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Nest Thermostat Error N72

FrankieJ
Community Member
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:
  • Y
  • G
  • Rc
  • C
 
From our very basic, boiler-and-radiator based oil heat system we have 2 wires:
  • W
  • Rh
 
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!
178 REPLIES 178

EricPizz
Community Member

Thank you for your persistence. It will be helpful to know the results when you receive and install the replacement. 

capecoddah
Community Member

OK Google, here's my latest adventure in this "plug and play" thermostat technology.   I put 2 additional relays between my AC unit and your "smart" thermostat.   They are respectively connected to Y, Rc, and G and your thermostat.  I powered the 2 relays with a separate 24V transformer and also provided your thermostat with a C wire from the separate, new 24 V source.   Then of course connected the respective relay contacts to the Y, Rc, and G wires on the AC unit.    The good news is I can shut off my AC for the winter, and still power your thermostat.    Also, the heat and air conditioner works.   The bad news is your thermostat still gives me the n72 error.    If you would just tell us what the algorithm for announcing an n72 error is, we could figure this out.   I am assuming it means that the thermostat cannot draw power between C and Rh because your algorithm assumes we all have one transformer systems.   Well in the NE USA, there are lots of 2 transformer systems and they are coming alive now as it gets cold.   I am assuming this error would happen in any 2 transformer system where the heating system is separate from the AC, because the C wire from the AC does not have the same return path as the C wire from the heat, and you have only one C wire on your back plate which announces to your software what's connected and what's not.   Know that many heating thermostats in the NE USA are 2 wires, so there is no C wire.  I am assuming you know this?  I am assuming a few lines of code would fix it -  if you can get power from the AC side in a 2 transformer system, tell the user that's OK.   If you can get power from the heating side of a 2 transformer system, tell the user that's OK.   If there is a C wire connected to just one of them, use that side to power the thermostat and perhaps I identify this and warn the user not to shut off the system that's providing the power.    That's assuming I have it right, which I won't know unless you tell us.    No wonder the professional installers are also flummoxed.   You have to tell people how it works, and you cannot change the basic operations later in a way the causes functioning system to fail.   This is a basic rule of IoT.   Stability of basic functionality is more valuable that any wiz bang feature.  We await a real response.

This is unbelievable that we have not gotten anyone from Google to comment about this. I think what you are saying makes sense about the issue. 

pem884
Community Member

Strongly agree with both @capecoddah and @Chobes 

Edited to add; I am back to using my old "dumb" Honeywell thermostat, which works fine-thank-you-very-much powered as in my existing setup (c wire included). No surprises there.

I am beginning to wonder if there could be some kind of issue with the equipment detection method (voltage?) when the Nest thermostat internal battery goes south. Not sure exactly how that would work and I suppose we could guess just about anything, given the complete lack of any valuable info from Google. I'm not extremely skilled but somewhat handy and willing to take measurements or try a few things if it could benefit the group... but capecoddah has really gone way above and beyond.

Unlike google.

DPM76
Community Member

I posted before but wanted to provide more specifics because I really do not think the N72 error has to with the wiring setup.

 

  • I have four Nest thermostats in my house and two of them are exactly the same - display model 3.4 (backplate 5.4).  One of these is upstairs and the other is on the main floor.  Software is up to date on both of them.
  • The upstairs display started giving me the N72 error once we turned our heating system on (worked fine hooked up to central air all summer) and it runs out of battery after about 24 hours and we have to manually recharge it every day in order to run our heating system upstairs.
  • We swapped the two units and had the same problem downstairs (with the upstairs display on the downstairs backplate) while the downstairs display worked fine (on the upstairs baseplate).
  • I have tried to reset the impacted unit but still get the N72 error.  It stinks because we are in the Northeast and it is starting to get cold so our heat is not running.  

Essentially, the issue is that the N72 error fails to allow the display to charge.  I have done a ton of research and it does not see any solution online.

Called Google and spoke with Marvin tonight.  He was was really nice and walked me through the troubleshooting steps and ultimately indicated that the N72 error is related to the display itself.  If the display is out of warranty, then they will NOT replace it.  Mine was purchased in 2017 so he told me that I would have to purchase a new display if I wanted to fix the issue.

Chobes
Community Member

it will be interesting to see what happens when @GS1 installs his replacement unit!

GS1
Community Member

Ok friends, I finally got my replacement intalled the same exact way as my old one. I have good news and some not so good news. The good news is that my heat and AC both work. The bad news is that I still have the N72 error. At this point, a month into this nonsense, I'm satisfied with it. But the problem is clearly with the actual thermostat and its software. I suggest of you're under warranty try to get them to send you a replacement. If youre not under warranty I'd still bug them to do the right thing and give you a replacement. Twitter really worked well for me getting them to respond quickly.

