Needs an electrician advise

I have a Sensi thermostat that supplies heat to my 2nd and 3rd floor. This is connected to the 24v from the old thermostat that supplies steam to radiators. Works great with ST.
I just bought a Nest that was open box at an great price.
My 1st floor and basement gets heat via Hot water baseboards supplied by a circulator pump. Hooked up to it is an old 120v thermostat that I would like to replace/convert to use the Nest.
I found this relay after searching but I would like some input before I purchase it.
RC840T-120 Electromechanical Relay on Amazon.
Anyone has any experience with this? Thanks

I don’t know exactly what your requirements are. I’ve never used that relay, but I have used this one that I got a year ago to connect my humidifier to my Nest. I tested it out and it works, however I never got around to hooking i up.

I have the 240V version Aube relay for my nest which I use with electric baseboards. Works fine, but it is a large relay. Had to install a dedicated box for it…

it will be installed in the basement in the boiler room out of sight so size don’t matter. My only concern is that it will work with the pump and nest.

Also was thinking if a 120v/24v transformer would work. Do I really need a relay? That current thermostat is like an On/off switch

I’d contact Nest & see what they have to say about a compatible relay.

They got no clue…Dude keep saying it will only work with low voltage system. Doesnt even know what a transformer does

Yes, you really do need a relay. The relay is for making the circuit complete for your heater. I highly recommend the Aube if you are going this transformer/relay route. It’s designed especially for this purpose.

The relay RC840T has bult-in 120V/24V AC to power the thermostat. And this is low voltage

Is this what you’re looking for?

Granted, it shows electric heater. But it’s just a relay and it will work with any load type, including a circulator pump.

You will need a relay for two reasons. The Nest cannot handle line voltage, and needs it to operate the internal electronics, I would also assume that the electronics can handle less than 1A switching current when switching a load, whereas it would need to be rated for 10-12x full load. Also, technically, in Canada (and presumably the US), the thermostat doesn’t fall under Electrical Code regulations because it is under 35 V.

They got no clue…Dude keep saying it will only work with low voltage system. Doesnt even know what a transformer does

Went through that myself when I had to call support. Informed them that it was connected in away that was reasonable and safe, and that it didn’t affect the issue I was calling support about (The measured temperature was not dropping below 20 degrees Celsius)…

There is no “official” support for a nest operating at line voltage, and no official relay that can be used to operate line voltage systems. The annoying part is that if the relay had been installed in a “central” location and 24V supplied to the thermostat location, they wouldn’t bat an eye at supporting it, but because I had to add the relay myself, they consider it unsupported. <\rant>

Thank you guys so much. Am a computer tech who doesnt know bull about Electrical stuff.
And thanks NomadTech, that diagram is exactly what I needed. I will go ahead and order the Aube.

It is cool that the advices from this thread helped you! In you place I would call the professional electrician.