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I'm using a Nest Thermostat to power baseboard heaters (240VAC).

I have 4 wires that I have access to:

  • 2 wires: Red/Black from a transformer which outputs 24VAC
  • 2 wires: 24VAC from a thermostat RELAY (Honeywell R841C1169)

I thought initially that the two wires from the thermostat relay connect to W1 and RH.

But then I wasn't sure how to do the common wire.

schematic

Then I thought about the C wire, and thought I could use a 24VAC transformer, but in my diagram, 2 hot wire were touching, and I wasn't sure if that was bad. Is it?

schematic

Somehow this doesn't seem good.

How can I get the common wire involved? I'm pretty sure the wires from the relay are OK, they just hookup to W1 and RH. But where do the wires from the 24VAC transformer go? One of them I'm pretty sure needs to go to the C wire, but where does that last wire go??

Thanks!

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2 Answers 2

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Since you have a R841C (with an integral transformer)

You have your terminals on the relay confused. Connect RH to the HOT terminal from the relay and W1 to to the NEUTRAL terminal from the relay. C on the Nest is left unconnected, and you do not need a separate transformer here.

If this was an R841D (standalone relay)

You have your terminals on the Nest confused here. Connect the hot wire from the transformer to the RH terminal on the Nest, the neutral wire on the transformer to one end of the relay coil (it doesn't care which way it's wired) AND to the C terminal on the Nest, and the other end of the relay coil to the W1 terminal on the Nest.

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  • Whoa, this solution is so smart. I think my mistake was that I thought that one of the lines of the relay was HOT - but I don't think this is true? If not, why does the relay need it's own separate transformer then?
    – Nick Lang
    Oct 3, 2016 at 2:50
  • If it's a standalone relay, neither line is HOT and it needs you to provide the transformer.
    – ThreePhaseEel
    Oct 3, 2016 at 2:56
  • Thanks so much for this detailed answer. Just for my own understanding, if have an integral transformer, and I only connect the W1 and RH to the relay, then how does the Nest get power? I leave my system OFF for long periods of time (it's a place in the country so I only go sometimes).
    – Nick Lang
    Oct 4, 2016 at 19:16
  • @NickLang -- my understanding is the Nest can trickle its own operating power through the relay without providing enough current through it to pull it in, basically. (I know some thermostats are capable of it.)
    – ThreePhaseEel
    Oct 4, 2016 at 22:17
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While it is technically possible to get Nest (24 V) to work with your baseboard heaters (120/240 V), I have concerns over the long-term reliability of the relay/transformer set up.

Cadet, a baseboard heater manufacturer, had this to say about using relays to control baseboard heaters with Nest:

"We suspect running a smart thermostat with one of these relays as the power source may lead to the draining of the thermostat’s internal batteries."
source

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  • I removed the part about your product since it doesn't answer the question. Please see help center and the pages it links to for more information. You're welcome to put that promotional information in the "about me" section of your profile.
    – Niall C. ♦
    Feb 21, 2017 at 2:33

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