Next time lets just all buy dumb thermostats and call it a day

capecoddah
Community Member

Thank you, this is very helpful and 100% in line with what I am experiencing with a newer thermostat.  I get the error, but heat still work and AC.  But it does appear that the Air Conditioner power must be on to continue to provide power to the thermostat if the C wire goes back to the air conditioner transformer, as I believe it does for those of us with this configuration.   

GS1
Community Member

I think I spoke too soon. 12 hours in and it won't connect to WiFi. JUNK 

Mikeya
Community Member

Is it because the “low battery”?

Wondering the same thing.   If the c-wire is from the air conditioner, and that's switched off for the season, then the thermostat might not be powered if it works the way (which of course I cannot verify with Google).   Might work for a while till the battery drains.   There's a tech screen if you scroll through on the device and a power selection, would be interested to know what that reads, might give a clue.   

My C wire is from the A/C and it had not been a problem for 3+ years until just recently. Heating and AC systems are fully functional...had techs come look at them both. They found no issues at all. Their opinion is that the Nest thermostat is faulty.

GS1
Community Member

Battery 3.926v

Voc 36.14v

Vin 36.14v

Lin 200

 

 

capecoddah
Community Member

Ain't power,  So much for that theory.   Sorry!

I'm sick of guessing, time for Google to fess up and tell us how it works!

GS1
Community Member

Nothing but problems with this thing. Its exhausting!

GS1
Community Member

So after my replacement thermostat stopped connecting to WiFi they agreed to send me ANOTHER ONE. I laid into them a little bit to the point where they said they were sending my case to their legal department and sent me a brand new thermostat as a replacement. Ive been using the new one for 3-4 days so far and I've had no issues. I still get the N72 error but my heat and AC work.... For now. I havent heard back from their legal dept but since they put me through a month long battle I am no doubt going to follow up regardless.

Mikeya
Community Member

Same here 😡 - two completely separate systems, AC and heat, and all of a sudden one of my thermostats started with the n72 error. Did anyone get anywhere with support?

GS1
Community Member

Unfortunately not. I got them to agree to send me a replacement unit since Im still under warranty.  Im expecting to get it sometime next week. Hopefully that fixes it. If youre still under warranty this might be the only option as everyone here stated the troubleshooting with support was a waste of time. 

capecoddah
Community Member

GS1, my thermostat is new (last month or so) exhibiting the n72 error.   If the new one you get corrects it, please let us know.  I felt the support person I worked with was really trying, but wasn't told how the product actually works either.    Also, since Google won't help us, it would be helpful to know if those experiencing this error have a) heat and AC work but error is shown, b) heat doesn't work, AC works,  c) AC works, heat doesn't work, d)  Nothing works, e) Thermostat goes dead, no power to after internal batteries wear out.    For the record, mine case is a).   

Hi, 

I posted this reply to someone above.  My heat works, but I can't test AC because it tells me it's too cold to do so (?!). (but looking at the image of my wiring, I'm thinking it won't work.  Who knows.  I do have an N72 error, but I read somewhere that if the system works, I can ignore that.  I bought the stat on Saturday and installed it with lots of trouble until I figured things out down at the furnace.  Battery is holding a charge at 3.8 or so.  

I also read that the Nest has an internal jumper between Rh and Rc, so don't need both as long as power to one?

"I don't know if this helps, but here's an image of my setup.  Just did this on Saturday.  I know that the Y1 is "redded" out, but that's for AC and I live in PA...so cold right now.  The Rc is also greyed out, so I'm not sure what's going on there.  My AC and Heat all patch into the same transformer, but my C terminal on the Transformer doesn't have a wire out.  The blue wire out of the transformer comes off of the R terminal.  So I can't figure **bleep** out...

All that said.  Heat works, I get the following on the power read out on the thermostat (all are within workable levels, though obviously it's not picking up a C wire power feed)

Battery 3.84

Voc. 35.87

Vin 34.80

lin 40mA

IMG_2545.jpegIMG_2550.jpeg

B and C are the same…but my AC work and heat doesn’t. 

EricPizz
Community Member

Sorry to hear that, GS1, but it was well worth the try. Really appreciate you documenting your efforts, has helped me and I'm sure others on this thread.  I now have the Honeywell T9 smart thermostat installed in place of the Nest and no issues whatsoever. Has been working perfectly for almost a month now, FWIW. I would love the Nest to work for me, it won't. So, I had to go to another solution. All the best.

Chobes
Community Member

How’s the T9? Hearing mixed reviews about it too. Is there a cheaper Honeywell that uses the same app so my two thermostats can be on the same ecosystem? Or would I need two T9’s (1 upstairs , 1 downstairs)?

EricPizz
Community Member

Hi Chobes...The T9 is excellent and the Honeywell app works just fine. I can also control it with Google Home and assistant. I paid about $75 after electric company rebate. Very pleased with it. It's a smart thermostat but not sure it had all of the learning capability of the nest. Control is via app, schedule, and remote sensors that detect motion. Also supports geofencing.

