Home Improvement--General/Air conditioning


When my A/c shuts off the furnace (forced air) makes a thud. I have been told my cold air return isn't big enough and my plentum needs to be reinforced. Which answer if any is correct ?

Thanks, Mark

Hey, Mark

That sound you hear is most likely the metal duct work shifting after the forced air system turns off. Just like a balloon inflates - so does your duct work when the system is running.  As soon as the unit stops running - the duct work adjusts.

This is very common in systems where the ductwork (specifically a plenum box) is undersized.

If you have received advice from a licensed HVAC contractor that your return box is too small - this explains the issue. Reinforcing a return plenum doesn't solve the problem it just eliminates the sound.

Think of the sound as a symptom - and the problem is a return or return plenum that is too small. Correcting this will not only eliminate the sound but also help your heating / air conditioning system to function more efficiently. Making you more Comfortable and spend less money.

Best of luck. Make sure the contractor shows you a Manual D calculation that justifies the size that the new return would be. Manual D is the accepted standard for determining duct work sizes.


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Anthony Procaccini


Residential Construction questions - specifically diagnosing an issue or concern in the home. My main area of expertise is Home Energy Efficiency and reducing your utility bills or increasing the comfort of your home.


I was a home builder for over 5 years. I have been doing energy auditing and analysis for the past 4 years.

Building Performance Institute, United States Green Building Council - NC Chapter

My blog - bsiofnc.blogspot.com

I am a Building Performance Institute Certified Building Analyst, Certified Manufactured Home Specialist, Certified Envelope Professional, and a Certified Quality Control Inspector

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