Ventilation question

My grow house was built in 1920 and has a nice basement that I am using. We had to replace the gas furnace and gas water heater and due to the newer, more efficient systems they changed the venting. In doing so, they put a 10" plug in the chimney where the furnace exhaust vent was, which is about 16 inches below the exhaust vent and there is a tube that extends further up the chimney.

I cut a 6" hole in the 10" cap and tried to vent using a 4" and 6" high output fans up through the chimney. Seemed like a perfect solution. Not so much.

What ended up happening is that there was a back draft the leaked through the edges of the cap, which due to the ducting coming out of it and the proximity to other ducting was hard to seat flush to the exterior of the chimney. It seemed to vent ok until the furnace kicked on, but being in Michigan that is a problem 5 months out of the year.

I was afraid this would be a problem as I recalled reading something about this before.

Any suggestions? Thanks in advance!!

maybe install a 6" duct damper, that way the flow of air can only go one way. do you think this would work for you. You might need to install a duct booster fan too.

thats just what i was thimkin as well ...

What fans specifically are you using? Due to the distance you are moving air, I would get a sizable 6" inline fan like an elicit, fan tech, or vortex. It's not really a matter of cfms. But one of static pressure or in other words....how hard the fans blows. These fans are around $200 but are worth every penny. Hope this helps.