Thermostat bad or OK

Discussion in 'General Electronics Chat' started by sangpo, Aug 26, 2013.

  1. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    helo all. With digital multimeter , how do we check thermostat for Ok or Bad.
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Depends what kind of thermostat.

    If you want/need a valid helpful answer you need to provide enough information to make that possible.

    Otherwise I could just provide this answer - connect digital multimeter to thermostat and see if it is OK!
     
  3. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    The old mercury bulb, 4 wire thermostats were simple switches which connected red to green for fan, red to yellow for compressor, and red to white for heat. All the rest of the logic and power switching is done in the noisy parts of the equipment. These can be examined with an ohm meter.

    Modern digital thermostats make the same connections but they are equipped (plagued) with time delays and resets that make checking all the triacs take nearly half an hour. You can not check them with an ohm meter. You have to apply 24 VAC and measure for an output voltage.

    If you have a heat pump or a fuel burning furnace, the Blue and Orange connections come into play as logic signals to the change-over switches and valves. Which kind do you have?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,535
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    Many digital thermostats use a Miniature relay, if you can get it to operate around the set point, listen for the relay click, check the output with an ohm meter, if it stay open, it is usually the relay contacts shot.
    Max.
     
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  5. #12

    Expert

    Nov 30, 2010
    16,283
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    You have been working on different thermostats than I have.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,535
    2,369
    Honeywell.
    Max.
     
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  7. strantor

    AAC Fanatic!

    Oct 3, 2010
    4,302
    1,988
    I have seen the honeywell relay thermostats as well, used in industrial water & hydraulic cooling units, but never seen them in residential applications.
     
  8. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    All the 24VAC thermostats I've used in homes use a small relay.
     
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