Home Improvement--General/Air conditioning

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Question
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

Answer
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.

Anthony  

Home Improvement--General

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Anthony Procaccini

Expertise

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.

Experience

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

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

Publications
My blog - bsiofnc.blogspot.com

Education/Credentials
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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