Installing thermostat for baseboard heater but there are four wires

[u/PersonalBusiness2023]

I have one room in our new house with a baseboard heater, which requires a high voltage thermostat. I’d like to either install a mysa, or a Nest with one of those step-down transformers they sell on Amazon for $70. The thing is, both the mysa and the adapter have inputs for three wires, and the existing thermostat has four wires. There are instructions on the mysa webpage for how to deal with four wires if two come up from the bottom and two come down from the top, but all four of these come down from the top.

Does anyone know how to do it? I’ve attached photos.

[u/ZanyDroid]

Link to the installation page, so those of us who haven't used the product but know the theory somewhat can help?

Pretty sure the 4 wire setup is due to this being a 240V heater. I see red labeling on the whites there. The reason I'm mentioning this, is b/c you have a lot higher chance of success if you map back to the schematic, rather than just random pattern matching of the numbers of wires.

"with one of those step-down transformers they sell on Amazon for $70" - These words don't make sense to me wrt how the electrical works here.

Also those Tan wirenuts are using a lot of volume, you can see about dropping down to the minimal size needed to hold 2x #12.

[u/PersonalBusiness2023 → u/ZanyDroid]

This is the transformer/relay i was referring to: https://www.amazon.com/Aube-RC840T-240-Switching-Electric-Transformer/dp/B0037MXM1C/

[u/ZanyDroid → u/PersonalBusiness2023]

OK what about the Mysa?

I suggest taking a step back and drawing a schematic of what's going on. If you're not able to do this or come up with a plan for probing the existing wiring with a multimeter... you'll have to find, I dunno, a video call on discord or in-person mentor to handhold you in real time. You may not converge on an actionable solution just doing a discussion thread.

[u/ZanyDroid → u/PersonalBusiness2023]

This isn't a step down transformer (well, it might have one in there, but that's only part of the function. I THINK it might combine a step down to power the nest, with a relay). Not sure I can blame you for misidentifying when they kind of F'ed up the schematic on the device.

This is a RIB-relay wannabe with a 24V control coil flipping a 240V load.

It is indeed kind of weird that it has 3 wires, and the schematic sucks for figuring this out. I would have to dig through more manufacturer docs or tutorials using this one.

(EDIT: just realized why it's not weird. You are allowed by code IIRC to switch a single leg of a 240V heater load)

Honestly I'd look for a different RIB-style device that is documented better (EDIT: the manual is fine on this thing, it's the on-device schematic that is rife for problems). For $75 there's no need for this level of mental difficulty.

[u/ZanyDroid → u/ZanyDroid]

https://morelectricheating.com/media/assets/product/documents/RC840T-Install.pdf?srsltid=AfmBOoqYaMbSCx6SXszSgu6I0aH49sDsyj26uXYBnUOGhAsNb7IY5hS0

Page 1 gives enough information to show how it works & to modify your existing wiring. An electrician would be able to do it. I'm also confident, from scrubbing the amazon reviews, that at least one of those user photos is correct and applicable to your situation.

If you draw a schematic of your existing wiring I will annotate it with how to use this. But I don't know how I can explain from what you have posted, how to do it (nor is it responsible for me to do so, there's a high chance to mess up).

[u/PersonalBusiness2023 → u/ZanyDroid]

I think I have it. Right now one black wire is connected to the “load” port of the existing thermometer. I think that’s the load. That makes the other black wire the line. And the remaining two white wires that are wrapped in red electrical tape and connected to red wires on the current thermostat, are the connection back and they go together with the red on the back of the mysa. Is that right?

[u/ZanyDroid → u/PersonalBusiness2023]

I had a hard time parsing that, also it seems wrong. One Romex is going to be the input and the other is going to be the output

Diagram and manual screenshots please

You can post them on imgur

[u/ZanyDroid → u/PersonalBusiness2023]

Would be helpful to reference this too https://pdf.lowes.com/productdocuments/b9f1c143-373d-4891-a9c7-9848d09c5117/43649512.pdf

FWIW your double 240v pole setup looks similar to the manual except which holes in the plastic box the romexes come out of

[u/Rivered_The_Nuts]

Honestly, based on your comments you shouldn’t be messing with electrical.

[u/ZanyDroid → u/Rivered_The_Nuts]

Yeah, I’m bailing from this thread since it seems like OP is only glacially moving ahead

[u/ithinarine]

Fucking hell dude, if you can't grasp the fact that all of the cables coming from the top doesn't mean anything, and that you could have 2 from the top and 2 from the bottom that are backwards from the installation picture, you need to step away.

2 wires come from the panel, 2 wires go to the heater.

It is not rocket science to figure out which are which.

[u/ZanyDroid → u/ithinarine]

I think you need to chill out, man. Yes, OP is a bit clueless and IMO has 80% chance of not finishing the project, but this was unnecessarily hostile.

[u/nhorvath → u/ZanyDroid]

was it unnecessary? it seems like op shouldn't be messing with deadly voltage if they can't figure out that it's switching both hot legs.

[u/PersonalBusiness2023 → u/ithinarine]

So which is which? They look the same to me.

[u/ithinarine → u/PersonalBusiness2023]

How the fuck am I supposed to know? You're the one there with a multi-meter who can test the voltage, not me.

[u/F_ur_feelingss → u/PersonalBusiness2023]

The back of existing thermostat says the top is load. I dont think you can trust that 100% though. You just need a voltage tester. With heater off the wire with power is the line.

[u/wyliec22]

I like the Mysa units - I have two installed. Simple and straightforward setup for 4 wires - two hots from panel (typically red and black) and two to the baseboard (load - also red and black).

One is controlling three baseboards but total load is within the Mysa specs.

When using the wire nuts (or however you make the wire-to-wire connections) make sure it is tight and secure as there will be significant amperage - loose connections are a fire hazard.

[u/xNOOPSx]

Use Mysa. You need to identify the line and load - in theory that's done on the stat, but I wouldn't necessarily trust it.

Black's go together with the black lead. Red from the panel is the hot. Red to the baseboard is the load.

Done.

No low voltage stuff. It just works.

[u/PersonalBusiness2023 → u/xNOOPSx]

Right now one of the blacks is connected to the port on the thermostat marked “load.”

[u/PersonalBusiness2023 → u/xNOOPSx]

I think I have it. Right now one black wire is connected to the “load” port of the existing thermometer. I think that’s the load. That makes the other black wire the line. And the remaining two white/red wires are the connection back and they go together with the red on the back of the mysa.

[u/xNOOPSx → u/PersonalBusiness2023]

That should be right, but you're depending on the installer to have cared which was line and load. The stat doesn't actually care and will work properly if backwards. Connect the Black's together and then connect what you believe to be the line. If the stat powers on, it's right. If it doesn't power on, that's the load. Swap and try again. Once you're sure it's good, then connect the load. Hooking up the line to the load shouldn't cause a problem, but it can.

[u/talljerseyguy]

It’s double pole 208-240 you will have one leg constantly hot to make this set up work and you have to run your own low voltage wires. My house is set up like this I went with this