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Jason, Electrician
Category: Home Improvement
Satisfied Customers: 4286
Experience:  I do all of the remodeling projects on my own home. Experienced in all facets of Home Improvement.
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New humming noise coming from outdoor air conditioning condenser

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New humming noise coming from outdoor air conditioning condenser -- Reason for concern? This condenser is less than 3 years old but has recently begun to emit an electronic humming noise when on which is about twice the volume it has been emitting for the last 3 years since installation. There is no disruption of air conditioning activity -- only the louder noise from the condenser which is only an electronic hum but nothing that sounds like grinding. Coincidentally, the electrical overhead lighting in the same area seems to flicker when the central air conditioning automatically turns on to regulate temperature in this hot and humid South Central USA environment. What might be going on with the air conditioning condenser to make it hum now twice as loudly as before (noting that outdoor temperatures have been slowly dropping lately, not rising, such that higher heat should not be a reason for the air conditioning condenser to strain / hum louder.)
Hello. Welcome to Just Answer.

Inside the condensing unit, there is a small relay, which is sometimes referred to as a contactor. It is what turns off and on when the thermostat calls for cooling. So basically, it's a switch, but it's operated electrically, rather than manually, like a standard switch.

This relay has a coil of wire that is necessary for producing a magnetic field, and the field is what pulls the relay in and out, turning it on and off.

When the coil on the relay starts to get weak, the relay will hum. This will get progressively worse, until the relay just won't work anymore. The contacts on the relay could need cleaning as well. There may be an insect in the relay, between the contacts. That happened to my air conditioner about a month ago. If there is an insect involved, the power has to jump across a gap in the contacts, and that can make lights dim, due to high currents from arcing.

The relays are not expensive, and not difficult to replace. To find it, simply open the wiring compartment on the unit, after turning off the 240 power first. Follow the thermostat wire right to the relay. If you plan to replace it, I would get the same model, and move one wire at a time from the old to the new.

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