# HVAC Members.. Need Help With Simple BTU Calc.

| Posted in General Questions on
 Fireplace heat exchanger… – 1200 CFM Blower > 3″ Square Duct Inlet > Air Heated Inside of Fireplace To (Estimated 400 degrees) > 3″ Square Duct Outlet. How many BTUs are being produced, assuming no duct restrictions… Can you even pump 1200 CFM through a 3″ Duct? Is there a limit… Wondering if I should run at 600 CFM instead to save power.

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### Replies

1. Expert Member
| | #1

The equation for heat transfer by air is:

transfer rate (BTU/hr)= 1.08*(airflow in cfm)*(temperature difference in F)

So if your air is at 400F and the room is at 70F that's 330F difference. At 1200 CFM that's over 500K BTU/hr. You have to make sure that the heat exchanger within the stove can actually support that, more likely it can't and your air will be coming out significantly cooler.

Can you push 1200 CFM through a 3" duct? With enough air pressure, sure. Do you want to? Generally not. The smaller the duct the faster the air has to flow, which means more noise. Also HVAC fans are not designed to operate at the pressure that kind of flow would require in that small a duct. The recommended minimum duct size for 1200 CFM is 16" round or 20x12 rectangular. A 3" duct is going to be whistling like a steamboat at 1200 CFM.

2. | | #2

I think you are more likely to get 1200 cubic feet per HOUR through a 3 inch duct…

3. | | #3

Thank you!!

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