Ken Thomure
: First, I am not a plumber. The home I bought had the gas water heater installed with the vent joining the gas furnace flue. We had a carbon monoxide problem and determined that the water heater could not vent properly when the forced air furnace was venting, therefore the water heater exhaust overflowed and could not go up the vent pipe. I dicussed it with my plumber at work (restaurant business) and eventually installed an in-line fan above the water heater before it gets to the T. I also changed the angle of the T so that the flow is smoother. The fan kicks on when the fan of the furnace comes on, problem solved. Be careful of this as it seemed to be the only solution.
