View Full Version : Return Duct and Return Grill Size
12-04-2006, 01:11 PM
I am having new duct work installed as I type.
The installer and I are discussing return duct and filter grill size.
My current return duct size is 14 x 20. Is this enough return for a 4 ton package unit?
I am having a 24 x 30 filter grill installed. Is this large enough also.
My installer says all is good. Is it?
Thanks in advance for all input.
Your rectangular 14 by 20 is 280 sqin. A round 18-inch is 255 sqin. Almost comparable to my own house, when I bought it -- my AC system was 3.5 ton with 255 sqin return. With a 4.0 ton air handler blowing 1600 cfm, I calculate 823 feet/min air velocity for you. Hard duct can work with higher velocities than flex for supply (but not for return), Manual D still recommends 600 feet/min and say 700 feet/min is max.
I imagine your tech will say he's installed many systems like this, and they all work fine. Perhaps true but he is not being faithful to ACCA Manual D principles. I bought the book and it's on Chapter 3, page 6.
Filter grilles don't get much larger than 24 by 30, at least not in the common sizes. That is 5 square feet of filter area, the 1600 CFM your blower is likely to run, means a face velocity of 320 feet/min. Manual D recommends 300, to me that's not a gross violation. I have heard rules of thumb (not Manual D obviously) which recommend a minimum of 1 square foot per ton -- you are within that rule of thumb at least. It certainly is comparable to what my own house had, and I did not hear any scathing criticism from several techs who have looked at it. However I have since installed additional returns and lots of grill area, with the result that a filter is unlikely to clog enough to threaten the mechanical health of the system.
Any chance you could downsize the system, push less airflow through the same ducts? That is my preferred solution to my own system which has puny supply ducts on a 3.5 ton 1400 CFM airflow. Sizing equipment via Manual J and Manual S might point to smaller HVAC equipment, making your problem go away. I calculate (independently of Manual J) that 2.75 tons is sufficient for my climate, a 3.0 ton system will be gentler on my ducts next year. Manual J tends to point to smaller sizing still, I am afraid to believe that without an *expert* pro to second that opinion.
You are getting these opinions from a homeowner (not a pro) who like you, has been worried about getting things right. In my case I have been studying it for several years.
Best of luck -- Pstu
[Edited by pstu on 12-04-2006 at 04:23 PM]
12-04-2006, 04:18 PM
if the tech is sure he's right - have him do static pressure measurement after the install to prove/disprove it
12-04-2006, 07:18 PM
Depends on the length and configuration weather or not that 14 x 20 is big enough.
As Jeff said, have him check the static when he's done.
Jeff learned the hard way.
12-04-2006, 09:59 PM
14x20 is not big enough,a 14 x 30 with a 16 in collar in flex to the plenum,could be enough,but your really pushing it.
>>14 x 30 with a 16 in collar in flex to the plenum, could be enough
I might be misunderstanding what you say here. Are you proposing a section of 16-inch flex between the return and the plenum? That would be way smaller than the 14 x 30 section. As I figure it, the 16 inch round would be 201 sqin, the 14 x 30 would be 420 sqin. Maybe *two* 16 inch flex ducts would be a better fit?
Thank you -- Pstu
12-05-2006, 08:10 AM
I will be the first to admit that I have a lot to learn and I appreciate all of the input. I used to do A/C and refrigeration work. Not sheet metal work mind you. I understand the importance of a full size return. The liquid refrigerant must boil off or over a period of time, good-bye compressor. Not to mention proper air flow. He is my hang-up. The return opening on the unit is 12 x 18. I understand a little about length of the return duct. With mine it's maybe 20ft with 2 90's. With the 24 x 30 filter grill and the 14 x 20 duct, is it not enough?
The installer is coming back today to finish up. I think this guy is doing a good job. I checked him out by talking with people he has done work for and others that know him that used to be in the business. All recomended him. He seems honest and sincere when we discussed the return size.
I'm a little confused. Thanks for your help.
[Edited by sc_dave on 12-05-2006 at 03:51 PM]
12-05-2006, 05:36 PM
With that length and 2 els you shouldn't have a problem with the return duct.
Your velocity through the filter grille will be alittle high.
Now tell us about the supply.
12-05-2006, 11:58 PM
i'm still missing the point of where the flex came into play. original message just said duct size and filter grill size. 4 ton unit. duct calculator says 14*20 = 2100 cfms at .1 he only needs 1600 cfm to leave the unit. what am i missing. or what are you missing. a simple i dont know would be a nicer reply. i bet the installer said websites are just stupid. heck just make sure your duct is the same size as the opening on the unit or bigger. thats a great way to know the answer. if you cant read a duct cal.
12-06-2006, 12:01 AM
oh i'm sorry i didn't read the hole post
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