how it scavenges power for a given wiring configuration? Hours on the phone with them, no one can answer this simple question. Like to hear more about the specific isolation relay coil that works. Thanks for all these posts, very helpful!

Re: Nest Thermostat Error N72

Fatima — Sun, 31 Oct 2021 03:11:51 GMT

My setup:

heat: oil based radiators. Two wires coming from the relay by the furnace, Rh and W1

ac: four wires coming through. Rc, G, C, Y1

C wire comes from the HVAC condenser but is installed on the heating site of the nest.

n72 error is a result of Rh not getting power.

I call rubbish.

all wires except C are generating. 24v. That is what is needed. I checked voltage at the thermostat and at the isolation relay box. Everything had the 24v (except for the main 120v at the relay box)

I removed all the HVAC wires, including the C wire. Heat doesn’t turn on, when it should.

bought a Honeywell stat, installed the W1 and Rh, again, 24v steady, voila, heats on!!

conclusion: NEST GOOGLE ITS YOUR DEVICE/ALGORITHM. You did something in the recent weeks to screw this up. For 5 YEARS all was ok until this season. Highly unlikely that there is a change in C voltage from before- it’s probably Google not accepting that 12v.

Re: Nest Thermostat Error N72

capecoddah — Sun, 31 Oct 2021 03:20:38 GMT

Thanks for this reply, so I get this straight, your system was working with a Nest thermostat until a recent software update. I'm new to the nest thermostat so this is great info.

Re: Nest Thermostat Error N72

Fatima — Sun, 31 Oct 2021 03:22:43 GMT

Correct. Worked fine for 5 yrs. Turned stat to heat this season/yr and it didn’t work.

Re: Nest Thermostat Error N72

capecoddah — Sun, 31 Oct 2021 12:47:50 GMT

Has anyone tried putting the C wire adapter on the heating relay, and disconnecting the C wire connected to the air conditioner? And does anyone know what the C wire adapter is? My guess is a resistor to sneak some current from Rh in our case, without going through W and flipping the relay. Again thanks for this thread, very helpful to know it's not just me.

Re: Nest Thermostat Error N72

capecoddah — Sun, 31 Oct 2021 20:00:58 GMT

OK, one more question, haas anyone tried the RIBU1C relay as described here - https://www.youtube.com/watch?v=IcvKK0yYtws. I have a TACO zone controller driving my Rh and W. A separate air conditioner with Y, G, Rc and C. I'm temped to try this but I am concerned that I've read that some opinions that the Nest chooses Rc and C for power if it is present. And it appears that the NEST somehow detects when a wire is present (perhaps by some sort of magnetic sensor looking at the base plate depressions? So I am worried, even if I do this, it will still want to scavenge power from AC and not give preference to my heating system and/or will just go into an error mode and let my house freeze. Any experiences appreciated. I've actually contemplated connecting a dumb thermostat in parallel with the next for Rh and W. Would be an interesting conversation piece for my friends. Whats with the 2 thermostats next to each other on the wall? Answer: the dumb one is because I don't trust the smart one to keep my pipes from freezing. Thanks for any hints!

Re: Nest Thermostat Error N72

SofianeMTL — Mon, 01 Nov 2021 18:04:48 GMT

I have the same issue, the Rh connexion is grayed and n72 error code. In my case I have:

1. Heating with baseboard

2. AC with forced Air system

Now when I heat the baseboards go on but the problem is that it starts the baseboard heaters with the Air furnace which makes no sense at all, I wonder why I bought a 350$ thermostats that does a worse job than the 50$ one i Had