What does it mean if the thermostat Nest has no power to RH wire?

Updated: May 20, 2023
6 min read
What does it mean if the thermostat Nest has no power to RH wire?
by Hanna Finley
Tech Expert & Writer
Checked by
Quality Control team

One day you have a message on your thermostat’s display, “Nest no power to RH wire detected”. It’s error E74. What does it mean?

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The Nest branded thermostat is an electronic device that is connected to a wireless home network. It’s a smart gadget for optimizing the operation of domestic heating systems. Also, this thermostat has a wired connection to your home's air conditioning or HVAC (heating, ventilation, air conditioning) system.

So, what to do if you see on the display of your Nest the “No power to RH wire” message or E74 error? This signals the breakdown or malfunction of your HVAC system. But maybe not everything is so bad. Let’s look at possible solutions to fix the E74 error on Nest.

Nest no power to RH wire 3880

Reasons for error 74 on the Nest Thermostat

So, your Nest alerts “No power to RH wire” with error E74. This situation’s pretty common, so you may see dozens of Nest E74 wiring reports on forums. Let's take a look at possible reasons for this issue:

  • Lack of electricity in the house.
  • Clogged or frozen drainage system.
  • No power from the HVAC system.
  • Broken wire contacts on the control unit.
  • Incorrect thermostat model.
  • Wrong wire connection.
  • Outdated thermostat software.
  • Voltage drop in the HVAC system.
  • Short circuit or breakdown of electrical elements in the HVAC system.

What is RH wire and why it facing a problem?

Nest no power to RH wire 3880

The RH wire is the physical wire that is connected to the back of the Nest thermostat and is usually red in color. This wire supplies voltage from the HVAC system to the Nest panel. The control panel has accumulators, so when there is no power to the RH wire, this way the control panel detects the breaking of the HVAC system.

So, answering the most common forum question “What does no power to RH wire mean?”, we can clearly say that it shows that your thermostat is not receiving power from the air conditioning system through the Rh wire. So, the source of the problem may hide in the thermostat’s software and hardware, a broken wired connection, or HVAC issues.

Step-by-step guide on fixing this error

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Prepare some tools like a multimeter, screwdriver, and side cutters. Examine your wires and thermostat. Charge the thermostat. Run a self-check test to factory reset the device and remove any software glitches.

Check for physical damage. Examine HVAC and clean drain pipes. When they are clogged, the system can mark them as insufficient. And it leads to various glitches, including the Nest E74 error.

Step 1. Examine the RH wire cords

To review the RH wire linkage, follow these guidelines:

  • Shut down your HVAC system by the switch. Your system may have multiple switches, or you may turn off the switch in the electrical panel.
  • Detach the thermostat display from the wall mount. While holding the thermostat ring, gently turn it towards you opposite the wall.
  • Press the latch on the RH wire to disconnect it.
  • Press the latch with the C symbol and disconnect the usually blue wire.
  • Examine the exposed RH wire and RC wire. They must not be corroded or polluted, broken or bent. Strip the RH wire and other wires if necessary.
  • Connect the multimeter to the RH wire as plus and minus to the other wire. Check for short circuits in the wires with a multimeter. Double check none of the wires are touching each other.
  • Turn on the HVAC system.
  • Check the voltage on the voltmeter display. The normal voltage should be 24V AC. If there is no voltage, it may be that the RH wire is broken somewhere on its length or in the air conditioning unit. If the voltage is much lower, this may indicate that the air conditioning unit is faulty.
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  • Turn off the power.
  • Return the RH wire and the other wire to your contact points. Check that the wires are held tight.
  • Return the thermostat panel back onto the wall mount.
  • Turn on the electricity to the air conditioning system, and wait until the thermostat is turned on. Check if the is gone.

If the Nest E74 note persists, go to the next step.

Step 2. Locate the thermostat

You have multiple ways to check the Nest thermostat. But in all cases, you should find where is located. To do this, use our instructions:

  1. Check the most common locations. It is usually installed in the hallway, living room, or in bedroom near the light switch on the wall.
  2. Look for the Nest logo: it’s a small round icon in black or white color.
  3. Look for wires. Older thermostats have wires connected to them. Look for a wire bundle and trace them from the HVAC system to the thermostat. Modern advanced thermostats have a wireless connection.
  4. Check the Nest app on your phone. If your thermostat is connected to your account, check its status on the official app. It should display the temperature and status of the thermostat.

