ac return duct sizing and more info

Old 03-19-11, 01:29 AM
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ac return duct sizing and more info

After giving a local hvac guy $1500 to install ac ductwork in the kitchen with the return near the stove, I decided to do the bedroom myself. I recently ran 3 10" flex ducts, about 25' each to the 3 exterior walls of my bedroom and now need to do the return. The heat gain calculation is about 18000 btu. Can someone tell me what size return(s) will be sufficient? Can I do harm by oversizing the return? All comments are welcomed and appreciated.
Old 03-19-11, 02:36 AM
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I'm no A/C person but I do know I never would have installed a return in a kitchen.
Now any time something burns on or in the stove it will be sucked in and blown out in the other rooms.
Plus the cooking grease in the air will get sucked in and clog up the filter faster.
Old 03-25-11, 05:40 PM
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What area U talking about 18,000-BTUH?

Hi zizanio,

I doubt the heat calculation would be anywhere near 18,000-BTUH, or 1.5-Ton
Three ten inch flex ducts, properly stretched & hung.(?)

Sq.ins., of a round duct: 10X10=100 *X's .7854= 78.54-sq.ins.,/144= 0.54544166-sf *X's 600fpm Vel= 327.25-cfm *X's 3= 982-cfm * 30-BTUH per cfm= 29,460-BTUH, way too much airflow.

First, the heat gain and Btu/hr of cooling is done for each room.
At 400-CFM per/ton of cooling, 12,000-BTUH / 400-cfm = 30-BTUH for each (CFM) Cubic Foot per Minute of Airflow.

The supply air diffuser should have a similar 'Ak' open-air-area as, say, the 6" duct, or an Ak of 0.19635-sq.ft.

You can oversize Return Air duct some, for around 400 to 500-fpm Vel for the required CFM of air flow.

As joecaption said, there should be NO Return Air register in the kitchen near the cooking range; we don't put them in the kitchen.

Last edited by HVAC RETIRED; 03-25-11 at 05:44 PM. Reason: Deleted words, & added a few.

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