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    bfmorgan's Avatar
    bfmorgan Posts: 1, Reputation: 1
    New Member
    		  
    #1

    Feb 16, 2010, 12:59 PM
    Power Stealing Thermostat connection to millivolt heater?
    I have an older home that has only a millvolt heating system and no central AC.
    I want to install a special thermostat that requires 24 volt power and supports power stealing. Since a millivolt system has no power connection, I wonder if I can use this thermostat by addint a 24V transformer plugged into the wall. I have tried this and connected wires a few ways, but with little success.

    The instructions for my thermostat do address the question of a millivolt heater in a system with a 24 V AC unit and show that the connections should be:

    Millivot heat is connected to the thermostat at RH and W.
    24V AC hot connected to RC and shows the other side connected to Y and G with a jumper to C (Common) optional).

    Since I don't have 24V AC, I tried connecting a transformer that converts 110V to 24VAC and connected the hot to RC and the common jumpered to Y, G, and the C (Common) connection on the thermostat.

    But this gives me more power.

    One night I got desperate and connected the 24V transformer hot side to Y and the common side to C on the thermostat. The thermostat did get power for a while, but then burned out my power supply, so I'm back to square one.

    Also, I know one solution is to find a different thermostat. But I have special need to use this one if at all possible.

    Any help is much appreciated.
    wmproop's Avatar
    wmproop Posts: 3,749, Reputation: 91
    Ultra Member
    		  
    #2

    Feb 16, 2010, 01:04 PM
    Good luck
    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
    Heating & Air Conditioning Expert
    		  
    #3

    Feb 16, 2010, 01:05 PM
    Just wire in a relay to close the millivolt power contacts when the thermostat tells it to. Use the on-off contacts in the relay to tell the millivolt signal to turn on and off thus the heater will turn on and off.

    You will need a 24 volt supply for the 24 volt thermostat and if you are smart you will also use a 24 volt coil relay so you can power both off the same transformer. Install a inline fuse to prevent transformer burnouts if you mess up the wiring. Good luck.
    tk03's Avatar
    tk03 Posts: 33, Reputation: 2
    Junior Member
    		  
    #4

    Feb 16, 2010, 06:07 PM

    Millivolt contacts are usually silver coated due to very low voltage. I don't know if anybody makes a spst relay for millivolt.
    KISS's Avatar
    KISS Posts: 12,510, Reputation: 839
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    #5

    Feb 16, 2010, 06:35 PM

    The Idec RM or RY series should work:

    http://www.alliedelec.com/Images/Pro...N/814-3072.PDF RM is preferable.

    What you don't want to do is use a relay reted for 10 A. The RM series has a contact resistance of 30 milli-ohms.
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