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Machinc, General Contractor
Category: HVAC
Satisfied Customers: 945
Experience:  Has written home improvement articles since 2008. Earned South Carolina Residential Builders License in 1991.
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When my ac system shuts off when the thermostat setting is

Customer Question

When my ac system shuts off when the thermostat setting is reached the scroll compressor makes a loud clunk/bang and a loud thud in the feed lines, however, if I turn the system off before then by adjusting the thermostat (even a couple seconds before it would turn off by reachingthe set temp) it turns off with s simple click and absolutely no noise from the compressor or feed lines. Why is this and how can I get if to stop making this loud shutoff noise? The unit is right outside my bedroom window so it is quite annoying. I've had professionals look at it several times and they insure me the coolant levels are fine and pressure is normal but the cannot provide an explanation for this issue. There is obviously some difference in the way the thermostat is turning off the system. Please help!
Submitted: 2 years ago.
Category: HVAC
Expert:  Machinc replied 2 years ago.

I will have your answer in a moment

Expert:  Machinc replied 2 years ago.
This problem can most likely be resolved by replacing the thermostat.
There could be a problem with the control board but the thermostat is the most likely cause.
The thermostat is trying to turn off the unit but it is hesitating and doing like an off /on/ off
sequence basically the same as if someone was flipping a switch off and on and back off
rapidly .replacing the thermostat (and possibly upgrading to a higher grade model) Is your
most likely fix.
I will be glad to answer any questions you may have.
Customer: replied 2 years ago.
Changing the thermostat did not change the issue unfortunately. Any other ideas?
Customer: replied 2 years ago.
I'm using and Emerson Sensi fyi
Expert:  Machinc replied 2 years ago.
In that case the problem is most likely the
control board. A much more expensive fix.
Check for loose connections before replacing.
Also make sure the wiring from the thermostat is not
loose or damaged at the control board end.
Customer: replied 2 years ago.
The system is only 2 years old and has been doing this from the beginning. Other than this issue the system cools and runs fine.
Customer: replied 2 years ago.
All connections look fine and board looks new and pristine. I would assume if there was a problem with the board it would affect the normal operations of the system overall?
Expert:  Machinc replied 2 years ago.
With its numerous functions
one component can Be affected and others work.
Your problem is specific to the unit turning off
when controlled by a sensor.
This would be in the thermostat,
Or the control board or a temperature
The control board can look good and
be bad.
The sensor could be the one located
Near or in the return small ( has two wires) coming out
To control board.
Expert:  Machinc replied 2 years ago.
The control board would actually test
ok as it is functioning.
The main relay us used when you
manually turn the unit off.
A small relay on the board could (chatter) a term some use
when a relay attempts to .operate and has trouble making contact.
This would not affect any other functions of the board .
Eventually it could fail completely but that could be way down the road.
As expensive as the board is it may be something you may want to compromise
On. It does work but it is noisy.
If the trouble is not in the temperature Sensor that is your next