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Troubleshooting a digital thermostat

Discussion in 'Electronic Repair' started by DGF, Jan 30, 2010.

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  1. DGF

    DGF Guest

    How do you verify that a battery-powered digital thermostat is working
    like it should? From what I've read, it seems that it is supposed to
    electrically bridge the connection between R wire and the W wire, when
    it turns on heating. I detached a thermostat from the HVAC system and
    tested the connection between the two terminals using a multimeter *when
    the room temperature was well below the heating set-point*; the contacts
    on the thermostat corresponding to the R & W wires didn't seem to allow
    current to flow through. Given that it's an electronic device and may
    not respond to the small DC voltage from the multimeter the same way it
    may to 24 VAC, I can't tell whether the thermostat is good or bad.

    Any ideas?
     
  2. Put it back on the wall. Run the temp way up. Go see if
    there is a 24 volt signal at W.

    --
    Christopher A. Young
    Learn more about Jesus
    www.lds.org
    ..


    How do you verify that a battery-powered digital thermostat
    is working
    like it should? From what I've read, it seems that it is
    supposed to
    electrically bridge the connection between R wire and the W
    wire, when
    it turns on heating. I detached a thermostat from the HVAC
    system and
    tested the connection between the two terminals using a
    multimeter *when
    the room temperature was well below the heating set-point*;
    the contacts
    on the thermostat corresponding to the R & W wires didn't
    seem to allow
    current to flow through. Given that it's an electronic
    device and may
    not respond to the small DC voltage from the multimeter the
    same way it
    may to 24 VAC, I can't tell whether the thermostat is good
    or bad.

    Any ideas?
     
  3. PeterD

    PeterD Guest

    Like making a test circuit that provides the necessary 24 volts?
     
  4. Gnack Nol

    Gnack Nol Guest



    That is correct thermostats electronic or mechanical always depend on the
    power source in the HVAC system they control for power and are never self
    powered.

    The battery is just there for power interruptions to keep the thermostat
    from resetting to a different temperature nothing more.

    To test it you will have to provide the 24v source for it to operate from
    otherwise nothing will happen.

    Gnack
     
  5. GregS

    GregS Guest

    Most thermostats have a relay. One has to know the circuit to troubleshoot
    it. Some of the best thermostats have power stealing circuitry
    and will opperate without batteries, but most of the cheap
    ones just have a small relay, and contacts could have some resistance
    over time.

    greg
     
  6. GregS

    GregS Guest

    I always assumed a relay, and yes they are loud. On a soild state unit I could also
    hear a click, but I think that was from current flow. If units use a switch, I hope
    they have a safety overheat breaker.

    greg
     
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