Line voltage thermostat to 24v thermostat

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Old 05-05-16, 11:06 PM
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Line voltage thermostat to 24v thermostat

Neee some help. I purchased a venstar 7900 wifi thermostat and went to install it today. When I pulled off the old thermostat, i noticed it wasn't 24v. There's a red and a black wire coming into the thermostat. Coming out is 2 reds and 2 blacks. Next to the thermostat is a wall heater that is electric and it has a fan. I'm presuming the 2 sets of wires are to power the heater and the fan. The 2 reds are connected to one terminal and the 2 blacks are connected to the other.

I read online that I can use a transformer / relay such as the Honeywell RC840T but I've got no idea how to wire it. I also don't know if it's 220 or 120. I would call an electrical contractor but I live in the SF Bay Area and construction is booming. Nobody is interested in a small job like this. I'm hoping you guys can walk me through this.
 
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Old 05-05-16, 11:12 PM
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Here's a picture of the wiring on the existing thermostat
 
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Old 05-05-16, 11:43 PM
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You can check the voltage by looking in the breaker box. Given the wire colors it should be 240 volts.
You can verify that bt checking the breaker is two pole.

If the heater is 240v then simplest is to use a transformer that has a 240v primary and 24 volt secondary. The relay you posted seems to be a 120 volt transformer. This is out of my area. I understand the wiring but am not familiar with available equipment. You will have to wait for the pros on that.

Basic wiring though is the 24 volts is run to the thermostat which then controls the 24 volt coil of the relay. The relay acts as a switch to turn the heater on and off.
 
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Old 05-06-16, 12:20 AM
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A Wifi thermostat for electric baseboard heat.

Yes the 840T will do what you want but it needs to be mounted to an electrical junction box. Where would you put something like that now ? Technically it belongs at the panel which means you'd need to run a thermostat cable. It certainly wouldn't be appealing hanging off the wall where the thermostat was.

What are trying to accomplish with a WiFi stat ?
 
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Old 05-06-16, 12:48 AM
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@Ray, I'll check and see if it's 240 or 120. They sell both versions of the RC840.

@PJmax, I just bought the wifi thermostat because it's color screen and it looks cool to be honest. But I figured it'd come in handy on the seldom occasions when it's too cold to get out of bed and turn the heater up. I thought the RC840 can just be mounted anywhere but I don't mind running wires. I was gonna shove it in the box in the pictures I attached and mount my new thermostat over it. I just don't know how to wire it to my existing system.
 
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Old 05-06-16, 01:02 AM
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You can't just "shove it in the box". There isn't enough room. You could mount an extension box on the existing box but it would be awkward looking.
 

Last edited by PJmax; 05-06-16 at 09:14 AM. Reason: typo
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Old 05-06-16, 06:34 AM
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Wow! That's a lot of control for 1 baseboard unit,be sure to check out page 78 in the install manual, I not sure that you will be able to get a common from that RC840T but there appears to be a module if needed.
 
 

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