Nest Thermostat E72

[KidSid]

Evening.

Installed Nest Learning, with C-wire.

Sometimes we get an E72 error message, with a note about the Rh wire not getting power. The strange thing is that this happens intermittently.

As it shuts off with this message, it would suddenly come back online.

Doesn't seem to be an issue with lose wiring at the thermostat or the HVAC board and the fuse isn't tripped.

What could it be?

[CoolingWizard]

Hello, KidSid,

Did your old thermostat have and was it using a Power Common wire or did you attach a wire that was not being used to the C terminal? If you have not tested the Power Common, it might not be properly terminated in the furnace. In which case the Nest Learning Thermostat is reaching the Power Common through the W or Y wire. In that case, when that wire is energized by the thermostat, it can no longer be used to bring Power Common to the Nest Learning Thermostat.

Ken, The AC Cooling Wizard

[KidSid]

Ken.

Thanks for the response. The C-wire is connected and it's been working fine for a couple of months. The thermostat shows it's getting power and the battery is also charging properly.

The HVAC turns on and off most of the time. The error message comes on every so often and the voltage goes to zero. After several minutes, usually, it turns back on showing full voltage. On a couple of occasions the thermostat will have an actual countdown after which it would resume cooling other times it shuts off almost right after it turns on.

[CoolingWizard]

KidSid,

Can you tell about your HVAC equipment? What type of cooling, what type of heating. If it is a heat pump, what type of auxiliary heat. Thank you.

Ken, The AC Cooling Wizard

[KidSid]

Electric HVAC. In South Florida, so haven't used heat yet. The inside air handler is a Carrier.

[CoolingWizard]

Since you have some type of heat, other than a heat pump, you should have the red wire connected to the RC if you have a nest learning thermostat. Then what you want to do is go into the menu select settings, rotate the ring to Ness pro, and then you will see a picture of the back playing. Look to see if the power wire is identified and that the little wedge with the letter on it is green. If it’s yellow rotate to the yellow, select it, and then to find what it does. This is should correct any problems you might be having

[KidSid]

OK. Made recommended changes.

Everything was green.

I'll update.

[CoolingWizard]

KidSid,

here’s a little bit of additional information just for educational purposes. The indoor unit has a control panel and a step down transformer located inside of it. The arm wire comes from that step down transformer and that’s also where the common is coming from. If the thermostat is reporting that the power is not there, there might be a problem with the electrical connection of either the R wire, or the C wire in the air handler. Sometimes because that’s also where the air conditioner coil is located, the moisture in the air stream will cause corrosion, particularly when you have a gas furnace also in the same airstream. In the HVAC industry, we work hard to seal the systems so that that air doesn’t slip down and cause these kinds of problems but sometimes the design of the equipment is such that it’s hard to do. So sometimes we find corroded wires that caused some problems and that might be yours but I can’t say for sure without seeing it. Just thought I’d pass this on to help.

Ken, The AC Cooling Wizard

[KidSid]

Thanks, Ken. I'll keep that in the back burner in the event the rewiring doesn't fix the issue.

I'll also double check to make sure corrosion isn't creeping into the air handler control panel and wiring.

[KidSid]

Well. Unfortunately, the Rc change didn't seem to address the problem.

The HVAC turns on and cools, but every do often I'll get the error message about having no power from the Rc wire.

The thermostat does a countdown, then turns back on.

[CoolingWizard]

Do you have access to a volt ohm meter sometimes called a multimeter? If you do you want to check the AC voltage between R and C. Sure that it is approximately 24 V AC not less than 17.8 V and not more than 27.4 V. This is beginning to sound like your actual see wire is not connected to see in your furnace/air handler controller.

The AC Cooling Wizard

[KidSid]

Good morning.

Made the suggested checks, at the thermostat.

The G, Y, and C wires come in at about 27.7

The W wire comes in at about 21.5

[CoolingWizard]

The only one that matters is R to C. The R is the Power In, and C is the direct connection to the Power Common. The Power Common is connected to all of the control relays for the Heat (W), Blower Fan (G), Compressor Contactor (Y). Hence the “common” aspect. The control wire is like this:

R—->W—>RELAY COIL<—COMMON. So when the thermostat has a need to call for heating, the thermostat connects the incoming power to the W terminal, and the W terminal wire carries that power to the heating relay coil. And, since the common side of the power is already connected on the other side of the coil, this will energize the coil and turn on the heating system.
So clearly, it looks like your power transformer is adequate and supplying the needed power for the system to run. Therefore it should have enough between R and C to charge the thermostat. So you now either have a problem with the backplate or the logic board in the thermostat itself.

The AC Cooling Wizard

[KidSid]

Thanks again.

Went to the air handler and it looks like there's no circuit board. Wires put together via wire nuts. 😐

[CoolingWizard]

In the air handler you’re going to see where the thermostat wires come in, the red wire will be connected to the 24 V power of the transformer. The second side will be a wire coming out of that transformer and you will notice it’ll be wired to the relays. That is your common side. You are looking for the Common wire color you are using and verifying what it is connected to.

The AC Cooling Wizard

[KidSid]

Hey Wizard.

Looking at the wiring diagram on the thermostat, I noticed the Rc going from green to red when the unit shuts off before reaching the designated cooling temperature. The rest of the wiring stays green. When it switches back, it's after the red indicator goes back to green.

So that seems there's an issue with the HVAC and not the wiring to the thermostat, correct?

What could cause that power shut down?

[CoolingWizard]

KidSid,

Without a properly connected Power Common, the thermostat uses the Y1 or the W wire to reach the common. When the system turns off, the thermostat starts the process of finding the common. That is what you are seeing in the color change. All good.

Ken, The AC Cooling Wizard

[EdwardT]

Hi folks,

@CoolingWizard, thanks for the help!

@KidSid, how's it going with your Nest Thermostat? Still need our help?

Thanks,

Edward

[KidSid]

Looks like things are working fine, thanks to Cooling Wizard's suggestions. 

Also, as a separate issue, people need to check on their Safe-t-switch. If your system isn't draining that switch can shut off your system giving you an error message on the thermostat. 

Checking that switch and drainage should be one of the troubleshooting steps if you get an E72 error message. 

[EdwardT]

Hi KidSid,

. You 

Thanks for the update. I'm glad to hear that everything is working fine. That step is documented on this article about power issues.

 I will be locking this thread in 24 hours. Please feel free to create a new thread should you have some more questions or concerns in the near future and we'll be more than glad to help.

Thanks,

Edward