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Only C and W wires, will it work?

Math_Dube
Community Member

Hello, I have a baseboard with a relay Rc840T-240. I have an old thermostat 24v connected to the relay. I only have two wires at the thermostat and according to the relay, I have C and W. I understand that W is for heat and C is for the power, so I guess it should work, but I just want to be sure before buying a new thermostat.

Will it work with the nest thermostat?

Screenshot_20221128-160228_Gallery.jpg

Screenshot_20221128-155933_Gallery.jpg

9 REPLIES 9

CoolingWizard
Silver Product Expert
Silver Product Expert

Math_Dube,

Normally the low voltage thermostat is connected to the R and W. Your picture appears that the two wire are on W and C. Is that how it is wired?
Ken, The AC Cooling Wizard

NestPro and HVAC company owner

Hello Ken, it seems that it's wired like this (W and C), the only labels available are on the relay, there is nothing written on the thermostat. The relay was changed recently and that's how the electrician plugged the wires in it. He didn't touch anything else.

CoolingWizard
Silver Product Expert
Silver Product Expert

Here is the wiring diagram for the installation and service manual for the RC840T-240.

FED32419-15CC-451F-9F81-0BFAC145E195.jpeg

The connection of W to C is essentially connecting common to common. And, this would not close the relay and thus energizing the heating element. it needs to be changed to R and W.

Ken, The AC Cooling Wizard

NestPro and HVAC company owner

Thanks for the diagram Ken, I understand what you are saying. With the actual setup, the baseboard is heating well, so I'm not sure I want to play with the wires, but I'll test it to see what happens. And what do you think about using a nest in my setup? Would it be possible?

CoolingWizard
Silver Product Expert
Silver Product Expert

The basic Nest Thermostat will work fine. It uses a couple AA batteries so you do not need a Power Common wire. It will replace that Honeywell T87 thermostat.

Ken, The AC Cooling Wizard

NestPro and HVAC company owner

Hello folks,

I wanted to ensure that everything was covered here.

Thanks for helping here, Ken.

@Math_Dube, I hope you've got the answer you're looking for. If you're still in need of assistance, feel free to let us know.

Best regards,

Mark

Hi Math_Dube,

I'm checking in to make sure that you've seen our response. Please let us know if you have any additional questions or concerns.

Thanks,

Mark

I'm good thanks!

Markjosephp
Community Specialist
Community Specialist

Hello Math_Dube,

Sounds great! I'm glad that you've got the answer you're looking for. So I'll lock this thread in 24 hours, and if you have any further questions or concerns, please make a new post.

Thanks,

Mark