What Does It Mean If the Thermostat Nest Has No Power to RH Wire?

Updated: September 19, 2022
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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? 

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 this issue. 

Reasons for error 74 on the Nest Thermostat

So, your Nest alerts “No power to RH wire” with error E74.

Let’s take a look at possible reasons for error E74:

  • 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  • Outdated thermostat software.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?

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. 

The RH wire

Step-by-step guide on fixing this error

Attention! These instructions assume that you have minimal skills in working with electrical systems. If you read the instructions and you did not understand much, it is better to contact a specialist.

Before starting to execute the instructions, read them and prepare some tools like a multimeter, screwdriver, and side cutters. 

Step 1. Inspect the RH wire connection

To check the RH wire connection, 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.
Checking the connection of the right wire
  • 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 E74 error is gone.

If the E74 error persists, go to the next step. 

Step 2. Check and change the fuse in the HVAC system

HVAC systems have 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 an  E74 error.

Follow the steps below to inspect or replace a fuse:

  • Please 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.
Checking or replacing a fuse
  • 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 3. Change the HVAC contactor relay

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 power to your HVAC system. 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.
Disconnect wire from relay contactor
  • 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.
HVAC Contactor Relay Replacement
  • 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 4. Check the float switchHVAC 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.

Checking the HVAC float switch

This is needed to protect your house from possible water overflow from the HVAC unit. 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 error 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 line 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.
Cleaning the switch from dirt
  • 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. Unclog the drain line

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. Approach the external valve of the HVAC system. Usually, it is outside.
  2. Connect the pipe of the vacuum cleaner to the external drain pipe.
  3. 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.
  4. Keep the pipe clean regularly to avoid the E74 error and more costly breakdowns of your HVAC system.

If in the process of performing one of the guides you need help, please call a professional.

How to сharge 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. ring, gently turn it towards you away from the wall.
  2. From behind the thermostat, you will see a USB connector. Charge it using your smartphone charger by plugging it into the connector. After a few hours, the batteries will be fully charged.
  3. Slide the Nest thermostat panel back onto the wall mount.

 

Conclusion

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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