# Thread: CFMs vs Square Footage

1. Professional Member*
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Originally Posted by duckman06
While I agree with George 2. While 10x12 usually needs a 120 cfm (not all the time but a good rule).

Are we talking about metal or flex ductwork? If flex then I would install a 7 inch with a damper. Metal duct work moves more cfm then flex so if metal is chosen then a 6" might work?

Do you know the static pressure of the system. This is important as with either metal or flex it would let you know what size duct to install. Based on .5 static pressure a 7" flex duct
will move/provide roughly 120-130 cfm if installed propely. A 6"metal duct drop will provide 85-100 cfm. Either way I install a 7" with damper.

If the room gets to much air flow/cfm you can also damper it back. Without a load cal. Ran for each room then the company is guessing.
Sorry......flex duct doesn't readily come to (my) mind because we use so little of it here.

Dampers are a must in every situation IMHO.

2. Originally Posted by arc8
I think he said 70K is the output. When figuring 60F TR I came up with 1166 CFM!

I thought you were referring to a minimum velocity needed to help "push" the air or some other notion.

70k input @ 95% eff, divided by 1.08, then by 60F = 1026CFM.

Of course you could also use a smaller furnace and AC. AC in northern climates frequently get oversized because too high of a design temperature is used. Most of Vermont is between a 81-84F design temperature with 69F wet bulb. 1.5 tons should be plenty. 2 tons at most if there were a lot of windows and minimal shade.

3. Originally Posted by M.Dubbs
Let me make sure I am understanding this correctly.

If I have three rooms that are each 120 sq. ft. then I should have a furnace that can produce 360 CFM. (enough for each room to recieve 120 CFM)

Also, what do you mean by "The furnace blower will slow down when in the heating mode, so it is not an issue for the heating." ?
A 120 sq ft room may only need 60 CFM. The sq ft of a room doesn't determine how much air that room needs for heating or cooling.

Your 28X8 supply duct is ok for 1200 CFM.

There are many 2800 sq ft homes that don't have anywhere near 2800 CFM of air flow.

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