Draft fan & gravity vent

I know that I have run across this before and never thought about it, but yesterday I ran across 3 draft fan induced furnaces and a gravity vent water heater connected to the same flue. The flue size looked OK (was bigger at each connection). It got me thinking though, that couldn’t the draft fans cause a back draft at the water heater draft diverter. I don’t recall ever hearing that you cannot connect both kinds to the same flue.



Gravity vented appliances and induced-vent appliances can share the same flue system.

Shouldn’t the smallest appliance (water heater) be “above” the others? This one seems to be “after” the others, but not “above” them. I’m not arguing the point, because I don’t know, but it does look like the furnaces could push their exhaust down to the water heater.

As long as the appropriate fittings are used, and the vents are sized and sloped properly, there’s not an issue.

Do you know what the proper slope should be? I don’t recall.

1/4 inch per foot minimum rise. . .

Did you check for back drafting?

The flue pipe must be the diameter of the largest appliance plus 50% of the other attached appliances.

I don’t like the looks of galvanized gas pipe or the extension cord power supplies to the furnaces that are not protected. Or the number of unsupported fittings in the flue configuration.

Every one of the furnaces also had flexible hose through the enclosure too.

I did check for any obvious back drafting at the water heater when at least two of the furnaces were running.

Thanks for the replies.

I just saw your good question and wanted to comment.
The assist fan pulls from the heat exchanger and does not create positive pressure in the flue. Type 1 vent is considered non-positive. So the way I understand it, the exhaust is not being pushed. It’s being pulled, caused by the air pressure difference

Till it shuts off…

Gas code says “no dips” nothing about minimum rise, installer uses 1/4" per foot rule of thumb to ensure there are no dips.