Help on wiring Electric Baseboard Thermostat

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Old 10-02-18, 11:40 AM
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Help on wiring Electric Baseboard Thermostat

Hi there,

I am trying to retrofit my existing electric baseboard heating in my small condo with a new "smart" electric radiator thermostat made by the company "Mysa." ( https://www.getmysa.com ) I am looking forward to more efficiently manage the temperature in the space. It's a midcentury modern condo building made in the late 60s.

I have been in touch with Mysa tech support and they have been helpful but have not been able to 100% guarantee how I should wire the thermostat because the following:

The previous singular thermostat actually controlled two electric baseboard heaters with two separate 240 circuits. There are four wires coming in from the panel and four wires going to the heaters in the same jbox. Mysa supplied me with the image here on how to wire the thermostat in a scenario when there are two heaters, but this presupposes that there is only one 240 circuit supplying the power.

The question I have then is how to wire it with the existing 2 x 240 circuits. Can I wire the two black hots from the panel together with the L1 from the thermostat and the two neutrals from the panel L2 connections.

I'm new here but I would appreciate any help!

Thank you so much,




 
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Old 10-02-18, 11:46 AM
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No, you may not put the other black incoming wire (from the panel) into the hole occupied by the red wire and the other black outgoing wire (to a heater) into the hole occupied by the green wire.

And no, the two incoming black wires (and also the two incoming white wires) in your setup may not be combined (paralleled).

The new thermostat can switch both heaters only if both heaters can coexist on the same heater branch circuit (one incoming white and black pair) in terms of watts or amperes draw and also to the limit the thermostat can handle iin terms of watts or amperes draw.

The old thermostat could have been wired to handle either one circuit and heater or two circuits with two respective heaters. Even for the old thermostat the preferred wiring is to handle just one circuit, whites and blacks connected to the thermostat, and however many heaters that one circuit will support to kick on and off together..
 

Last edited by AllanJ; 10-02-18 at 12:02 PM.
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Old 10-02-18, 05:34 PM
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What was the model number of the old thermostat ?
 
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Old 10-02-18, 08:19 PM
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Thank you so much, Pete and Allan! I am glad that I asked before just trying it.
Here are some pictures of my old thermostat.
It is a "549G Listed Room Thermostat" with four terminal connections.
Any ideas on a way to make the new one work or am I completely out of luck?
Thanks in advance for your time,
Flint



 
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Old 10-02-18, 09:49 PM
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549G is a UL listing or certification.

Here's the problem. With most if not all the four wire thermostats I know of........... one pair of switched lines is controlled thermostatically. That means on a call for heat...... only one pair closes. The other pair closes when the thermostat is turned on and opens when turned off. That means it can't control two circuits.

That's why I was trying to figure out what you had there.
 
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Old 10-03-18, 06:12 AM
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I have worked with 4 wire line voltage thermostats where both pairs of switched lines were controlled thermostatically. There was one possible problem; the two pairs did not switch on and off at exactly the same temperatures. Performance with two heater circuits depended on which heater was closer to the thermostat.

If your old thermostat had one pair of contacts that was on all the time unless the thermostat was turned all the way down then one of your heaters would have been on all of the time and you would have noticed almost immediately that something was way amiss.
 
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Old 10-10-18, 11:40 AM
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Hi there,
I finally have a day off work so I can focus on this again. Thank you so much for your help thus far.
The two heaters are equidistant from the thermostat.
I have drawn a small diagram of the way that the old thermostat was wired. You can disregard the letters near the lines, that is just how I have them labeled the wires on site.
Does this help at all with wiring my new thermostat?
Thank you, again, for your help. I am very appreciative.
Flint

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Last edited by PJmax; 10-10-18 at 10:22 PM. Reason: cropped/reoriented picture
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Old 10-10-18, 10:27 PM
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I thought we had covered this.

As far as I know..... there is no smart thermostat that will control two separate 240v heater circuits.
The only way you can use that thermostat is if you can run both heaters off of one circuit.

What size is breaker 5/6 ? and 7/8 ?
What is the wattage of the heater on the 5/6 breaker ?
What is the wattage of the heater on the 7/8 breaker ?

You can run a maximum of 3840 watts on one 240v 20A circuit.
 
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