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Discussion Starter #1 (Edited)
I had a new HVAC system installed last year with I believe a 5 ton AC unit. The home is 2200sq ft. I have two return air ducts that are non-flex 10'' ducting. One is located pretty close to the unit, the other is way on the other end of the house. Both units whistle on and off when the AC or HEAT is on.

The AC does not cycle at all. It's almost impossible for it to cool the house it will just maintain the current temp.

Is my return ducting too small? How big should they be? Is 2 enough?
 

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2-10" are almost big enough for 2 tons.

A 12 and a 14" or 2-14" round return ducts. Plus you need the return grilles to be big enough.
 

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Can you fit a 14" flexduct where your existing 10" aluminum duct is? If so, use 2 of these. Flex duct has more resistance to airflow than smooth aluminum but 2 14" returns should be sufficient.

This may or may not improve your cooling performance but should get rid of the whistling and allow the blower to work within its design specs.
 

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Discussion Starter #6
Can you fit a 14" flexduct where your existing 10" aluminum duct is? If so, use 2 of these. Flex duct has more resistance to airflow than smooth aluminum but 2 14" returns should be sufficient.

This may or may not improve your cooling performance but should get rid of the whistling and allow the blower to work within its design specs.
Yes, there is a good amount of room in the attic to fit bigger ductwork.
 

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Something simular to that return can will help.
BUt, get it insulated. It helps to keep noise down.
 

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You can use metal or flex.Either one will be find. Flex won't transmition noise as much as metal.
 

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Discussion Starter #9
So it's a 5ton unit not 3.5 like I originally thought. Theres two 11-12inch return air ducts right now.

What size should I take it up to based on this new info?
 

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2-14"
 
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