Should AC compressor hum

Hello, I have a Janitrol AC system (2 1/2 ton) I turned the unit off at the circuit breaker, in order to replace the disconnect box next to the unit. Someone bypassed it, when it burned a connection off.
With the circuit breaker off, I hear a low humming from the compressor area. Is this normal?
Thanks in advance!
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That depends upon where your control transformer is located. If it's in the outdoor unit, then no, it shouldn't be making any noise at all -- unless it's not really off. You'll find that out pretty quickly when you start breaking connections <G>.
If it's in/near the indoor air handler (common), you need to turn off the breaker to that, also. (It might even be on a different circuit - like mounted on the wall near the air handler - check that, and make sure it's off.)
OR... you've got musical bugs/rodents inside the unit.
LLoyd
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I'm sorry

Why didn't the stoopid SOB replace it when it burned???

If its humming with the breaker off, either you didn't turn off the correct breaker, or it forgot the words. I would very highly recommend that you check with a meter before sticking your fingers in there, unless you are up for being nominated for a Darwin Award.

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I would say you are just hearing the contactor (another name for a large relay) humming. It is what switches on power to the compressor when the thermostat calls for cooling.
The thermostat is most likely powered by a transformer located in the air handler (which is on a different breaker). When the thermostat calls for cooling it activates the contactor by sending 24VAC through the two small wires that go to the outside unit. Those two small wires, or a wire pair, supply power to the contactor coil
Some contactors make a humming or buzzing sound depending on the age and state of wear of the contactor.
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sometimes contactors do hum. Its time to worry when they whistle.

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Soon they'll be whistlin Dixie...
--



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