# CFM ratings of inline duct fans far exceed the ratings for their intended duct sizes. What are we to make of this when designing a system?

My garage is 25×29' with 10' ceilings, so 7250 cf. I'm hoping to be able to turn over the air every 3 minutes or so, so aiming at using 2 fans rated at a total of 2400cfm.

The fans I'd like to use are 10" and rated for 1200 cfm each. They attach to 10" ducts, which themselves are supposedly only capable of moving 325 cfm. The manufacturer has these fans being used in 10" ducts. They were made for them. My plans have been to have one 12" exhaust vent on the side of the building connected to a 5-10' section of 12" solid round pipe, which then connects to a wye that drops to 10" arms. From there, I'd have equal length 10" solid pipes connected to the inline fans, which in turn will have 24×24 registers to grab fumes, etc. from the ceiling.

Will this work ok? My guess is it may be loud and working harder than is good for it, especially at its highest speed. Any advice would be appreciated. Here's the fan for reference.

• What static pressure drop per length of ductwork is that 325cfm figure at? – ThreePhaseEel May 13 at 13:58
• Honestly not sure. It seems that all available info on cfm of ductwork is in agreement though. I'm surprised at how little information is available to the general public on hvac components and makeup. Is it that the math is too complicated to learn? I know Green Building Advisor always recommends hiring an engineer when designing such systems. – Chris S May 13 at 14:51
• I think it's more likely that I just dont understand the basics re static pressure. Do you have any good resources in mind? – Chris S May 13 at 14:55
• Well, I can assure you, based on fluid dynamics, mass through a pipe is very, very much a function of pressure. The role of pressure is to overcome the friction of boundary layer effects - air getting dragged down by the pipe walls. So the 325 CFM numbers are definitely based on an assumption about pressure. And pressure is what fans do, so Occam's Razor says this fan makes more pressure than the industry assumptions. – Harper - Reinstate Monica May 13 at 18:30
• Under what conditons were the fans rated? often it's something utterly unrealistic - i.e. here's a duct fan, if you can find the fine print the rated CFM is for no duct at all, and in real life the flow will be less, dictated by the duct resistance. They seem to claim about 4 inches of water (1059 Pa) for pressure, but not clear if that's dead-headed or actually moving air. – Ecnerwal May 13 at 19:22