It appears that our exhaust fans that draw in cool night air are also creating negative pressure, and that's affecting the gas water heater draft. Down cellar, I felt a LOT of spillage from the draft hood.

That inspired me to go look at the installation manual that I got with this WH, which (urk) I sort of didn't bother to read at the time (ack), and what I found concerns me even more.

I can probably deal with the draft problem for the summer just by keeping the a cellar window or two open.

But the manual says that the WH shouldn't share a flue with a power vented gas furnace. And that's exactly how it's set up - it shares an old masonry chimney (I think it's lined, but not sure) with a mid-90s vintage 80% efficiency power-vented furnace. That's the way the old WH was installed, and when I installed this one a few years back, I just popped the old flue pipe right on top of the new WH.

What do the experts here recommend? Is this a serious situation? What can happen when you connect a draft-vented WH to the same flue as a power vented furnace?

Do I need to consider a different WH? I'm really not interested in a tankless WH, but I might go for a power vented WH, if I can vent it through the sill with PVC. Especially if it's a two-pipe or coaxial direct vent (drawing combustion air from the outside).

If I do go that route, what kind of pitfalls am I in for with a direct vented WH? Any recommendations as to brand and model? Where would a DIYer find a PVWH or DVWH?

Thanks!

"What can happen when you connect a draft-vented WH to the same flue as a power vented furnace?

Not much.
It will probably only kill you.
------------

You're only option is to install a 2nd flue for the water heater or install the power vent.

Make sure you pull a building permit and have the work inspected this time.

But ... really? You're supposed to pull a permit to replace a WH with a functionally identical unit? Where the installation outline is essentially nothing but disconnect the pipe unions, screw the hot and cold nipples out of the old WH and into the new, rejoin the unions, and switch the flex gas line over to the new WH?

I never knew that such a simple and straightforward job, with no structural change, required a permit.

I learned several new things today. Thanks.

Your mid-eff furnace will have an inducer motor that is there only to overcome the resistance when it is initailly creating draft. It induces draft, it doesn't create pressure in your venting, which will spill out your HWT. If you post your furnace make and model, we can confirm this. You can have a mid-eff furnacce and HWT share a common vent. It is perfectly legal and safe.

If you have a newer built home, or one that is sealed well, or your appliances are spilling, you should have combustion air brought into the area with the furnace and HWT. It is as simple as running an insulated pipe from outside to inside. The pipe must be sized properly.

Splillage could also be cause by a blocked vent, oversized chimney, undersized chimney, or a deteriated chimney. These are all easily diagnosed by a HVAC proffesional. You should call one ASAP. Your life may depend on it.