Nest Thermostat Error N72

[FrankieJ]

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!

[Jenelyn_O]

Hi folks,

Thanks for reaching out. I'm sorry for the trouble this has caused you. You'll see the code N72 if one of the wires in your thermostat is receiving power, but it's less power than expected.

Giving these steps a whirl might help: 

1. Turn off power to your system at the breaker box, fuse box or system switch. This helps prevent damage to your system and keeps you safe.
2. Pull off the thermostat display.
3. Check that all connectors with wires are pressed down.
   • If they aren't, re-insert the wires. Press the connector button down and insert the wire as far as it will go. If the wire is in correctly, the button will stay down when you let it go.
   • You may need to strip the wire ends. There should be ⅓ to ½ inch of straight copper wire exposed.
4. Press the display onto the base until it clicks into place and turn your system power back on.

Let me know how it goes.

Best,

Jenelyn

[wsl52]

Hi Jenelyn,

Your suggestions don't work for me.  I have Y, G, Rc, Rh, and W wires connected.  If I disconnect the Y wire, it will display the Rh wire is connected.  But if I connect the Y wire, it will show the Rh wire not connected.  I was on a call with the Nest support and they said the only way is to get a C wire connected.  Not sure if that will work, but I'm going get that installed and see.

[Markjosephp]

Hello there,

Thanks for looking into this, Jenelyn.

@wsl52, we appreciate your updates. Could you help us gather your thermostat's power readouts in its Technical Info settings with and without the Y1 wire? Also, what's your support ID number, so we can check the status and see what else we can do?

Regards,
Mark

[wsl52]

With Y wire connected, I see Battery 3.815V, Voc 37.08V, Vin 35.48V, Iin 40mA.  Without Y wire, I see Battery 3.788V, Voc 4.26V, Vin 4.26V, Iin 8mA.  

[Cip54]

I had the exact same experience. For me, the N72 error had no effect on my system, just a nuisance. Do you have functional issues? As I mentioned earlier in this post, I found that the Fast Stat common maker has the Nest believing there is a C wire, and I have found the result to be very stable. The Google power adapter also is working for me on a different Nest.  But neither cures the N72 error. Just makes it irrelevant.

[wsl52]

For me it works, but seems to be running on the battery backup power as my Rh wire is not connected.  For the last 2 weeks, I would need to take the display off about every 7 days and use the USB to recharge it.

[Markjosephp]

Hi everyone,

I appreciate the details. In addition to the message code, it appears to be an early warning or a heads-up that a power-related issue is causing a report with the Rh wire. With this, if your system works normally, you don't need to take any action but might want to contact an HVAC technician to check the system.

You can try checking the actual voltage readings by using a Voltmeter or a multimeter, and if the code keeps showing, we'd suggest reaching out to a Nest Pro or Technician for a system inspection.

Best,
Mark

[Markjosephp]

Hey folks,

I don't want to be a nudge, but I'd like to check if you were able to get your thermostat working. Feel free to reach back if you still need help.

Thanks,

Mark

[wsl52]

I'm still waiting for my brother to help me hook up the c-wire.  Will let you guys know if that fixes the issue.

[Markjosephp]

Hello wsl52,

Sounds like a plan! Keep us posted on how the installation goes, and we would be glad to know what the result will be. Also, you can check out this article about the common wire, or C wire, for reference.

Best regards,
Mark

[Markjosephp]

Hi wsl52,

Checking back in — how's it going with the installation? Did that help?
Thanks,

Mark

[SofianeMTL]

For me no, so what I do is during winter I unplug the wire that starts the fan and in the summer I put it back in order to allow me to have air conditioning.

[TimW08]

For anyone running a Taco SR50x-EXP-4 switching relay (x = number of zones and it doesn't change what's needed as the controller unit is simply expanded to allow for more zones)  and want to run a Nest thermostat but receiving a the error for Rh low voltage (n72 or n73). A 1000Ohm 1/2 Watt resistor needs to be added between W and C from the 24V zone outputs that connect to the thermostat. 

Will be trying this in the next few days and plan to report back with results. 

Here's a video that explains the process and also shows the relay hook up. 

https://www.youtube.com/watch?v=IcvKK0yYtws

Here's the manual from Taco also explaining this resistor. 

https://www.tacocomfort.com/documents/FileLibrary/102-389.pdf

Taco's product page for documentation.

https://www.tacocomfort.com/product/enhanced-switching-relays/

[mmetz5]

RESOLVED - So I had the same issue except my Nest worked for 9 months before issue started.

