Nest No Power to Rh Wire | E74 Error – How to troubleshoot and fix

The Nest thermostat is an excellent smart home device to control the temperature of your home remotely. On a warm day, you can adjust the thermostat to cool the house remotely. Thus, you get the convenience of a smart device as well as the functionality of a thermostat. However, even smart home devices tend to run into hardware issues. One such case with Nest thermostat is Nest has no power to RH wire. 

Many Nest thermostat users have reported the wiring problem. This issue is also commonly referred to as error codes E74 and N72. You’ll encounter these codes when the Rh wire is receiving low power. Here’s how you troubleshoot this issue:

  1. Turn off your system’s electricity at the breaker box, fuse box, or system switch. This helps in protecting your system against damages and keeps you secure. 
  2. Remove the thermostat display. 
  3. Inspect all connections and wires, and ensure that they are positioned as described below:
    1. Re-insert the wires in case they are not properly inserted.  Push the wire as far as it will go by pressing down the connector button. If the connector button remains in its position when you let it go, this means that the wire has been properly inserted. 
    2. The wire ends may need to be stripped. A total of 1/3 to 1/2 inches of straight, corrosion-free copper should be exposed.
  4. Now, press the display back to the base until it fits into its place. After that, power on the system.

No Power to Rh wire | e74 error

Nest states that this error code (E74 or N72) occurs when no power is supplied to the RH wire. However, understanding why an issue is transpiring does not necessarily tell you how you can resolve it. This is why, in this article, we will explain what you can do to solve the issue effectively.

The Rh wire is a type of red wire, which delivers 24-hour electric power from the transformer. In contrast to your cooling system, the Rh wire links to your heating system. This wire may be red without an “H” connected to it based on whether you have a dual transformer configuration. The Rh wire connects to your thermostat’s RH terminal.

How To Fix Nest Thermostat Not Detecting RH Wire

Here are the probable causes and fixes for Rh wire connection issues with your Nest thermostat.

Loose connection at thermostat

Check for and repair loose connections on your Nest thermostat to resolve no power to the Rh wire problem. If that doesn’t work, clear the HVAC drain line and inspect if the condensate pump connected to your HVAC is jammed.

HVAC float switch

A condensate overflow switch comes bundled with your HVAC system. This switch is positioned near the air handler and intends to keep water from overflowing into your home. 

After you’ve checked for loose connections and the state of your drain pipes, you should inspect your HVAC float switch.

You must ensure that it is turned on and operational. Or else, your Nest will display numerous error warnings and will not function due to no power. 

If the switch is turned on, it should create a clicking noise after a little rumbling. If it produces a sound, it signifies that the float switch is in the correct position.

Use Shop-Vac or “drain cleaner” to unclog the drain line

If you discover that your HVAC drain pipe is clogged, you do not need expert assistance. There is a simple fix that you can do at home. 

You have to go near the outside valve and suck all the gunk out with a vacuum. It may take some time, and you may require a strong tool, such as a shop vac.

It is essential to clean drainage pipes regularly because these pipelines discharge all types of fluids. As a result, the pipes get clogged and become the main reason behind the malfunctioning HVAC system. 

This is also hazardous to your HVAC system. It could end up damaging your compressor, which will be far more expensive to get repaired.

You can also hire a professional to clean these pipes. A vacuum suction every few months should be sufficient.

HVAC main breaker/fuse

If there are power fluctuations and loose connections, you must check the control unit fuse as it has a tendency to blow often.  

After you’ve determined that there’s no problem with your wiring, the next thing you should look for is power fluctuations in your system. 

Please note that this fuse is connected to the control unit within your HVAC and is a little valve positioned on the right of the HVAC.

How to run a quick check to know if your HVAC control is working fine:

  1. Turn off the power to your HVAC system. 
  2. Remove the fuse. 
  3. Inspect the connection center for any damaged or waterlogged components. 
  4. The fuse is blown if a u-shaped wire is broken in any way.

