I keep getting e24 error message, there is no power in the Rh wire

[ndalimonte]

What is very weird is after it shuts off for several hours, it comes back on?! That doesn't make sense... unless the unit has gone fickle. The same generation in another part of our home works just fine. I've reset the nest, unplugged/replugged the router, etc. and nothing works to clear the E message.

Can anyone offer some help? Much thanks in advance for your reply.

[kcruzgonzalez]

Hi @ndalimonte,

Thank you for posting in the community. I understand you're consistently seeing an E24 error ("no power in the Rh wire") on your Google Nest thermostat, which then oddly shuts off for hours before restarting. It's confusing, especially since another Nest in your home works fine. I apologize for this persistent inconvenience after your troubleshooting efforts.

Let's focus on troubleshooting steps to address this particular error and the power fluctuations:

• 1. Gather power readouts from your Nest thermostat.

Your Nest thermostat can provide specific details about the power it's receiving, which can help diagnose the issue.

• On your Nest thermostat display, go to Settings > Technical Info > Power.

• Please note down the values you see for:

  • Voc (voltage open circuit)

  • Vin (voltage in)

  • Lin (current in)

  • Battery

• Share these values with me, as they can indicate whether the thermostat is receiving a stable power supply.

• 2. Perform a Diagnostic Swap with Your Other Nest Thermostat

Since you have another working Nest thermostat of the same generation, this is an excellent way to determine if the E24 issue is with the specific thermostat unit or the wiring/power supply at that particular location.

• Turn off the power to both HVAC systems at their respective breakers.

• Carefully remove the display from the problematic Nest thermostat.

• Carefully remove the display from the working Nest thermostat.

• Swap the displays: Attach the working display to the problematic location's base.

• Turn power back on to the HVAC system at the problematic location.

• Observe: Does the E24 error appear on the previously working display when it's connected to the problematic location's base?

  • If the E24 error appears on the previously working display when it's in the problematic location, this strongly suggests the issue is with the wiring or power supply to that specific thermostat's base (e.g., the Rh wire itself or the furnace control board).

  • If the previously working display works fine in the problematic location, and you can test the problematic display on the working base (with power off again for the swap), and the E24 error follows the thermostat display, then the display unit itself might be faulty.

• If the issue persists, visit our help center to learn more about troubleshooting thermostat power issues during hot weather (E3, E4, E23, E24, E73, E74, or E195 help...

Please let me know the power readout values and the results of the swap test. This will help us pinpoint the cause of the intermittent power and the error message.

Regards,

Kevin

[ndalimonte]

Hi Kevin, thank you for your reply! I followed your instructions and the information for the Battery: 3.765 V; Voc: 0.10 V; Vin: 0.10 V; Lin: 8 mA (o). 

I also swapped out the other nest, after turning off the breakers of both units, and both units work on the main level but NOT on the upper unit.

My guess is what you wrote about the Rh wire - it is faulty itself or from the furnace control board. Sadly, I’ve now had 3 furnace technicians out to our home and first two said everything was fine, and the third one said I needed a low-volt separate transformer for the air scrubber, attached to the furnace. He attached it (and it wasn’t free) but 2 ½ hours, the unit turned off again. Clearly, I’m dealing with incompetence but I am at their mercy. Another “senior” tech is supposed to come back tomorrow. This issue has been going on for more than a month now. Thanks, Nancia

[kcruzgonzalez]

Hi @ndalimonte,

Thank you for the update and for providing those power readouts and conducting the diagnostic swap test. I truly understand how frustrating it must be to be dealing with this issue for over a month, especially with your home being so warm despite multiple technician visits. I'm very sorry you're going through such an inconvenience.

Your detailed observations and the results of the swap test are incredibly helpful and confirm our suspicions:

• The power readouts (Voc: 0.10 V; Vin: 0.10 V; Lin: 8 mA) are extremely low. These readings definitively show that the Nest thermostat at the upper unit location is receiving virtually no power from your HVAC system's Rh wire, which directly correlates with the E24 error you're seeing. For proper operation, Voc and Vin should typically be above 24V AC.

• The diagnostic swap test further validates this. The fact that both Nest units work on your main level but neither works on the upper unit confirms that the issue is not with the Nest Thermostat itself, but rather with the wiring or the power supply coming from your HVAC system to the thermostat's base at that upper unit location.

Given this clear evidence, the problem lies with your HVAC system's wiring or the furnace control board. The previous technician's addition of a "low-volt separate transformer for the air scrubber" might not be addressing the core issue of insufficient 24V AC power to the thermostat.

This information should help the senior technician focus their efforts on the actual power delivery from your HVAC system. Please let me know how your discussion with the senior technician goes.

We'll continue to support you in getting this resolved.

Regards,

Kevin

[ndalimonte]

Thanks so much for keeping me in mind over this discussion. You are correct, the problem was not with our nest thermostat (s), it was a faulty safety switch on the furnace that kept flipping on/off at random. Because it was working sometimes and then not others, it made for a frustrating experience. Add to that, 4 hvac technician visits came up empty with a solution this week… it took the fifth technician today, a supervisor, who immediately figured out the cause of a nearly 2 month issue. What a fiasco, not to mention many sleepless nights in the basement to escape the heat. Thankfully, this issue is now closed.