How to fix it: Furnace control unit w/o 24V power

[Pavarotti]

Hi all and thanks for your interest! I want to replace my old Honeywell with a Nest Learning thermostat. It's connected to the furnace's motherboard with a three prong cable: R, W, C.

After installing, I got a category E error message (Nest doesn't get power). So I checked with a Voltmeter and there's zero Voltage on the R cable. In a second step, I checked the R output on the furnace's control unit. Again: zero. 

My conclusion is that there's a broken fuse or a broken 120 to 24V transformer. My old analog Honeywell thermostat doesn't need power so it doesn't care:  Honeywell only needs W to switch the furnace on/off. R and C aren't really needed. However, when I connect Nest, it needs the control unit to consistently deliver 24V through R and C and since fuse or transformer's broken, it can't work. 

Am I on the right track or am I missing something?

Pic from the furnace's control unit: Voltmeter shows 0V when attached to R and C (or R and an earthed metal screw. The control unit is plugged in and active with fuse/circuit breaker ON)

[listairee]

Hello @Pavarotti,

Thanks for reaching out. Did you use the compatibility checker to check if your system is compatible with Nest thermostats: Compatibility Checker. Also, as you did not provide an error code, it is difficult to help troubleshoot but here is a list of common E error code that you can refer to help troubleshoot. 

Hope that helps! 🙂

[CoolingWizard]

Your meter is set for 12 Volts DC voltage range and you need to be set to 200 Volts AC voltage to get a proper reading.  And your Honeywell did indeed need power to turn on your Furnace.  The basic thermostat takes in 24Volts of AC power on the R terminal, and then when heat is needed, it sends that power to the Furnace for heating on the W wire.  When the power arrives at the IFC on the W terminal, the processor initiates the heating cycle.  

Ken, The AC Cooling Wizard

[Pavarotti]

Thanks Ken, you were right. After switching the multimeter to AC it detected the current. That helped a lot and opened the door for further testing with the resistance/Ohm-meter where I found out that the blue C cable does not work. While red and white conduct voltage/have resistance, blue does not. It must be broken somewhere in the wall. My only solution for that is to replace the cable with a new one.

[CoolingWizard]

With your meter set for AC voltage, touching the red wire, and the blue wire at your thermostat and the cable does not show you any voltage? This is not about resistance this is about voltage. At the thermostat the R and C should show you 24VOLTS +/- 4 volts.

Ken, The AC Cooling Wizard

[Jake]

Hey there,

Thanks for the help Ken.

I wanted to follow up. Please let me know if you have any questions from here, as I would be happy to take a closer look and assist you further. 

Best regards,
Jake

[Pavarotti]

Hi Jake, thanks for following up! My new learning thermostat is working.

Background: Between furnace and thermostat was a cable that consisted of 3 connectors: blue, white, red. I found out with a multimeter that the blue wire was broken and didn't conduct any current (a rare and unlikely occurence and since it was in the wall, I couldn't identify the reason). Red and white connector were working (my old thermostat only needed red and white which is why I never had a problem with a broken blue one before). 

This was a perfect use case for the Nest Power Connector. After placing the Power Connector between the thermostat and the furnace's control unit everything worked like a charm.

[Jake]

Hey there,

I am glad to hear you were able to resolve the issue, and everything is good to go. With that being the case, I am going to go ahead and lock the thread at this time. If you have any questions from here, please feel free to create a new post.

Best regards,

Jake