Chobes
Community Member

I purchased the T9 and installed it. I wired it with the same connection points as the Nest before it started having issues. And voila it works and haven’t had any issues after a day of use! It didn’t automatically detect my heater so I had to go in equipment and configure it but it works well so far. 

Mikeya
Community Member

Not sure if anyone has done this yet, but I suggest doing it if you have some time. Just like everyone else, my two thermostats started having the same issue after their latest update. I finally got someone on the phone after opening up a case online, to admit that it just be a software update that is causing the issue (he looked at the date the issue started from the thermostat and correlated that with the latest update). Anyhow, he escalated the case to engineering, I think the more of us complain and escalate the quicker they might push to resolution. He thinks the new update screwed up with the power readings.

EricPizz
Community Member

I initiated a case. Let us know if there is an update from your side. Thanks!

Mikeya
Community Member

Definitely will do! 

Chobes
Community Member

That’s great to hear. How do you open a case and what did you say just explaining about the N72 error. We should be consistent with each other so they know they are related. 

Mikeya
Community Member

I started the case online through Twitter, but continued over the phone. I basically explained that both my thermostats were working without any issues for 2 years with the config I had (no changes made, and it’s a brand new construction, so new heating system and new AC, no maintenance needed there). The tech looked for when the Rh wire started not being recognized, and then I mentioned the latest update they had, and he saw that it was the same date. He made me go through a few troubleshooting steps (switching some wires and what not), all stupid steps if you ask me, but he needed them to take some pictures and be able to escalate the case.

Chobes
Community Member

I’ve been interested in trying an additional relay to see if it would work. I followed this post using a PAM-1 relay. https://www.reddit.com/r/Nest/comments/9hp0lb/fixed_nest_thermostat_causing_chattering_relay_on/

Im pretty sure I wired everything correctly and plugged in my charged NEST. I ended up getting a Rc no power E73 error. I checked breakers and fuses and condensate pump on my AC system but I didn’t see any issues. I gave up and rigged my old dumb thermostat so I can continue to have heat. Not really sure what my options are now besides getting a different thermostat. Doesnt look like Google cares about making good on what we all believe to be their mess-up. 

pem884
Community Member

Yup. I got so frustrated I ran a whole new C wire from my aquastat just to try it instead of the one that comes from my AC (the aquastat/boiler only had 2 conductors coming so I figured I'd want more eventually; pulled 5)... wasted a couple hours (inevitably a couple things went wrong)... no joy; nothing changed. I'm back to my old dumb Honeywell, too.

I have considered going through support, would really love to increase their awareness of the issue, but I always have other things to do... really annoying because I am also working on other smart home projects.

Chobes
Community Member

@Jake @GarrettDS  care to look into our issue on this thread?

Jpbone120
Community Member

Completely pissed and frustrated. Like you all I have a separate central air system from my hot water baseboard furnace system.  I have four wires for the AC and the red and white for heat. Sometime in October I started getting the same error messages that flip flop between the white and red wires. I troubleshoot the issue by reseating the wires in place at the baseplate this worked for the past month. Last night however I was not calling for heat and my boiler kept sending heat until it got to 80 degrees. I almost died from the heat it was unbearable. **bleep** Nest **bleep** Google … I’m going to Honeywell. I love the nest app and the product integration from the door bell to the smart locks but this has left a sour taste in my mouth, After this experience I’m considering getting rid of all my nest product

Yanikb
Community Member

Switched thermostat from nest 2 to nest 3 all of a sudden n72 error code rh wire greyed out power not detected, switched back to nest 2 thermostat no error code no greyed out wire. Switched back to nest 3 again n72 error code rh greyed out. WTF is that?

Chobes
Community Member

Thanks for letting us know. That is helpful to know it is something with this thermostat 

Fatima
Community Member

Ok! After several weeks of patiently waiting, still no response or Hope insight from @GoogleNestTeam … I switched to the Amazon thermostat.

a fraction of the cost, same functionality, sleek, support is AWESOME and most importantly it WORKS! I usually don’t follow a thread this long but @GoogleNestTeam is not going to admit to this ridiculous, simple mistake.  Next step, class action lawsuit anyone!?

pem884
Community Member

Thank you so much for the update... I'll have to check that device out.

For anyone interested... I mentioned previously that I installed another C wire, this one going to the aquastat of my boiler.... well... it's a Honeywell L8148J which only puts out 0.8a, so the thermostat doesn't like that either (this time, understandably, IMO)

Now it's just a matter of deciding how much more time, and/or money, I can afford to waste on this... definitely going to look at the Amazon one.

Chobes
Community Member

I’m game. Tell me where to sign! Haha