Cold Air Return

17 years ago

I have a friend who has an older home and had a new furnace put in about 3 years ago. Her house is approximately 3200 square feet. After the new furnace was installed, the room farthest from the furnace got much colder in the winter and much warmer in the summer. Two different furnace repair people have sait that it is fine the way it is.

When they put in the new furnace, they put in a new cold air return from the ceiling to the side of the furnace - the furnace is in the basement and is an upflow gas furnace. The new cold air return they installed is 24 inches by 12 inches - 288 sq inches. The old return was round and was at least 24 inches in diameter, so at least 450 square inches. We are talking about putting in a new cold air return that will be 30 x 17, or about 510 square inches, hoping that this will solve her problem. Does anyone have any thoughts about this? Personally I think the furnace repair guys who said this is not a problem don't have the foggiest idea what they are talking about, or else they just don't want to mess with it.

thanks

Anne

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Comments (2)

udarrell_2007
17 years ago
What tonnage is the A/C?

A 5-ton 2000-CFM; 4-ton needs 1600-CFM of airflow.
The Returns should be sized so the velocity will be 500-FPM or less, & free-air filter rack sizing is below.
TWO RETURN AIR FILTER RACKS - NONE IN FURNACE
Typical disposable 1-inch capacity return air filter is 2 cfm per square inch of gross filter area. Recommended filter velocity is 300-350-fpm, lower is better.

Velocities higher than 500 fpm will decrease filter performance. Increase flow resistance, and possibly
blow off collections of dirt. Measure Velocity 1" from RA grille face.

Average Free Air area of most Return Air grilles about 75%.

A 4-ton 1600-cfm / 24"duct is 3.13888sq.ft.= 510-fpm velocity.

A 5-ton 2000-cfm /24" duct 3.138887-sq.ft.= 637-fpm velocity.

A 5-TON 2000-cfm /288sq.ins or 2-sq.ft.= 1000-fpm - udarrell

Here is a link that might be useful: Sizing Duct Systems & Return Air Filter Racks
anneljohnson
Original Author
17 years ago
Turns out the tag on the AC unit is not readable. It is probably at least 20 years old (and needs to be replaced - there is a leak somewhere, and the repair folks told her that it is most probably in the compressor unit, and recommended replacing the unit and the lines. Of course I suppose the evaporator could be leaking as well. After recharge, it runs for a couple of weeks and slowly stops cooling. They want $150 for leak testing - is this reasonable?

So anyway, I don't know the tonage of the unit. The house was built in 1925 (2000 square feet of it) and the rest was added on in about 1985. While the old part has been insulated, the new part is much better insulated, and so does not require as much capacity per square foot as the old part.

I notice your web page mentions Bloomington - would this perhaps be Bloomington Indiana? If so, can you recommend someone competent to install the AC and also check out the ducting (in Indianapolis)?
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