No heat, tempory fix with plug in transformer?

Discussion in 'HVAC Heating & Cooling' started by mtnman1000, Mar 14, 2019.

  1. mtnman1000

    mtnman1000 New Member

    Joined:
    Oct 23, 2018
    Location:
    59701
    One of my gravity in floor furnaces stopped working and it does not appear to be the thermostat The pilot is on but it doesnt heat up even when jumping the R and W wires.

    Is it possible to use a MGT2440 transformer I have just to test since it would be difficult to get into the crawl space at the moment where the transformer is. I also have a Sensi thermostat available as well as the 2 wire Honeywell that was removed for diagnostic

    Thanks
     
  2. Stuff

    Stuff Active Member

    Joined:
    Mar 7, 2013
    Location:
    Pennsylvania
    Transformers don't fail very often. First verify the furnace is getting power then get a multi-meter and measure the voltage between the R and W wires.
     
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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Checking voltages can be misleading if you don't understand what's going on. A thermostat is a temperature controlled switch. The R and W wires will read zero volts ac between them IF the thermostat is calling for heat. You'll measure somewhere around 24vac if the thermostat is not calling for heat. If you measure across the R and W terminals of the thermostat when it is calling for heat, any voltage you measure between them would indicate a poor contact (switch) in the thermostat...it should go to zero or very very close. When you're measuring the wires, you're measuring from one side of the transformer, through the coil of the relay, to the other side of the transformer...not directly across the transformer, but with no current flowing, the coil's resistance is much less than the meter's circuit, so it will look like nearly the full voltage across the transformer.

    Not understanding this can lead to erroneous conclusions.

    Using a transformer attached to the thermostat won't generate a complete circuit...the other side of the coil is not readily available to you except inside of the furnace.
     
  5. WorthFlorida

    WorthFlorida Broad-Wing Hawk

    Joined:
    Oct 28, 2009
    Occupation:
    Retired
    Location:
    Orlando, Florida
    If the transformer is open and not providing power, it also means the control board in the furnance doesn’t have power nor does the gas valve to open. Temporary placing one at the thermostat most likely will not work, plus it could be dangerous with a gas.
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    A second transformer is sometimes used at the thermostat when the thing needs power and you can't get the common from the heating device (easily). Smart, WiFi enabled thermostats generally need power. But, then, it uses its internal contacts to energize an isolation relay since mixing different sources of 24vac control signals is unreliable and usually won't work except in an unusual situation.
     
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