Return Air - methodology

Old 07-22-08, 02:34 PM
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Return Air - methodology

Started browsing this section today and have a question.

What is the calculation for determining more/less return air? It seems the consensus on the majority of the post regarding return air is "go for it, most new system installs don't have enough return air."

I have a 3 ton unit on 1,000 sq ft. 5 total rooms. 1 centralized return measuring 18x24.

Install was just "thrown" in from previous owner/contractor. Working on balancing system and increasing efficiency.

I am interested in adding (1) 6" cold air return to each room minus the bathroom for a total of (4). Don't want to add 1 in the kitchen, but it gets full sun most of the day.

Thanks for keeping such a great site handy. Please advise.
Old 07-23-08, 12:45 PM
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Entire books have been written on duct design and it's been too many decades since I have done duct design to remember all of the ins and outs. Pretty much it is true that you cannot have too much return air and the limiting factors are often budget and room to install.

That stated there is no use in installing more RA duct than can be accommodated by the blower inlet. You want to keep velocity low enough to keep noise levels at a minimum and also to promote as much as possible laminar air flow and reduce turbulence. This generally requires ducts of larger cross-sectional area and gradual turns or the use of turning vanes inside the ducts. Few residential systems will ever use turning vanes and if a sufficient number of individual ducts are used the velocity will rarely be a concern.

One thing to be careful of in what you propose to do is to ensure that the individual returns are run as close to the blower (furnace or air handler) inlet as practicable. If you have to run them to a trunk duct or extended plenum ensure that trunk or plenum is of sufficient size to handle the airflow from all the individual ducts. Also, if your current centrally located RA grille also contains the filter for your system you will need to either incorporate filter grilles at each new RA inlet or install a new filter rack at the air inlet to the furnace or air handler.

Most public libraries will have several books that cover HVAC duct design. Look in the applied technology section, Dewey Decimal Classification 620 through 630 (I don't remember if it is 621, 623, 629 or some other number.)
Old 07-24-08, 02:43 PM
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You may find this handy:

Also, don't know if your return is a filter grill or not, but many contractors figure 200 square inches per ton of AC return so it would seem that a 3 ton unit would require 600 square inch of return grill. Also, many contractors figure 400-450 CFM supply per ton of AC and some even go as high as 500 CFM per ton of heat pump.
I know this because I'm in the process of retrofitting a heat pump onto an existing gas furnace system in which the duct work is too small.

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