If this part is broken or flooded, our best recommendation is to get a completely new HVAC control unit. 

Such components are quite difficult to mount, even when performed by highly trained technicians, so if you are hesitant, get professional assistance and have them replace the breaker or fuse. 

Faulty relay in outside unit (condensing unit)

This is related to your air conditioner’s main unit outside. It usually occurs after years of use, and the relay can simply go bad after a while. A malfunctioning relay coil may trigger the fuse in the AC control unit to blow as well.

To replace the relay, we recommend that you get professional assistance from a qualified technician. When you repair the relay, you should be able to change the control unit fuse as well. The contactor relay may differ depending on the model and capacity of the device. 

Charging the nest thermostat manually

If your Nest’s power has been gone for more than 12 hours and there are no indications of it detecting the RH wire, you may try charging it through USB. 

You need to plug the thermostat into a USB-enabled device, such as a desktop or a charging station, and charge it for a few hours. 

If the display remains blank after charging, you must manually charge the battery.

Nest thermostat manual charging
Nest thermostat manual charging

The instructions below will show you how to manually charge the battery in your Nest thermostat using a USB connection. 

  1. Remove the thermostat from its wall mount 
  2. Look for two connectors on the back panel
  3. Find the micro USB port. 
  4. Connect to a device with a USB port
  5. Allow it to rest for a few hours to completely charge the battery

Frequently Asked Questions

What does it mean when Nest says no power to Rh wire?

Nest Thermostat No power to Rh wire means that there is a wiring problem with your Nest. The specific error occurs when the Rh wire is loosely connected and receiving low power. Your device shows this error when no power to Rh wire detected. 

What is the Rh wire on a thermostat?

The Rh wire is a kind of red wire that provides electric power from the transformer 24×7. Unlike your cooling system, the Rh wire connects to your heating system. If you have a dual transformer setup, this wire may be red without an “H” linked to it. The Rh wire connects to the RH terminal on your Nest thermostat.

Are R and RH wires the same?

The power wire for your heating and cooling system is the R wire. If you have a single R wire (no Rh or Rc), it can connect to the Nest thermostat’s Rc or Rh. 

Without an Rc wire, the Rh wire serves as the power wire for your thermostat’s heating and cooling systems. The Rh wire is the power wire for your heating system if you have an Rh and an Rc wire.

How do I check my Nest Thermostat battery level?

The Nest Thermostat will notify you when the battery is low, but you may manually check it as well. 

Open Menu View > Settings > Battery. If the battery level is “very low,” then you must replace the batteries. If your batteries go low frequently, it could be a wiring issue blocking ample charging. 

How do I fix Nest error E74?

Check and repair loose connections on your Nest thermostat to fix an E74 error or no power to the Rh wire issue. If it renders ineffective, clear up the drain pipes and check if the connected condensate pump is clogged. Get it cleaned, and it should fix the issue. If not, then you must seek professional help or contact Nest support. 

What does E74 mean on Nest thermostat?

The Nest E74 error indicates that the Nest thermostat is not detecting the Rh wire electrically. These faults can be caused by various factors, including the weather, loose connections, clogged drains, and more.

What is the difference between RC and RH on a Nest thermostat?

The RH wire stands for “red heating,” which implies the connection required to power your AC unit’s heating system. 

The RC wire, on the contrary, stands for “red cooling.” It’s the same as the RH wire, but it powers the cooling system.


To conclude, we must reiterate that it is common for wiring issues to pop up over time. If that happens, you don’t need to panic. 

If you have no experience connecting wires, you must seek professional help to fix the issue. But if you do have experience handling wires, then our troubleshooting methods should suffice to solve the wiring issue on your Nest thermostat. 

We hope our guide helps fix the “Nest no power to Rh wire” issue on your Nest thermostat. Let us know in the comments!

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About Melissa

Hey! I'm Mel, I write the occasional article for Spacehop, amongst other things. I love meeting new people and chatting, so if you have any questions don't hesitate to ask.

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