Located in the garage are my water heater and forced air heater (FAU). The FAU exhaust ducting is 5" in diamater and runs about 8' before connecting into a double insullated "T" that then continues via double walled venting up through 2 stories. The take-off at the "T" is 4", and is fed from the hot water heater. That hot water run is about 10' and uses single, not double walled ducting.

Problem: When both the hot water heater is no and the space heater (FAU) is on, is a slight backpressure that builds up in the hot water ducting. We tests air flow at the top of the hot water unit flue. I think if I change the "T" to a "Y" (Why), the venturi effect will eliminate that problem. It's very slight... but I just want to improve the situation.

Your thoughts?. but if I do that and can find the correct "Wye" (5x5x4), should I replace the hot water ducking with double walled ducting? How about the FAU unitl... should that be double walled also?

Orange Homeowner.

Double wall or what is called B vent needs to be used in all enclosed areas (hidden from site) for the flue pipe for any gas burning appliance. All flue gas is to exit the building to provide safe operation of appliances/equipment. Back pressure problems can be caused by many different items like lack of enough combustion air, to small a flue pipe, negative pressure inside of home etc. If the water heater is 4 inch and the furnace is 5 inch and you are stuffing it into a 5 inch flue you are asking for trouble since no matter what you do the flue is to small to start with especially since the garage is probably cold where the units are located causing a fast drop in flue temperature from both units. The minimum temp for flue gases must be maintained.

For more info read your manufactures install instructions that came with both pieces
of equipment.


One side note: You may end up having to use B vent for the entire length of the pipe for both units inside of the garage due to the temperature situation. This will not forgive the possibilities of having a main flue that is too small to take care of both appliances/units.