Background / Set up:

 - Gas boiler , baseboard hot water heat in basement (3 heat zones + hot water heater also - so really 4 with Taco controls)
- Separate HVAC with air handler in attic

- Nest thermostat controlling 2nd floor heat zone and 1st and 2nd floor HVAC - air cond unit

- Winter no issue, thermostat remained charged all winter.

- Summer - would work for several days and then shut down as it was not charging the internal battery

Bottom line (prob obvious to everyone now) - the smart thermostats require a COMMON wire period.

So I had no desire to try and run a plug in charging solution through my walls to an outlet - horrible solution AND my thermostat is not sitting directly above an outlet and certainly not one I want always taken with a thermostat charger (eye roll).

Solution - FAST STAT Common Maker - HVAC Wiring extender

1. Not expensive ($40 home depot, etc)
2. Hooks to thermostat AND then to one of your systems
3. You do need to know your own wiring a bit, but they provide several wiring diagrams with kit AND more on line on their site AND great tech support (called me back in 5 min - I called just to make sure I was using the correct wiring diagram
4. My wiring was a little weird but I was able to figure it out pretty quickly

               - Heat thermostat wiring for 2nd floor went first to attic to the air handler then back to the thermostat

               - So thermostat has Rh, Rc, W, G, Y - Rc was Red wire and Rh was Blue - confused me a bit at first but in attic I could see the Red from basement connected to the Blue back to thermostat . And the actual Blue common on the air handler wiring not going to the thermostat. Technically they say you can take the Green (lose control of fan) and use that as a common, but I was not a 'fan' (lol) of losing option to run fan in winter.

Added FAST STAT common maker and voila , thermostat now has a common and is staying charged.

[EmersonB]

Hi everyone,

@mmetz5, we're thankful for what you've shared and we're glad that it's working fine now.

I appreciate your response, SofianeMTL and TimW08.

Let us know if you have more questions in mind.

Thanks for your assistance, Mark and Jenelyn.

Regards,

Emerson

[FrankSt]

I have a similar issue. 2 Nest Learning Thermostats and one Nest E. They control a 3 zones gas hydronic baseboard heating and additionally the 2 learning thermostats control 2 AC Units. All worked normally for 4yrs+

One of the Learning thermostats started to behave erratically when in Heating mode. It calls for heating but doesn’t send anything to the Taco Board controlling the heating zones (boiler doesn’t start and zone doesn’t open).  The AC function works normally in this unit, just the heating fails. All other thermostats work as intended for both heating and cooling. 
if I take the thermostat that isn’t working and plug it in the base of the one working, still doesn’t work, but if I take the working thermostat and plug it in the base of the one failing, it works. This proves it’s not the base but the thermostat itself. 

have tried restarting in and have tried a full reset with no luck.

• Display 3.4
• Software 6.2-27
• Battery 3.92v

[FrankSt]

@EmersonB help here please 

[Ether]

I had experienced the same thing on one unit and the other unit kept cycling my boiler on/off/on/off where i can hear the taco valve cycle.  What this tells me the solid state switch responsible to turn ON/OFF rH or rC is in leaking in the off state.  The fix was placing a pull down resistor.  for the other unit it would not turn on at all.  this is after 4 yrs of service using 3 nest to control 3 zones, troublshooting with Google 'tech thermostat' replacing the base, etc.  After much frustration especially due to the need for proper heating is of course when you experience these problems. I went to Honeywell smart thermo which uses a traditional airgap contactor, not a solid state component to toggle rH rC, besides without a C on a Nest unit, it parasitically charges the batt from rH/rC.  When you jump at the taco board, it works fine.  I gave up on Nest, not worth the time and a freezing home nor the lack of real support. When you speak to Google Nest customer support, it is all scripted troublshooting.  No real support besides processing of refurb units.

[FrankSt]

thanks for your response. I ended up giving up on Nest too. Bought an Amazon thermostat and it’s been working great. Only cost me $30 at Woot.com vs the $250 for the learning thermostat, that is not getting support, further development and probably will be dropped by Google too; wasn’t worth the investment to replace the faulty unit