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Years and years ago a HVAC tech told me that one of the best things to do for a HVAC system was to add a second air return. He stated, basically, that doubling the size of the HVAC filter area would make the system quieter and put less stress on the HVAC components.

That was years ago, so...

Q: In a standard new, stick built home[1] what are the ramifications of adding a second air return, that doubles the filter area, to the HVAC system?

Notes:

Generally this is easy enough to do for the average home owner and costs < $50 for a “20 X 20 Steel Return Air Filter Grille for 1" Filter” (see Amazon).

[1] Any reputable builder who’s won something like: Professional Builder magazine--Builder of the Year or National Association of Home Builders (NAHB)--National Housing Quality Award


Edit 2018-08-27 11:11, Clarifications in reply to comments and answers:

Since there are multiple possible HVAC build outs where a homeowner can achieve the same thing, I was trying to not use any one specific build out. As it seems my question wasn't clear enough, I'll add what the 'goal' is (which I should have done originally) and one specific build out for reference. Hopefully that'll make a better understanding? I'm going to also add a second, related question at the end of this edit.

Goal:

The HVAC change is to allow the homeowner to increase (double) the size of the return air filter area for less than $100.

Alternate Q wording:

Is doubling the size of the return air filter area, with no other changes, good or bad for a HVAC system?

A specific Example:

The existing HVAC return air filter is in standard(ish) drywall 'feed' ducting to the HVAC blower unit. In many new homes this return is located in a central hallway, or other central area, of the house and there is plenty of 'blank' drywall in this drywall 'feed' ducting to install a second 'Return Air Filter Grille for 1" Filter' assembly [1].

Worst case, to see exactly what the drywall 'feed' ducting looks like, you'll need to goto a stick built model home, and stick your head through where the HVAC filter is. Think of a rectangular return duct built with drywall...

Two other clarifications:

  • The existing air filter is in a wall, and there is no air filter at/on the HVAC system itself.
  • The new return is added to the existing return 'duct work,' so the only true change to the HVAC system is there is now twice the filter area.

[1] I don't know what the entirety of this component is called, it's the steel frame that holds the filter and has an airtight fitting into a wall. A unit that holds a 20x20x1 filter is less than $50 at Home Depot, Lowe's, Amazon, etc.

Second, related Q (feel free to expand as desired! ..might need a physicist, as this is definitely outside of my math skills..):

If the existing filter area is 20x20 (400 sq in.), and the builder recommends a MERV 8, now that the filter area is 2 times 20x20 (800 sq in.), can filters of MERV 11 or 13 be used without restricting the airflow below the original airflow?

  • Hello, and welcome to Stack Exchange. Good question: keep 'em coming! – Daniel Griscom Aug 24 '18 at 17:55
  • The details of what you are suggesting are not clear to me. The size of the filter is not usually increased if the opening to the plenum is increased. When I had a new a/c and furnace installed in 1991 the HVAC installers did cut another opening into the plenum below the filter unit. This did not increase the area of the filter because that is part of the steel case of air handler, but very well could have decreased restriction. At that time we had the pleated paper SpaceGard filter unit installed and have used it since 1991. – Jim Stewart Aug 24 '18 at 18:32
  • @Jim ah, yes, sorry for the confusion :( I added a much expanded edit for clarification. – Michael Aug 27 '18 at 17:18
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It is true multiple points of return will increase the efficiency at a very low cost. Here is a common example from 1960's era tract homes. The single air return was normally in the hall way that lead to the bedrooms all rooms had heat registers. The kitchen and living rooms had large open areas that connected to the hall. When doing remodels we added a secondary return at the opposite end of the home usually 14x18" if memory serves, this helped to balance the draw through the home especially if the bed and bathroom doors were closed and reducing the draft and a minor reduction of noise at the return. Many homes only have 1 return, and many only have filters at the furnace. By installing filters at the original return and adding a second return with filters we reduced the pressure drop and kept the ducts clean compared to filtering at the furnace. We started doing this in the late 70's on remodels, it was not that expensive back then, even cheaper with flex ducting today. we started doing it to create a more modern look and eliminate drafts, the clean ducts became part of the resale pitch later. The largest win in my opinion is drawing multiple points keeps the smells in the kitchen...

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One of the best things to do for a HVAC system is to have it properly sized and balanced. And generally, neither of those things happen.

If you have a central return and return ducts (doubtful), you'd screw up the balance on those returns. If you just have a central return, it will be less restrictive and therefor quieter, but unless the ductwork is flippantly undersized (which they probably are, to some extent), the extra work the blower has to do is negligible and not really the fault of the intake size.

Is it too noisy? It works okay, right? When you said "ramifications", I though you meant doing it right, which means tearing your house open and adding return ducts. The ramifications of doing that require every trade except a plumber - if you're lucky.

And obviously, now you'll go through filters two at a time.

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I have heard of damage to air handler motors being caused by changing to an excessively restrictive filter. We use the Aprilaire 201 pleated paper filter which is listed as 20 x 25 x 6 inches, listed as MERV 10, but the effective surface area is greatly increased by the fact that the filter is pleated. I could not say whether doubling the surface area would allow you to go from an 8 to a 10 MERV. But going to MERV 11 or 13 sounds to me like a big jump.

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