Nest "No power to Rh wire" message
"No power to Rh wire detected" is a Nest help code (E4, E24 or E74). The thermostat cannot sense power on its Rh wire, which powers the heating, or heating and cooling when there is no Rc wire. It is a critical code: you cannot control heating or cooling until a licensed HVAC technician fixes the cause.
What does "No power to Rh wire detected" mean?
It is one of the help codes a Google Nest thermostat shows when it finds a problem. Google says the thermostat or the app shows a message with a code when it detects something wrong, and this one says the thermostat cannot detect power on the wire connected to its Rh terminal.
The code is in Google's E group. Google describes E codes as critical and says they must be fixed as soon as possible: while one is on the screen, the thermostat will not let you control heating or cooling, and the message stays until the cause has been fixed. That is why the heat or air conditioner (AC) seems dead while the message is up.
What is the Rh wire?
It is one of the two power wires a thermostat can use. Google's wire guide explains them like this:
- If you have an Rh wire and no Rc wire, the Rh wire is the power wire for both your heating and your cooling.
- If you have both an Rh and an Rc wire, Rh powers the heating system and Rc powers the cooling system. Google calls this a dual transformer system, because heating and cooling have separate power sources, and it strongly recommends a professional for these systems to prevent damage.
- If you have a single wire labeled just R, it can sit in either the Rc or Rh connector on a Nest.
So "No power to Rh wire" can mean the thermostat has lost power for everything, or only for the heating side, depending on how your home is set up. You do not need to look behind the thermostat to find out. A licensed HVAC technician can tell from the wiring.
Which codes show this message?
Google's help code table lists the same message under three code numbers. The table has separate columns for each Nest model, so the number you see depends on which thermostat you have. Several related messages sit beside it:
| Help codes | Message | Type |
|---|---|---|
| E4, E24, E74 | "No power to Rh wire detected." | E code: critical |
| E3, E23, E73 | "No power to Rc wire detected." | E code: critical |
| E195 | "No power to R wire detected." | E code: critical |
| E2, E22, E72, E402 | "No power wires detected. Rc or Rh wire required." | E code: critical |
| N2, N22, N72, N402 | "Power wire Rh detected. Not connected to equipment." | N code |
| E298, E448 | "No system power." | E code, with its own article |
Write down the exact code and the message wording. That is the most useful thing you can hand to a technician.
Why would the thermostat lose power on Rh?
Google's explanation for this group of codes is short: you get one when the thermostat does not detect power to a wire, or cannot detect the wires connected to it. In other words, the thermostat is reporting what it sees at its terminals. It cannot tell you where along the way the power stopped.
Google's page names a few places it can stop. The power to the heating and cooling equipment can be switched off. In hot weather, Google says your system can break down and stop powering the Nest thermostat. Google also explains that if the drip pan or drain tube clogs and water backs up, an AC or heat pump may shut off to prevent an overflow, using a safety switch on the drain called a float switch.
Each of those sits in the equipment, not the thermostat. Google says that if you get an E3, E4, E23, E24, E73, E74 or E195 code in the summer, you will likely need to contact an HVAC professional. On this site the advice is the same in any season: a licensed HVAC technician should find and fix the cause.
How is it different from the N2 message?
The N2 group reads "Power wire Rh detected. Not connected to equipment." Here the thermostat does see power on Rh, but Google says it is less power than expected. Google does not describe N codes as critical in the way it describes E codes, and it says that if the code stays on the screen while your system runs normally, you can dismiss the message.
How is it different from the "No power" alert?
The "No power" alert, with codes E298, E448 or M20, has its own Google article for the Nest Thermostat and the Nest Learning Thermostat (4th gen). Google says it means the thermostat is not getting power from the heating and cooling system at all. On the thermostat it shows under Settings and then Equipment, with an exclamation point on the Settings icon that reads "Equipment error". Google lists causes from wiring to a missing C-wire (common wire) to the system being switched off.
Is it just a slow start instead?
If there is no help code and your system simply takes a few minutes to start, that is a different situation. Many thermostats wait on purpose before restarting a compressor, which is compressor protection, and they also wait for the room to drift past the setpoint by the temperature differential. A Nest that shows a countdown instead of a code is covered in Nest thermostat says "Delayed".
What should you do now?
Does the code start with E (E4, E24 or E74)?
- Yes
The thermostat cannot control heating or cooling until the cause is fixed. Note the code and your Nest model, and call a licensed HVAC technician.
- No
Is it an N code, such as N2, and is the system running normally?
- Yes
Google says you can dismiss the message. If it keeps returning, mention it to a technician at your next service.
- No
Note the code and the message exactly as shown and call a licensed HVAC technician.
- Yes
For other Nest messages and settings, see the Google Nest hub.
Frequently asked questions
- Can I keep using my Nest thermostat while it shows E4?
- Not to run your heating or cooling. Google says E codes are critical, that the thermostat will not let you control heating or cooling while one is showing, and that the message will not go away until the cause has been fixed.
- Why does the Rh code show up in summer?
- Google says an air conditioning system can break down in hot weather and stop powering the thermostat. Some systems also shut themselves off when condensate water backs up. Google says that an E4, E24 or E74 in summer will likely need an HVAC professional.
- Is "No power to Rh wire" the same as the "No power" alert?
- No. The "No power" alert is a separate message with its own codes, E298, E448 and M20, and Google covers it in a separate article for the Nest Thermostat and the Nest Learning Thermostat (4th gen). The Rh message names one specific wire.
- Will restarting the thermostat clear the Rh code?
- Google does not list a restart as a fix. It says E code messages stay until the cause is fixed, and the cause here is the power reaching the thermostat, which is outside anything the thermostat menus control.