Thread: New here - CFM best practices

1. Professional Member
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New here - CFM best practices

Looking for the pro's advice on the best way to measure CFM on a forced air furnace? Other than ESP with the manufacturers fan chart...

Best method? Most accurate tool?

I would really like to incorporate airflow measurements in to all of my calls, but want and relatively speedy and accurate way of doing it. (like those two always go together...)

Thanks!

2. Calculate based on heat rise and BTU output of furance. Not 100% accurate, but can get you in the ballpark quickly.

3. Originally Posted by 54regcab
Calculate based on heat rise and BTU output of furance. Not 100% accurate, but can get you in the ballpark quickly.
I have wondered about this. What if gas pressure has been turned down, does this method still work?

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Place an electric heater (edit: like an old dryer element or even a couple of hair dryers on high) in the duct and measure delta T, calc airflow based on volts, amps plus air properties at your altitude, and barometric pressure using psy' chart (and humidity if you want real high precision number). Very easy to do with HP with aux heat <G>

5. If you do this with a HP/Electric heat, verify the amp draw of the elements. It isn't uncommon to have a sequencer bad causing some elements not to work. 5-10KW will heat a lot of houses so possibly no service call for low heat, even if 15-20KW is installed. Even if partial heat isn't quite enough to do the job a customer may not call in until it gets too cold for the elements that are working to keep up.

6. You can clock the gas meter for btu's input.

btu' output
________________ =cfm's

temp rise x 1.08

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Originally Posted by Kevin Weaver
You can clock the gas meter for btu's input.

btu' output
________________ =cfm's

temp rise x 1.08
Is this fairly accurate? So lets say my rise is 45 and my output is 100000. My cfm would be 2057? Did I do that right?

8. You need to clock the meter to determin input btu's , then if your furnace is a 80% then multiply input by .80 to determine out put. Its the output you want. Or if you are not trying to be super accuarate, look at name plate. Say it is a 100,000 btu. 80%. then the output is 80,000 btu'

80,000
______________ =1646 cfm's

45x1.o8

same with electric heat amps x volts = watts watts x 3413= btu's

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I use either the Testo 416 mini vane anemometer or I have the Energy Conservatory True Flow plate. Depending on space, duct design or other factors determines what I use. Sometimes both just to be as accurate as possible. Unless you know the btu of the gas being delivered I don't use gas meter clocking. You can also try flow matching with a duct blaster if you have access to one. supposed to be very accurate but time consuming. I have not done that one.

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