# Size of AC return

• posted on February 16, 2005, 8:02 am
We have a 5 ton AC unit with a single 20" * 20" return filter, everyone agrees that is too small. I can double the area of the filter without too much trouble, however there is a single return that is simply a passageway that goes between the sheetrock and 2*4's, that passageway has a cross section is 16 * 32 inches. My question is is the return air duct large enough, the return goes from the 1st floor to an attic above the second floor.
Peter
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• posted on February 16, 2005, 8:18 am
Peter Wells wrote:

There is no way we can figure that out from here. It takes a few measurements and then a calculation called a Manual D to determine the proper sizing and design of the entire duct system. You can't just look at a part of it.
When you say everyone agrees it is too small, who is everyone? Are they saying it is the filter that is too small or the ducts? How much too small?
--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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• posted on February 16, 2005, 8:32 am

Everyone agrees the filter is too small - I'm just looking for a rule of thumb regarding the return duct size.
Thanks

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• posted on February 16, 2005, 2:42 am

This is Turtle.
I call 20" X 25" return air grill as small as i will accept but like to use the 24" X 24" or 20" X 30" if possible. Now if doubling up return air grills i like a pair of 20" X 20" .
TURTLE
TURTLE
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• posted on February 16, 2005, 12:52 pm

With a 5x400 = 2000 cfm airflow?

Sounds like Sir TURTLE wants a max 2000cfm/4ft^2 = 500 ft/min face velocity.
Nick
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• posted on February 16, 2005, 10:16 am
Peter Wells wrote:

Who is everyone? Uncle Fred or a HVAC professional?

There is no rule of thumb. It takes a proper evaluation to get a proper answer.
Frankly I am still not sure what exactly you want to do. If the filter is too small how big is the duct? How are you going to increase filter size without modifying the duct?

--
Joseph Meehan

26 + 6 = 1 It's Irish Math
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• posted on February 16, 2005, 10:25 am
wrote:

Is it noisy? As in......if there were a tv there, would you have to turn the volume up each time the A/C came on? Yes? Then its too small. Bubba
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• posted on February 16, 2005, 11:40 am
That's an interesting question because it is not noisy, in fact if you open the filter holder door there is little pressure drop across the filter (i.e. it is easy to remove the filter with the fan running). The fan has been checked so maybe I have a leak into the return so air is coming in without going through the filter.
Thanks for the input,
Peter

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• posted on February 16, 2005, 12:35 pm
Well, if its not noisy and very little suction holding the filter then its most likely sized somewhat close to what it should be. However, you may have a leak somewhere before that grille. Its all just guessing though from this end of the internet. You know the problem with "rules of thumb"............everyone has 2. Bubba
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• posted on February 16, 2005, 5:15 pm
Peter Wells wrote:

Or the ductwork is so restrictive the filter adds little additional pressure drop.
Please describe the return again. You seemed to say it was sandwiched between the shhetrock and 2x4's in the wall cavity, which could mean the effective area is more like 3.5 * 14.5 per inter-stud space.
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• posted on February 16, 2005, 5:34 pm
wrote:

You may be on to something. To test it, put a garbage bag over the filter and see if it shuts off all the air.
DK
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• posted on February 17, 2005, 12:39 am
Thanks to those who responded...
I can see that what I said about the return passageway could be misinterpreted, the cross section of the passageway is 16" * 32".
I put a garbage bag over the filter and while the suction did pull it into the frame there was still a good amount of air coming out of the vents, perhaps it was cut by 50%, I think I need to persuade one of the children to clamber up the return passageway and see if they can see any holes.
Regarding who said the filter area is too small it is the various Heat & AC service folk that have been out, and I didn't see them calculate anything!
Thanks again,
Peter

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• posted on February 16, 2005, 6:11 pm

This is Turtle.
I can't see it from here but here is what is suppose to be going on.
If you have 5 tons of cooling ability which will call for min. 1,800 cfm to max. 2,000 cfm and you put a 20" X 20" filter or Plain Grill in the return air of the system. Then you say the filter is easy to pull out with the blower running. Well All I can say is you have a air flow problem. The filter should slap in there with the fan running and should be making a wind tunnel noise. Now here is some cause of this not pull good air to the unit.
Blower speed set too low. Return ducting size coming to return air grill is too small. Restriction in the return air ducts. Plugged up evaperator coil. Paper or dirt clogging up the blower wheel of furnace or blower wheel running in reverse. The furnace is getting a lot of return air from leaks or loose ductwork. If the return air grill is not making a wind tunnel noise -- your letting about 1/3 the return air by-pass the return air grill and come from other sources.
I think you have another bigger problem than just a return air grill too small and is costing you big time on the cost to operate this system. you have a restrictive air flow problem to say the least. You need this looked at and get a good hvac service company that knows what they are doing to diagnosis the problem.
TURTLE