ventilation question

[SquareOneBeads]

My husband and I are working on putting together a ventilation system for my torch and had a question about the size of the ducting. We want to tap the ventilation duct into the pvc exhaust pipe for our water heater which is 3" in diameter. My husband is trying to figure out how many CFM's we will loose if we have a 8" exhaust fan with 500 CFM's and over a 9ft span reduce it from 8" to 3". Should I have more than one fan?

Any help would be greatly appreciated.

Thanks,
Heather

[Dale M.]

My husband and I are working on putting together a ventilation system for my torch and had a question about the size of the ducting. We want to tap the ventilation duct into the pvc exhaust pipe for our water heater which is 3" in diameter. My husband is trying to figure out how many CFM's we will loose if we have a 8" exhaust fan with 500 CFM's and over a 9ft span reduce it from 8" to 3". Should I have more than one fan?

Any help would be greatly appreciated.

Thanks,
Heather

THE 3 inch PVC duct is to small. It is probably at "capacity" just venting water heater. I think this is a bad idea, prone to failure and unsafe.

You probably need at least a 6 inch duct for the 500cfm fan, 8 inch would be better. And it needs to direct vent outside. By putting a large cfm volume into a small 3 inch duct that already is being used for water heater (and sized to match water heater) you put your self in danger of blowing gases back through water heater and pumping the gases into the room you are trying to evacuate them from. What I am saying is what you are trying to do is not a safe solution... Bite the bullet, use 8 inch duct the whole way, make 8 inch hole in wall (or roof) and do it right....

Dale M.