"Chuck" > wrote in message
...
> Going to have to install range hood with outside exhaust (through the
> roof) this weekend...
> I'm having new roof installed and want this in prior to new roof so
> I'm pushed for time.. My current hood is just a "blow steam and grease
> in the top of your head while cooking type...
>
> My length of ducting is well below the max for either.. 7ft of 7"
> round duct with no turns. Possibly needing a rectangular to round
> adapter depending on model of hood
> It's a 30" hood.. I see some with 220 cfm and 390 cfm for twice the
> money.. Is the 390 cfm needed?
> $380. for the 390 cfm
> $130 for the 220 cfm
> I wouldn't mind the 390 cfm as much if it's strong enough to pull in
> fresh air if something get's "funky" smelling.. and I open a couple
> windows..for a few minutes though I know it's not a whole house fan...
> I guess it's just sticker shock.. Nice hood and fan for %70 of cost of
> new standard range would cost..
> Thanks..

A 220 cfm is fine if you also install an air source near the stove (a run
from the outside to the appliance, usually on a side other than the vent
side, to provide make-up" air, with a "trap" in that line.)
If you use leaks in your house and the furnace air intake to get make-up
air, then the larger size is usually needed, since you won't be moving
anywhere near the rating because of extra "head loss".
(think four cubic feet vs six cubic feet per second in a 7" pipe - a LOT
of air IF there is air available over the stove. A vented stove typically
has around 16 cu ft over it, and a typical kitchen has 800 cu ft - and then
consider that if the fan actually exhausts 390 CFM, it has sucked ALL the
air out of the kitchen EVERY two minutes. Obviously, it doesn't do that if
outside air is not provided)

It is very important to make sure that your fan does not suck air
backwards through your heater and furnace vents - in other words, fans that
exhaust to the outside will draw the-air-they-exhaust from any opening - and
a vent pipe for a furnace or water heater, and a fireplace chimney, are
openings to that fan.

The make-up and exhaust-volume sizes should be sized together.

fwiw....

> Chuck (in SC)