Need help with venting design for new grow room

There is an existing exhaust duct that serves the hot water heater and the hvac heater in my basement. The existing duct is 6" - 8" in diameter and runs from the basement up through the house and out a roof vent. I believe it to be perfect for venting my grow room.

I am running into a problem designing the ducting so that the high cfm fan does not create positive pressure in the duct and push the heater(s) exhaust back into the basement. It is not really an option to vent the grow room exhaust any other way, except splicing into the existing duct.
With you help, I need to determine a way to do this while maintaining a negative pressure in the duct so the hot water heater and the hvac heater draft their exhaust correctly.

Here are a couple of set ups I have thought of so far...

Your input is greatly appreciated!!

Originally Posted by fluffygrrrl

bump bump bump

I would believe that if you had your fan exhausting below your heater it would only assist in pushing it up.... heat is going to rise... that's all your hot water tanks are basing their exhaust on is that the heat will rise... if you have a fan pushing air up as well it shouldn't cause a negative.... I would think that the hot air would just get pulled in and exhausted as well... who really knows... try something and if you test with a match above your hot water tank to see if you're getting a backdraft... this could also cause your hot water tank to flip a safety which would mean you might have to call the plumber....