ours is run generally on the low setting. Its not common to have to turn it up to high

todds:

You have hit upon the biggest issue with residential make-up air (MUA), the variability of the exhaust flow that the MUA is intended to replace. For modest cooking and modest odor removal, low fan speeds may suffice. For broiling, frying, wokking, full flow may be needed. Commercial systems often just tune the MUA for replacement of full hood power because the hoods are normally used at full power.

Variable MUA can be a pain if one intends to adjust the MUA flow rate manually. Or a chill if the MUA is set to handle modest flow rates and the window is expected to accommodate the rarer high flow cases. An automatic corrective system that adjusts the MUA such that the house internal pressure is kept at a minimum differential with the ouside air is desirable. When mine is working I will be happy to describe how it ends up being constructed.

If MUA fan noise and excessive air heating in winter are not issues, you could imagine having the MUA set for maximum hood exhaust, and then installing a weakly sprung damper that leaks air to the outside when the internal house pressure rises above the external pressure by a small differential. This would keep furnaces, fireplaces, and water heaters from back drafting (the Code purpose of MUA) while also keeping the house pressure from pushing too much moisture laden air into the walls in winter. It would also aid the flow out of the hood.

To answer the stated question, I've run my 1500 cfm hood fan between minimum and maximum, as well as my 1000 cfm over oven fan between minimum and maximum. (Actual flow is probably 2/3 these numbers.) Bathroom fans and furnace can add a few hundred cfm to these values depending on their state.

kas

I needed more than 600 cfm, and the next size they offered was 1200 cfm (two 600's working together). That's internal blowers venting straight out the back, so there isn't really a drop off in amount of air moved like one would get for extended ductwork. There's a continuous speed control knob. I don't know what the lowest volume is, but I'd guess, comparing my previous hood, that it's in the range of 450-650 cfm.

If I'm just stirring up a simple sauce, sauteing an onion or boiling up steamy stuff, I run my hood pretty low. Partially that's for the noise, but really it's all that's needed. If there isn't any steam or smell going around the house, that's all I care about. For frying, boiling down stock, or cookingpalooza for the whole gang, I run it at medium high to high. Medium high is usually enough unless I have all burners going. If there's a correlation to how far I turn the knob it's probably around 900-1000 cfm.

I don't have a make-up air system. The experts didn't think it was necessary given the particulars of my house. There's little opportunity for backdrafting, though I have sometimes wondered about the pressure thing. When I have run it at full power, I have sometimes felt that the oxygen was being sucked out, but that might well be part neurosis and part from having dozens of people inside with the doors and windows shut and damp out (which is why the shut doors and windows). At that point, the gas burners and the ovens are heating the kitchen anyway, so I'll just open the little casements in my garden window (like wing windows on an old sedan). There isn't a lot of in-sucking at the windows, however, so I think it's more the people breathing the air rather than a real problem with vacuum/pressure. The house is insulated but not air tight.

Hope this helps.

The whole makeup air thing is a dilema since no answer truly exists for residential. My house was built in 1969 and even though I am renovating, air sealing as I go I will not have a tight house by any means. But, it is always better to control where that makeup air comes from and the primary goal would be to temper the air like an HRV or ERV would vs. just pulling unconditioned air from the outside affecting room temp, etc.

Kas, if you do end up with a solution I would love to know what you came up with. I am not sure what My makeup air requirements will be but I kind of need to plan during the renovation cause I am not ripping stuff out to accommodate later ;).

Ah, the benefits of living in a moderate climate. "Unconditioned air" doesn't present a problem. I didn't have heat last Winter (long story), and while it was a little miserable, I don't think the indoor temperature ever got below 57ðF. I understand the point of MUA, but don't really need it. :)

Todds, This article was discussed in the Kitchens forum. http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods You might find it helpful. The middle to end of the second page of comments seemed particularly interesting.