How do I figure the proper amount of intake area for my central air conditioner?
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The return air grill should measure one square foot per ton. Of course that does you no good if the return air ducting does not match. Usually, though, the return air has no ducting and the return air chamber is just below the furnace.
The return air grill is in one of the walls of the chamber. An old rule of thumb someone taught me.
Your house already had heat loss calculations done, I'm supposing, and has existing air returns. That should be good enough.
A better air return for your cooling needs is high registers.(hot air rises).Cool your hot air(second floor), not your cooler (basement) air.
Cooling capacity sizes eg., 1 1/2 ton for a 1,000 sq. ft. house is the only other thing you need to know if you are installing air onto a heat only system.
As I said to someone else earlier, set your air registers to cool only where necessary, sealing or partially closing rooms you don't need it in. (different than winter heat requirements.)
The intake for a blower is calculated by using 500 to 750 feet per minute as a maximum velocity across the filter area.
That 500 feet per minute velocity in true in all ducts of a low pressure blower, both intake and output ducting.
So lets say you have a blower with a capacity of 1500 cubic feet per minute. Then 1500/500= 3 square feet of duct at intake. If you figure the velocityh at 750 feet/minute then the intake size would be 1500/750 or 2 square feet of duct size.
Try this manual for sizing ducting and filters for your AC
About page 30 to 35 or so. It gives homeowner's method to size ducts for your AC.
www.thermopride.com/manuals/mg...
If you have an older home, this is a very good question, because most older home returns are not sized properly. The recommended size for return air is 600 sq. inches per ton. If you are installing a new a/c, then simply make you return big enough to accomadate this, however, if you are enlarging an existing , then this can get difficult. To find the tonage of you a/c go outside, and look at your condensor (the outside unit). Look at the model number and somewhere in the number you should see some numbers such as 024, or 042, or 048, and so on. These represent the BTUs of your system in 1000's. 12,000 BTU's equal 1 ton. So the 024 would be 24,000 BtUs, devided by 12,000 equals 2 tons! Older units were made with the air handler quite low in the closet, and sometimes major work has to be done to raise the unit to allow for sufficient return size, however, some have plenty of room, and all you have to do, is make the hole bigger. Whatever, you do, make sure the return area is sealed good.
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