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Thread: Hooking up 240 VAC relay transformer to 24v thermostat

  1. #1
    DIY Junior Member wombat's Avatar
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    Default Hooking up 240 VAC relay transformer to 24v thermostat

    I'm doing this so that I can use a modern thermostat and not have to rely on the very limited selection of line-voltage progammable thermostats out there. According to the instructions for the RC840T, it cautions about adding an overload protection and cut off. As this will be in my house, it will already be on the home circuit breaker ( Of course this is very old and I don't trust it that much). Am I incurring any more risk than was there already for the old manual Cadet thermostat?

    Just as an example, I am looking at the AUBE/Honeywell RC840T, On/Off Switching Electric Heating Relay with Built-in 24 V Transformer.

    Besides the standard precautions of putting everything in an electrical box, please let me know if you have any suggestions.
    1. If I can fuse this what kind of fuse can I use? (Most 20AMP fuses look pretty large)
    2. Are there any recommendations for other relay transformers?
    3. Will the relay transformer put off a lot of heat to distort the thermostat?

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    Throughout life we come up with ideas concerning electrical installation that seem like a good idea and pursue making installations that end up being very dangerous.

    No matter which thermostat you choose to use the bottom line is the installation must conform to the minimum safety standards set forth by the electrical codes in order to have a safe installation.

    Should you choose to pursue this installation please understand that some form of disconnect must be installed within sight of the heating unit that disconnects all incoming lines that supply the heating unit or a lock out device must be installed at the breaker that supplies the circuit. This disconnect must have an absolute off position that cannot come on automatically.

    1. If I can fuse this what kind of fuse can I use? (Most 20AMP fuses look pretty large) (Answer) The overcurrent device for the circuit will be all that you will need as long as the device is not larger than 20 amps
    2. Are there any recommendations for other relay transformers? (Answer) Yes leave everything as it is and donít mess with it
    3. Will the relay transformer put off a lot of heat to distort the thermostat? (Answer) Letís hope and pray that you donít try to install the transformer in the same enclosure as the thermostat.

  3. #3
    Electrician ActionDave's Avatar
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    What kind of heat?

    Adding relays to heating systems can be problematic since there are steps that need to occur in order to make things safe and comfortable.

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    Moderator & Master Plumber hj's Avatar
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    A thermostat is merely a somewhat intelligent on/off switch. As such, connecting it to the 24 volt transformer circuitry is not a problem. It will open and close the relay according to its settings. The 240 volt circuitry is where you have to do everything correctly.
    Licensed residential and commercial plumber

  5. #5
    DIY Junior Member wombat's Avatar
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    Quote Originally Posted by ActionDave View Post
    What kind of heat?

    Adding relays to heating systems can be problematic since there are steps that need to occur in order to make things safe and comfortable.
    It's yee olde simple baseboard electric. I've have already installed a digital programmable thermostat (Honeywell TL8230A1003) on it, but the programming features of it are very limited. It only lets you choose two different temperatures during the day and it forces you to have exactly the same temperature for sleeping as when you are out of the house. How crazy is that?

    What kind of steps might I be missing?
    Last edited by wombat; 10-27-2012 at 05:10 PM.

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    Electrician ActionDave's Avatar
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    Well if it is plain olde ye olde baseboard then there are no steps, just an on and off. A relay should be fine.

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    Moderator & Master Plumber hj's Avatar
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    Most programmable thermostats need more than two wires, unless you get one has a battery for the logic circuit.
    Licensed residential and commercial plumber

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    Jack of all trades DonL's Avatar
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    You only need two wires for the relay coil that the thermostat controls.

    That is the only thing the thermostat should be controlling. Heat only if I understand.

    Then the Elements would be switched on the relay contacts.

    I would use a 4 Pole Relay and "Double Up" on the contacts, and get more Current and Safety capacity.


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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by hj View Post
    Most programmable thermostats need more than two wires, unless you get one has a battery for the logic circuit.
    You are correct, It needs to have a battery, or just be a mercury or Mag switch using 2 wires.

    I like the Thermostats with AA batteries, They Last about 3 Times Longer than the AAA for the same money.
    Theory only works perfect in a vacuum.

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