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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
Satisfied Customers: 852
Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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My ac unit is not running outside trying to diagnose a pr

Customer Question

my ac unit is not running outside
trying to diagnose a problem
Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.
Hi I'm Brian and I'm here to help! The humming you hear is probably the contactor being energized by the thermostat with control power from the indoor unit. The fact that nothing is running usually indicates no line voltage power but could be other things. You could have a bad contactor or even ants under the contactor points (believe it or not that is extremely common!) I could be that you only have 1 phase of the 2 phases of line voltage coming into the unit. You should check for 240 volts ac nominal on L1 and L2 of the contactor. It will be the two line wires that come into the unit from the disconnect. See if either of those issues may be the case and let me know what you find and we can go from there. Thanks, -Brian

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