Once you found out the thermostat, you can continue to fix the Nest E74 error.

Step 3. Charge the thermostat using USB

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You may face a situation when your thermostat just doesn’t have enough battery to turn on. In this case, you just need to charge the device using a USB cable. Stick to this instruction to get rid of the E74 nest error:

  1. Get a USB cable that is compatible with your thermostat. Look for a cord that has USB-A on one side and micro-USB on another.
  2. Shut down the HVAC system. You can do it by turning off the breaker switch for the air conditioner or furnace.
  3. Connect the USB-A side to the power adapter and micro-USB to the thermostat.
  4. Charge the device. You should see a battery icon that shows you this process.
  5. Disconnect the USB once you fully charge the device. Usually, it takes from 35 minutes up to 4 hours to charge the thermostat. You can leave it overnight to ensure that is fully charged.
  6. Reattach the Nest thermostat to its base and turn the power back on.

Use only compatible chargers to ensure that you don’t damage the thermostat.

Step 4. Reset Nest And run self-tests

Try to run a series of easy tests to find out what was cause why your Nest thermostat has no power. the problem. In this case, your thermostat runs mini-tests to eliminate software glitches:

  1. Turn down the HVAC. Use the breaker switch for the air conditioner or furnace.
  2. Press and hold the Nest thermostat’s screen for 10-20 seconds until it goes black.
  3. Release the display and wait for 1-2 minutes until your device will reboot.
  4. Choose the gear icon to open Settings.
  5. Scroll down the menu to find Reset.
  6. Run a series of self-tests by following the instructions on the screen. Your Nest thermostat should start running a self-test automatically. If it’s not, your device faces serious glitches. Contact Nest’s customer support.
  7. Reconfigure the Nest thermostat to your needs by following the on-screen instructions.
  8. Turn the power back on. Turn on the HVAC system and test your Nest thermostat.

Beware that your thermostat will erase all its settings and configurations. So make sure that you write them down before restarting the system. If you face any glitches during the self-tests, contact Nest support as soon as possible.

Step 5. Check and change the fuse in the HVAC system

HVAC has a fuse that protects the system from various failures. If it is broken, then the HVAC system has failed. These can be voltage drops, short circuits of wires, malfunctions, or overheating of various elements of the HVAC system. Especially in very hot weather, this fuse works most of the time, saving your system from serious damage. A broken fuse also leads to the Next thermostat E74 error.

Follow the steps below to inspect or substitute a fuse:

  • Shut down the electricity of the HVAC system. Double check the power is off and the fans are off.
  • Locate the control unit of your cooling system and remove the protective cover.
  • Find the fuse on the board. It will be similar to the fuse used in cars. Usually, they are rated at 3A.
Nest no power to RH wire 3880
  • Inspect this fuse for signs of blackening or burning. Just pull it out of the board and check if it is burned out. Check if the fuse is good using a multimeter.
  • If the fuse is broken, take it with you to the store and buy another one.
  • Replace a good fuse back into its position.
  • Close the protective cover of the HVAC unit.
  • Turn on the HVAC system.
  • Check system performance. Check for error E74 on the control panel.

Still see the error message? Go to the next step.

Step 6. Change the HVAC contactor relay

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Also, error E74 may appear due to the failure of the relay contactor. This can happen due to the deterioration of the relay itself or other electrical elements, power surges, and the influence of an electromagnetic field. This relay is controlled by a thermostat and turns on or off the power of the HVAC. The failure of the relay also leads to a malfunction of the HVAC fuse. We recommend contacting a professional to replace the relay.

But if you have an electrical engineering education, you can replace the relay yourself. For this, do the following:

  • Please, shut down the electricity in your HVAC system by the switch. Double check the power is off and the fans are off.
  • Remove the protective panel on the main unit of your HVAC system.
  • Find a relay contactor.
  • Write down or take a picture of the wires the relay contacts.
  • Disengage wires from the relay contactor.
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  • Remove relay. If you do not have a new replacement relay, you can write down the required specifications or take the relay as a sample and purchase the same good relay.
  • After that, replace a relay.
  • Connect the wires to the same contact points of the relay where they were.
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  • Also, check the health of the HVAC system fuse. How to do this is described in the previous instructions.
  • Close the protective cover of the HVAC unit.
  • Turn on the electricity.
  • Check the operation of the HVAC system and see that the E74 error is gone.

