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    domsmith's Avatar
    domsmith Posts: 2, Reputation: 1
    New Member

    Jan 24, 2008, 05:54 PM
    Old thermostat wiring- C, H and V terminals
    I'm replacing an old General Controls T-90 thermostat (2 wire) with a new programmable thermostat in use with an old heat-only floor heater (no fan). The two wires coming from the wall are not color coded and I have not been able to track down the old terminal letter designators. The old unit has "C" "H" and "V" terminals with one wire connected to "H" and one to "V". I suppose one will go to "R" and one will go to "W". Any suggestions?

    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
    Heating & Air Conditioning Expert

    Jan 24, 2008, 06:10 PM
    A new programmable thermostat may not work for you. In most cases you need a constant 24 volt hot leg which you do not have with a 2 wire. You might be able to get away with a battery operated unit.OR run a third wire.

    Follow the new thermostat directions for hooking up to single stage heat only. The paperwork you have will show you where to land the two wires you have for heat only. Do not worry about the old thermostat and where the wires were.You only have 2 wires so it is just about impossible to make a mistake.
    acetc's Avatar
    acetc Posts: 1,004, Reputation: 79
    Ultra Member

    Jan 25, 2008, 07:14 AM
    Obviously hvac1000 did not see that you have a FLOOR furnace and gave you bogus information. Because this is a floor furnace you have a millivolt system which means you need a millivolt thermostat and cannot or should not use a 24 volt thermostat, Good luck, Mike
    therinnaiguy's Avatar
    therinnaiguy Posts: 153, Reputation: 5
    Junior Member

    Jan 25, 2008, 07:41 AM
    Not all floor furnaces use a millivolt system. I suggest to test the contacts on the existing thermostat to see what the voltage is. 18-25 volts AC, listen to Hvac1000; different reading, listen to Acetc.
    domsmith's Avatar
    domsmith Posts: 2, Reputation: 1
    New Member

    Feb 4, 2008, 06:13 PM
    Thanks for the info guys. I purchased a battery operated Honeywell programmable (RTH2520B) and it works just fine. The directions were clear (Old "V" equals Rh and Old "H" equals W) making it a very simple hookup.

    I haven't used the programming function yet, but with the digital temperature adjustment this is at least as good as the old one and looks much more updated.

    Thanks again,


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