Chattering Furnace Contactor

Discussion in 'HVAC Heating & Cooling' started by gransom, Jan 7, 2017.

  1. gransom

    gransom New Member

    Joined:
    Jul 5, 2009
    Location:
    Texas
    I have a 8-year old Trane electric furnace. The issue is an intermittent chattering contactor switch which controls the heating elements. I have instrumented the 24v voltage on the contactor coil and the voltage drops when the switch is chattering. The chattering only occurs when the system is closed up, never with the covers off. The low voltage path on the contactor is transformer -> thermostat -> fan switch -> limit switch (155) -> contactor coil. In attempting to isolate the problem, every component in the above path has been replaced. In addition, the filter is new I have cleaned the evaporator coil, and checked that wire terminal screws were tight. The problem seemed to be fixed for a week, but my renter says the issue is back after a night where the temperature low was 18 degrees. The only thing I haven't replaced is the thermostat wiring, but that seems unlikely since the issue doesn't happen when the system covers are open. No issues with fan operation or the prior summer air conditioning. When I temporarily bypassed the limit switch (after the new one), the issue didn't occur but I may not have ran the system long enough for it to happen. What am I missing here in debugging this problem?
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    With the power off, I'd isolate the various bits and use an ohmmeter to see if any one of the interlock switches had a high impedance.

    The contactor could have a bad diode and you may need to replace that contactor.
     
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Joined:
    Oct 28, 2009
    Occupation:
    Retired
    Location:
    Orlando, Florida
    It quite common problem for contactor's that wear out and cause that buzzing noise. I had one chatter for well pump and they are nearly all the same for 24 volt residential systems, both heating, cooling and pumps. Just two months ago I replaced one on my own AC unit. They are quite cheap, well under twenty dollars, usually around $12 (at suppliers and on line) and every AC mechanic will have a few on hand because they fail quite often. For someone to replace it it may run $150 range.

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