The story is that a big blower on low will be better than a smaller one on high. It's the physics. Those bends are very important too, as in the length of your run. Variable speed is nice. Blower in the attic is a lot quieter than blower right over your head.

I have 1200 cfm right over my head, blowing right out of the wall. I only crank it up full when I have 4-5 pots going full blast, but I've never felt like it was too much.

A $100 savings doesn't sound worth stinting.

Saw your post on kitchens. I chickened out and went lower on CFM due to MUA issues in our area. If you live in an area that doesn't require or need MUA - go bigger. Otherwise, check on the cost of a MUA system before choosing.

BTW- the appliance dealer gave me the heads up on MUA and our GC said that recently the enforcement has been very strict.

It would be nice to have more CFMs but I didn't want to increase the costs that much.

Most northern climates require MUA above 300 CFM.

Good luck

Instead of "The story is that a big blower on low will be better than a smaller one on high," I would prefer to say that the story is that a larger flow rate blower on low will have lower noise than a smaller one on high. This is due to reduced blade tip velocity and hence lower turbulence. Noise is a principal driver of hood blower design. Otherwise, we could use screaming-mimi axial fans with much smaller duct sizes.

For best noise reduction, if there is space for it I would put a silencer in the duct between the blower and the hood. See Fantech for silencer options.

All blowers have a "zero static pressure" rated flow rate (CFM) that is measured with the blower effectively hanging in the air. When included as part of a ventilation system with other components causing air friction, there will be pressure losses. The greater the pressure losses, the lower the flow rate. The performance (flow rate) versus pressure loss is usually plotted in a "fan curve." Kitchen hood blower fan curves generally bend monotonically from the rated CFM at zero pressure loss to zero CFM at some maximum pressure loss, e.g., 2 inches of water column (w.c.). This behavior leaves the fan stable at all settings, i.e., the fan speed won't "hunt."

Your pressure losses will mainly be from:

• getting air into the kitchen from the outside

• getting air through the hood baffles

• getting air through the roof cap (with in-line blower use)

• getting air through the duct bends and hood internal transitions

• (optional) getting air through the silencer

This list is nominally ordered as most to least pressure loss without a make-up air (MUA) system; that is, just depending on house leakage or open (but screened) windows. With an MUA system, the first bullet may move to a different location in the list.

Your nominal 1000 CFM blower may pull only 400 CFM using house leakage, and maybe 600 with an open screened window.

There is a hazard here. Pulling down the house internal pressure below 0.03 inches w.c. can back-draft combustion appliances and pull carbon monoxide into the house. The pressure loss across the baffles will be greater than this at higher speeds. Hence, an adequate MUA system for the flow rate that is desired is necessary unless there are no combustion appliances pulling air from an air volume connected to the kitchen, and an MUA system is not required by local code.

How much net CFM do you need, you might ask. My answer would be not less than 90 CFM per square foot of hood aperture. With decent MUA, this should be achievable with 1.5X that value as a rated static pressure flow rate of the selected blower.

kas

Thanks, Kas. I tried to say it right, and knew I wasn't doing it well, so figured I'd mention it and let you say it correctly. :)