I have a Electrolux 6-burner cooktop that has a maximum output of 70,000 btus. The store that I bought it from recommended a Zephyr Tornado I (600 cfm) for the ventilation system with 6" ductwork. I live in an old house and the old kitchen duct is a 3.25 x 4. This duct is obviously too small, so the only way to vent is horizontally through the upper cabinents. The way that is shortest w/less bends (1-90, 3-45's and end cap - 10' total length) will come out about 2' from an on demand water heater vent. Another route is to have 3- 90's, 2-45's, end cap w/18-20' total length will come out at the base of the house. The manufacturer recommends not exceeding a total of 100 equivalent feet.
Different companies have recommended anywhere from 6" - 10" duct work and yes or no to whether to have fresh air brought in.
I am confused. Any ideas? Also if you increase the size of the recommended duct work can you extend the length of the duct and will it make it quieter?