No power to RH wire detected? If you are a Nest Thermostat owner you may see this error on your display. Even the traditional HVAC controllers will stop working at this point. Thanks to the smart thermostat controllers, they can pinpoint the error.
How do you fix this e74 error on your Nest thermostat? What’s the reason for this Rh Wire power loss error? We will see in detail.
When we go this error for the first time, we got panicked, we thought we are going to lose another couple of hundred dollars to the Ac Service guy. Anyway, that’s not the case always, Most of the time this is going to be a simple fix.
Check Your RH Wire Connection
When you get this error, you can check the connection diagram and see the RH wire missing display, Since no power to the RH wire, it’s marked as Red.
You can take off the display part and check the RH wire. Make sure your wire is tight enough and properly inserted there if you are installing the thermostat the first time. Follow the steps below if your thermostat was working fine and the issue started now.
Check HVAC Float Switch
The condensate overflow switch located near the Air Handler unit, inside your home. This is to protect your home from possible water overflow from the HVAC unit. If your drainage pipe is not clean, or if there has any clog, the overflow switch will disconnect the power and turn off the AC.
As you see in the image, there is a floating mechanism that will disconnect the internal switch when it floats in the water and move up. This works as an AC switch.
To make sure that the reed switch still working, you can hear a tiny clicking sound when you move the float back and forth.
You can confirm the issue by following the steps;
- Make sure the AC drainage pipe is clean.
- Take out the float switch and move the float to the bottom.
- Wait for few seconds and check the RH wire status on the Nest Thermostat display.
If this fixed the issue, you have to drain out all water from the AC unite water collecting pan and make sure no block in the drainage pipe. Once you clean the system, put back the float switch and your AC starts to work as usual.
Check Nest Thermostat Features
Check HVAC Control Unit Fuse
We found on our system, the control unit fuse was blown. This may occur very rarely and we experience it. Most of the time if you see a fuse failure, they may have some other issues with your HVAC unit.
The color code for the fuse meant for the current rating. When you replaced the fuse, make sure to turn off the AC power. And get the same rated fuse for the unit.
- Size: 0.75*0.74*0.2 in, Material:Zn alloy/Plastic.
We found some products from Amazon, that comes with a range of fuses that you can get. Please check if there has an underlying issue, with the help of a licensed AC technician before you replace the fuse. If you are not licensed to do perform this task, get help from your AC technician.
Replace the HVAC Contactor Relay.
This is with your AC main unit outside. This may cause by aging and the relay contacts may cause to generate some sparks,. Also, a faulty relay coil may cause the to blow the fuse out from the AC control unit.
- Single , (1) Pole 40A 24V C140A Contactor
We recommend getting professional help from a licensed technician to replace the relay. Most of the time, when you replace the relay you can replace the control unit fuse too. The contactor relay may vary depends on your model and the capacity of the unit. Just to get some idea, we are listing one similar product here.
Charge Nest Thermostat with USB
Well during this time, the internal battery will run to, if you take more than one day to fix this. Once the RH power back, the thermostat unit will charge the battery and turn on the display. But if you see the display is still out, you can manually connect t the display unit to a micro USB port as shown in the picture.
The display will come up in a couple of minutes and boot the system. During this time, if you want to cross-check that the RH still has the power;
- Get a multimeter.
- Set unit to AC ~
- Connect the probs to RH and C wires.
- The meter will display the AC voltage between the RH and Common wire.
The voltage will be more than 15V AC, and it slightly varies depends on the AC unit. Once you make sure all goo, you can insert back the Nest Display unit and get back to work!.
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Most of the time, the RH Power Loss issue will get resolved in the first part. Ned to clean the drain pipe and empty the water out. That would be enough. But if you want to replace the fuse or relay, please get help from a licensed AC technician.
Hope this saves you some time and at least a couple of hundred bucks to fix the Nest Thermostat (E74) No Power to RH error.