Buddy of mine had downdraft issues and before buying 1000 bucks of new pipe.. he bought some kind of fan motor, we cut a hole in the pipe above the stove... no more downdraft, if the day is acting up, flip the motor on. Done.
ONly thing I wished the motor would have an even slower speed than its slowest... as sometimes it needs only a tiny bit of help.. and could run just barely...
Greenies can KMA
