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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5648
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Air handler has a high pitch noise when first comes on. i

Customer Question

air handler has a high pitch noise when first comes on. i checked and changed the piston but stii does it. i also disconnected the inside fan to see if all cap tubes frosted up and they all did.
Submitted: 2 months ago.
Category: HVAC
Expert:  Phil replied 2 months ago.

Hello, is this a variable speed air handler (starts up gradually) or the older PSC type, motor starts very suddenly when cooling is called for,

So far I suspect a variable speed blower electronics pack for the high pitched noise.

Customer: replied 2 months ago.
It is a variable speed motor. Iran with fan only didn't do it. Disconnected fan turned unit on and it did it
Expert:  Phil replied 2 months ago.

Good testing, thanks.

Since the variable speed is adjusted electronically depending on a range of factors, including temperature differential, or in your test, FAN ON operation.. it could still be the fan motor making the noise as it ramps up (maybe a 30% chance of that)

See if you can change the sound by clamping a thick wet rag over the piston metering device...if you can, then that is the source most likely.

it could also be sound telegraphing up the liquid line... to test that, try running the system with the liquid line service valve on the outside unit only 2 turns open. If there is no change, the ECM motor then becomes the primary suspect.

Customer: replied 2 months ago.
I will try them test and get back with you
Expert:  Phil replied 2 months ago.

OK, try throttling the liquid line at various ranges that will create flash gas ahead if the piston orifice, and change or eliminate the sound.

We would not want to leave it that way, its only a test to prove or disprove the piston orifice as the source.

Customer: replied 2 months ago.
Put wet rag around piston no change. Closed valve and open it 2 turns no change.
Customer: replied 2 months ago.
Fan motor is a multi speed psc
Expert:  Phil replied 2 months ago.

Thanks, ***** ***** us with some unlikely potential sources of the sound, we can take a few stabs in the dark.

Use a mechanics stethoscope ($20 at an auto parts store) to listen for the sound on the piston orifice liquid at the A coil, and then again on the metal body of the compressor. This could be a scroll compressor noise telegraphing up the liquid or suction lines.

Try clamping your copper tube flare block tool onto the liquid line ahead of the piston orifice. That would change the resonance potential of the system at the piston orifice.