Replacing mechanical thermostat with digital thermostat

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Old 01-07-21, 03:04 PM
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Replacing mechanical thermostat with digital thermostat

My garage has a Cadet T521, line voltage, mechanical, single pole, 120v/208v/277v 22 amp thermostat with a low setting of 50deg. I would like to replace it with a digital one that has a low setting of 40deg. Digital thermostats are reputed to be more accurate, which I want when setting temps a few degrees above freezing. Trouble is, I can't find a line voltage digital thermostat, and I'm guessing the low voltage, battery powered ones won't be compatible. Appreciate any help.
 
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Old 01-07-21, 03:40 PM
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What are you controling with it? 240V?
 
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Old 01-07-21, 03:45 PM
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Except for electric baseboard I don't know of any furnaces that have line voltage thermostats. Without a big market I assume there aren't many/any being made. You might need to use a relay circuit to handle the 120V for the thermostat. Before going that route I'd look at commercial products to see if there is anything that will work for you. Maybe there are greenhouse heating systems use line voltage and you could get lucky.
 
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Old 01-07-21, 05:02 PM
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Digital baseboard thermostats work quite well,
Most are two wire do not need an extra wire to power them.
They are also proportional which means they will operate the baseboard at only enough power to satisfy the thermostat and have an indicator to show relatively how much power they are using.

 
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Old 01-07-21, 05:13 PM
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Typically the mechanical thermostats can handle more current/wattage than the digitals.

See if you can confirm how many watts/amps you need to switch.

Broan line voltage mechanical (down to 40f) - 22A/5280w
Honeywell line voltage digital - 15A/3600w
 
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Old 01-11-21, 07:04 AM
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The thermostat controls three 220v Cadet heaters which are rated at 18amps each, for a total of 54amps. I called the previous owner and asked him how a 22amp line voltage thermostat could control 54amps without burning up, and he told me there was a relay. I wanted to do this without an electrician to save some bucks, and most of the on-line videos say it's a simple procedure, but the more I learn the less I know.

Thanks to all of you who responded. When I have some free time I will click on the links you guys have provided. Maybe the answer is in one or more of them.
 
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Old 01-11-21, 07:49 AM
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Do you have a model number for the heaters?
 
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Old 01-11-21, 09:29 AM
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If you have an operating system with a relay..... you'd know it. The relay would have to be fairly well sized and somewhat noisy every time it closed. Check near the panel for a box containing a relay.

Pretty strange having a high current/high voltage thermostat controlling a relay.
 
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Old 01-11-21, 10:05 AM
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Are you sure that the wall thermostat controls all three baseboard heaters and that two of them do not have an internal thermostat?

One thing that might help you sort this out is to know what breaker(s) supply the cct to the thermostat and heaters.
It would be possible to have 3 breakers and three relays controlled by one thermostat.
Or, it's possible someone either did not care or know not to pile all of this one one circuit breaker.
It is common for a lot of higher amperage heaters, especially ones with fans to have 240 volt elements controlled by a relay that has a 240 volt coil in the relay.
This would require either another relay and a 24 volt thermostat or a 240 volt thermostat to directly control the relay.
 
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Old 01-11-21, 04:28 PM
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Heaters are Cadet Model: RCP 5025, Volts: 240/208 60HZ, Amps: 20.8/18.0, Watts: 5,000/3750
 
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Old 01-11-21, 04:38 PM
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You have 240v there which means that each heater is 5000w or 20.8A.
Each heater needs to be on it's own circuit.

You will need to investigate further to determine the wiring.
 
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Old 01-11-21, 04:40 PM
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Old 01-12-21, 08:46 AM
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Yes, they are RCP502S. My mistake.
 
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Old 01-12-21, 08:50 AM
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Those look like they come with a plug. Are yours plugged in or are they hardwired?
 
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Old 01-13-21, 01:20 PM
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I had an electrician look at it. The Cadet thermostat is 110vac and the heaters are 220volt There is a relay for each heater. I printed out the specs of the Honeywell RLV3120 thermostat (GregH included a photo of it in a previous post) and asked the electrician if it would work. It all looked good until he came to a list of systems it CANNOT be used with. One of them was: "A system driven by a contractor or a relay (inductive load)." So I need to find a digital line-voltage thermostat that will work with relays.

Gagecalman: The heaters are plugged


 
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Old 01-13-21, 02:35 PM
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Ok.
The mechanical thermostat does not care what voltage is switched as long as it does not exceed its rating.
What exactly is the coil voltage on the relays?

The reason you can not switch a relay with that thermostat is because it uses a semiconductor to do the switching.
A pain for what you want to do but good because it allows for a variable output and will modulate the output of a baseboard heater

If you research electronic 220 volt thermostats you might find one that uses a mechanical relay and can switch your heater relays.
 
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Old 01-13-21, 02:55 PM
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Make and model of the relays would also help.
 
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Old 01-14-21, 09:38 AM
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I was hoping the Honeywell RLV3120 would work because it sells for $30, but it won't work in systems with relays. But there is a Honeywell TH114 that does not have the same "won't work in systems with relays" warning, but it costs $63. The garage actually has two separate but identical systems, since the workshop is walled off, so I'll need to buy two of these. I'll try one for a week before installing the other one, just to make sure it works.

Thanks to all of you for your help!
 
 

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