hum from outdoor heat pump

Discussion in 'HVAC Heating & Cooling' started by GregO, Sep 24, 2013.

  1. GregO

    GregO Member

    Messages:
    73
    Location:
    Virginia
    Hi,

    I've got a Trane XL13i - outdoor unit has a low electrical hum, even when the power is off, meaning even after tripping the electric breakers for the indoor furnace and the outdoor unit. The hum best mimics that of a fluorescent light tube hum; it's not real loud and the unit seems to work (cycle) perfectly fine, although the capacitor has been replaced twice and the unit is 10 years old.

    Any idea what is causing this hum? Thanks!
    Greg
  2. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,509
    Location:
    Houston, TX

    It may be the 24 Volt transformer that is on another breaker.

    Is it popping your breaker ?

    What is your complete model number ?

    What is the value of the cap you are replacing ?


    Good Luck.
    Last edited: Sep 24, 2013
  3. kyleanderson

    kyleanderson New Member

    Messages:
    9
    Location:
    manchester, kentucky
    Hello
    There is power to indoor unit send low voltage to contractor. Maybe you are missing an breaker. There could be two branch circuits to indoor unit one being 60 amp and another being 30 making indoor unit 15KW backup heat kit installed. What you are have is contractor calling for heat or cool. On the outside unit take the yellow low voltage wires and disconnect them and buzz should stop.
  4. GregO

    GregO Member

    Messages:
    73
    Location:
    Virginia
    There's a 40-amp double breaker for the indoor furnace and a double-60 amp breaker for the outer unit...I think...I say that because that's all the breakers obvious/labeled for the HVAC. The system seems to work fine otherwise, but I don't recall hearing that humming sound ever when the HVAC system wasn't running at all.

    Greg
  5. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,509
    Location:
    Houston, TX
    Your 24 Volt transformer that the thermostat uses may be on a different breaker and will normally be feed by 120V. Some are shared with the Doorbell.

    When the gas in the unit is equalizing it will make noise, but not really a hum.

    Can you post audio ? Is it 60 Cycles ?

    The crankcase heater may hum a bit, but power would need to be applied.


    Good Luck.
    Last edited: Sep 25, 2013
  6. Bobelectric

    Bobelectric Electrical Contractor

    Messages:
    39
    Location:
    Eighty Four,Pa. 15330
    Those days of a shared xfmr are long gone. Never saw a 24 volt doorbell.
  7. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,509
    Location:
    Houston, TX

    I think sharing is a common practice. May not be the same transformer, but the same circuit.

    I have seen transformers located in the Convenience outlet for a furnace or air handler.

    24V is a standard MAX voltage. Most Doorbell chimes will have a on board regulator. As do the electronic controls in the newer HVAC systems.

    It may share the Alarm / Fire system , and is on a always on circuit, Nice ones have battery backup, and the transformer charges the backup batteries.


    Have Fun.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    In electronics, a few of the more common things that can create a hum generally contain a coil. Things that contain a coil are relays, contactors, transformers, and solenoids. THey only create a hum when there's power going through them (it's caused by the alternating current, and in the US, that's typically 60Hz (but sometimes you get a harmonic at 120Hz). It may not indicate a fault, or immanent failure, and can be the result of age as things loosen up. But, if it's the result of the thing drawing more current (maybe because it is sticking), it can be a problem.


    Finding the offending thing is easier if you have good hearing to help isolate it, but the sound can propagate through panels, making that difficult. If something is sticking, sometimes a light knock with something like a wooden mallet will work, and help isolate it (you don't want to be mucking around with something that conduct in there unless you really know what you're doing!). Once you've isolated the thing, many things like relays are fairly easy to replace, but putting the leads back wrong, can really mess things up. It can be more complicated if the replacement part is equivalent, but not identical - then you have to know and understand more to get it installed properly than just plug and play.
  9. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,509
    Location:
    Houston, TX
    My expansion valve makes noise after the unit turns off.

    But it is nothing like a 60hz or 120hz electrical Hum.

    Sound does propagate very well on Copper Tubing and duct work.

    The hum could be coming from the fan motor, but it would need to be getting power from somewhere.


    I would look at the cap that was replaced twice. That may be a clue, of things to come.

    It would be a good idea to make sure everything is grounded properly.
    Last edited: Oct 11, 2013
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    The valve that switches it from heating to cooling has a coil in it, but it is also changing the fluid flow direction...the expanding gasses through an orifice can make noises, too. But fluids aren't electronics, and my comment was on that type of component.
  11. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,509
    Location:
    Houston, TX

    Power has to be coming from some place.

    The waterfall on my pond was buzzing when I had it turned off, and it ended up being a Bee Hive.


    I just let them Bee.
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