How to Calculate CFM?

Engun....Eeer it!

Have you tried that wonderful facility called GOOGLE?

GOOGLE is always my first stop in research.

2nd hit from GOOGLE query "cfm hvac calculation"

http://www.ehow.com/how_5677999_calculate-cfm-hvac.html

Regards Woody

It is generally frowned upon to use a gun for HVAC work. Although if you do, make sure not to over-size or under-size.

Air velocity x Cross sectional area of Duct.(appropriate units has to be used)

You will need a good thermometer.

First, "clock" the gas meter. You do this by running the furnace with the heat on and counting the number of revolutions of the smallest capacity needle on the gas meter over a defined period of time.

You will have to figure out how to convert how many revolutions this needle makes into Btu/hr of gas input. Since it is possible that your local gas company has varied some important parameters of the gas and delivery pressure this may not be conclusive, but it is all you have got. You may have to assume that there is somewhere around 1050 Btu/Cu. Ft. of natural gas.

Since you probably do not have a gas pressure manometer you will also have to assume that the gas pressure in the burner manifold is correct.

If you look at the manufacturer's nameplate on the furnace, it will show the BTU/hr input and probably the output. If the Btu/hr output is not shown multiply the manufacturer's stated input by the presumed efficiency of the machine.

You are trying to get how much gas the furnace used and what the temperature rise across the furnace heat exchanger is. With this information you can calculate the volume of air moving through the furnace using the equation CFM = BTU/Hr Output/ (tout-tin), making assumption that the altitude above sea level at your location is less than 1000 feet so that no altitude correction factor is required.

If you get a Dwyer magnehelic differential air pressure gauge and the right probes you can also measure the pressure drop across the furnace heat exchanger and look up the volume of air moving through the furnace on the manufacture's tables.

This is what they are teaching HVAC technicians today. I have not done either of these 'experiments' yet, but I have seen several furnace installations where incorrectly sized furnaces, read blowers, were installed. I am hoping that when I do come across an installation where the furnace (blower) is incorrectly sized, I am able to determine what the correct sized blower would be. Note that we do not change blowers in furnaces, we install new furnaces.