Relatively simple duct question

Old 11-06-15, 08:02 PM
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Exclamation Relatively simple duct question

I'm remodeling my living room and consequently laying down a new subfloor. I have one return air in my whole home and it is basically just sheet metal nailed underneath the subfloor (to the joists). The furnace is in my basement and the return air drop assembly goes right up under those floor joists and connects to this "duct" formed by the sheet metal (old house, I know). This trunk duct is 8" by 18".

I have a 90,000 BTU furnace, temperature rise of 45-75. Using the formula
CFM = BTU / delta T x 1.08 = I get... 90000/(60*1.08) = 1388 CFM

I assume that I would need at least 1388 CFM of return air for the furnace to be running optimally. I read somewhere that for every square inch of return vent, you get 2 CFM. So that would mean I'd need a return vent that is something like 26 x 26 which equals 676 sq inches, 2CFM per square inch equals 1352 CFM of flow. But if my trunk duct is only 8" by 18" how is it even possible to get 1388 CFM of needed flow. Why can't I just put an 8" by 18" vent on the floor which matches the max width and height of my trunk duct?

As you can tell, this is not my day job. I am either miscalculating, or just wildly wrong. Please set me straight -- feel free to insult my intelligence first, but please do give me a thorough answer so I can understand what I need to do. Thanks so much...
Old 11-06-15, 10:04 PM
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Welcome to the forums.

My specialty is servicing..... not ductwork but I have a question.
You're going to put the return in the floor ?? Is that your only option ?
A floor return picks up an inordinate amount of dust and dirt from the floor.
Old 11-06-15, 10:27 PM
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Hi AMP and another welcome.
HVAC is also not my specialty but I can answer part of the question. You ask "But if my trunk duct is only 8" by 18" how is it even possible to get 1388 CFM of needed flow."
What happens when the supply and return sizes (capacity) are not matched is (in your case) the supply side pressurizes the house. That increased pressure increases the return flow, although not up to the desired rate, but up until the supply and return match, which physics requires.

Now here is the problem, and your house is common, that increased house pressure also forces expensive heated air out through the many leaks throughout the house. Besides being expensive, that warm air also carries a lot of moisture which can get deposited inside walls and ceilings where it reaches a cool surface. In addition, that lost warm air must be replaced somewhere within the system with cold outside air.

The HVAC pros will be along and provide guidance on what is needed and perhaps how to get there.

Old 11-07-15, 08:55 AM
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Thank you both for the replies.

PJ, Yes, the return must go in the floor with the way the duct runs under the floor (sheet metal between floor joists). The vent will be tucked in a little used corner of the room. Also, I've heard that a return on the floor (or baseboard) is preferable to one on the ceiling in heating situations (heat rises).

Bud, that makes complete sense. I think my concern really relies along two questions... is an 8 x 18 trunk duct, 7 ft in length, able to flow 1300 cfm coupled with an 8 x 18 floor cold air return vent. If so, why does everything I read suggest that floor vents are only capable of 2CFM per sq inch -- which in my case would require a vent size of 650 square inches.

The reason this is even an issue is because while remodeling and exposing the underfloor ducts, I noticed an increase of airflow to my registers once I removed the small baseboard return vents and opened up the floor. Almost as if the furnace was getting more air -- my wife thinks it's all in my head and it might be. The old return was a single 6x30. Again, way undersize with my calculations.

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