If you need help while following these guidelines, please contact a professional. If the E74 error persists, go to the next step of our guide.

Step 7. Check the float switch HVAC system

To do it, you need to find the overflow switch. It is usually located next to the air handling unit somewhere in your house.

Nest no power to RH wire 3880

This is needed to protect your house from potential water overflow from the HVAC. Especially if your drainage pipe is dirty or if there is any clog, the overflow switch will shut down the electricity to prevent damage to the entire system. This will also lead to error E74 – “No power on the RH wire” on the Nest display. Inside the float switch, there is a floating mechanism that floats in the water and, when lifted up, turns off the internal switch. It has a reed switch. To check its performance, move the float back and forth. If it is working, you will not hear loud clicks.

In order to double-check that the system is working properly and does not lead to the Nest wiring report E74, follow these instructions:

  • Please shut down the electricity HVAC system by the switch.
  • You need to find the condensate drain line HVAC system. The drain can be located in different places, depending on where your air conditioner is located, for example, in the attic or the back room.
  • After that, you need to see the condensate overflow switch. You can see the condensate overflow switch around the air handling unit of the HVAC system or the outlet of the condensate drain. There is a little low-voltage cable leading to the switch, by which you can identify it.
  • Check the drain pipe for clogging and that it is clean.
  • Remove the float switch if necessary, clean it of dirt and place it on the bottom.
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  • Check if error E74 has disappeared on the Nest control panel.
  • If this has fixed the error E74, empty all water from the A/C unit water collecting pan and inspect if the drainage pipe is clean.
  • If necessary, clean the drain system and then return back the float switch.
  • Check if your system is working duly.

If you still see the E74 error message, go to the next guide.

Step 5. Clean out drain pipes

In case of clogging your drain pipe, you need to clean it. Otherwise, clogging can lead not only to error E74 but also to more serious damage. To do this, you can contact a service organization or specialists.

Also, if you have a powerful vacuum cleaner, you can do it yourself. For this:

  1. Turn off the HVAC.
  2. Approach the external valve of the HVAC and locate the pipes. Usually, it is outside or in the attic.
  3. Remove the drain cap from the pipe. It’s usually located at the end of the pipe.
  4. Clean the blockage. You can use a wire brush, plumbing snake, or vacuum to the external drain pipe.
  5. Turn on the vacuum cleaner for a few minutes. Depending on the clogging of the pipe, a certain amount of dirt and water may accumulate in the vacuum cleaner.
  6. After clearing the blockage, wash the drain pipe with water to remove the remaining debris. You can use a bucket with tap water or connect a hose to the faucet.
  7. Place back the drain cap and wait for 30 minutes until the pipes will dry.
  8. Turn the power back on and test the HVAC system.

Keep the pipe clean regularly to avoid the E74 error and more costly breakdowns of your HVAC system. If you see mold or some water damage on the HVAC unit, we recommend you call your local technician to fix this issue.

How to charge the Nest thermostat accumulators

Suppose you come home and find that your Nest thermostat is not powered. There are no indicator lights on it, and when touched, it does not react. Probably the batteries in the thermostat are dead. Also, if there was no power for a long time through the RH wire, then with time, the thermostat would turn off due to a dead battery. After power is restored on the RH wire, the thermostat battery will charge itself. But you can also charge it via USB charging.

Follow these steps to charge your thermostat:

  1. Take off the thermostat from the wall mount. While holding the thermostat ring, gently turn it towards you away from the wall.
  2. From behind the thermostat, you will see a USB connector. Charge it using by plugging it into the connector. Wait for 2-3 hours until the battery is fully charged.
  3. Slide the Nest thermostat panel back onto the wall mount.


To summarize, we have considered the possible causes and solutions to the Nest thermostat “No power on the RH wire” issue. To perform troubleshooting effectively, you need to have minimal electrical skills and knowledge of your HVAC system. Otherwise, you may not fix it, and even more, break your HVAC system.

Ideally, it is better to carry out regular maintenance with the help of professionals. And in the future, you will not worry about problems like the Nest having no power on the RH wire or other breakdowns with your HVAC system.

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