The Main Cause Of Loud Noises When The AC System Is Running
I remember sitting to watch TV after dinner during the summer time and finding myself unable to hear the TV only when the AC system kicked on. My AC system was so loud it sounded like I was sitting in a wind tunnel and every time it came on I had to turn the TV volume higher, then when the AC shut off, the TV was too loud and I'd turn the volume back down. I remember not thinking anything about it at the time, only that it was a slightly annoying habit. Little did I know what was causing that wind tunnel like noise was actually hurting my AC system.
It was only year later when I started Green ID and performing energy audits and retrofits that I realized what was actually causing this to happen was an undersized return. Undersized returns plague the Phoenix market in homes of all ages and all types of HVAC systems. HVAC contractors and installers never seemed to get this right and the fact that Arizona homes are typically built with only one return per AC unit, this problem is still going on to this day.
Why Is An Undersized Return Hurting The AC Unit?
If you've ever been short of breath, had asthma or felt winded after an intense workout you know what it feels like to be starved for air... it seems like you can't get enough air into your lungs. That's what happens all the time when the AC system has too small of a return installed. That loud sound I was hearing was actually my AC system wheezing for air, it was literally trying to inhale more air than could fit in the ductwork. This makes the AC system work a lot harder, reduces it's capacity and creates a loud, wind-tunnel like noise at the grill.
How Do You Know What Size Return Is Good Enough?
We use a rule of thumb per the table below for the return duct size however the problem can often lie in other places like the elbow type, return air can, filter, grill, kinks in the ductwork and plenum size. A simple check you can do yourself is with the air or heat on, remove the grill and see if the noise is reduced. Then remove the filter and see how much that improves the noise. The solution to undersized returns is to make the return size larger. We can do this by adding a larger return, a second return, installing a return plenum or changing a twist elbow to a side by side elbow. Having a comprehensive energy audit on the home will present the best options because contrary to what your HVAC contractor may believe, it's not always as simple as adding a second return.
Return Size Chart
5 ton should have at least a 22" return
4 ton should have at least a 20" return
3 ton should have at least a 18" return
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