Hi everyone, I have a gas boiler heating unit, a gas space heater and a gas water heater in my basement that all vent their exhaust ducts into an old brick chimney running up the side of my 2-story building (the old fireplaces on the first and second floors have long been blocked off from the chimney). I want to add a bathroom exhaust fan to an existing bathroom in the basement, but have nowhere to vent it to except that same chimney. I was planning on making a separate penetration into the chimney below the gas ducts and installing a 'flapper' valve to stop backflow. Is this dangerous at all? Could I potentially backfeed the exhaust fumes into the basement? Additionally, will the bathroom fan work if the duct from it has to angle downhill to get into the chimney below the gas appliance ducts? Thanks for your help!
Do not do this unless you have a death wish, it is very dangerous especially since "the duct from it has to angle downhill". Put the vent out the side wall like a clothes dryer. You should never vent an exhaust fan of any kind into a chimney thats in use by appliances (water heater, furnaces, etc.) or fireplaces.