Neg pressure in my house,what can i do?

What should i do to solve the problem?I have a room in my basement split in half which houses the veg and flower room.I have an 8" 750cfm exhaust fan in the flower room which exhausts the air outside(Its on a speed controller slowed to about half and ive messed with it several times with different settings).There are passive intakes between the veg and flower room.In the veg room i have a 6" 400cfm(not sure on exact cfm)pulling air from the second floor and into the grow.My problem is the neg pressure in my house pulling air down the furnace/hot water heater exhaust chimney.PLEASE HELP ME FIGURE THIS OUT!! Can i completely seal my rooms with no intake or exhaust and just use co2?And maybe run an exhaust fan 15min every couple hours?BTW my boiler/furnace and hot water heater are in the basement 20-30 ft away from my nearly sealed grow room(leaks very little air around the entry door).Please and much thanks to anyone that could help me out.Im seriously considering shutting this whole operation down completely but with the amount invested id rather find a solution.

*bump Anybody?

Exhaust in ur house for the grow room, I have my air dumping in another room.I just shut the heat off in it and let the light exhaust heat it. Works fine in my house

it seems you could somehow take that negative pressure and use it to your advantage with proper exhaust fans

Sounds like you just need larger passive intakes.