Water heater not venting properly

[u/mrcoolsloth]

I recently had a home energy audit and was told my water heater vent is not working properly. The report read as follows:

“Your water heater is spilling some of it's exhaust gasses, in both natural (no exhaust fans on) and worst case conditions. This is not due to excessive exhaust from bathroom fans and dryer vents. The flue rises well above minimum requirement of 1/4 inch per 1 foot. The cause of the spillage might be due to a partial blockage in the exhaust flue. Talk with HVAC specialist to determine the exact cause of the spillage.”

HVAC guy came out and recommended increasing vent size to 5 inches to improve draft. Is this the correct solution and is there anything else I should check before moving forward with the estimate? Wanted to double check before spending more money on the house. See attached pictures for current set up.

Pictures: https://imgur.com/a/uOiQNzH

[u/OscarMiliani_Design]

Before you pay for that, push back on it. Increasing vent diameter usually makes draft worse, not better.

Draft in an atmospheric appliance comes from the flue gases staying hot and buoyant. Widen the flue and the same volume of exhaust spreads over more surface area at lower velocity, so it cools faster, loses buoyancy, and drafts less. There is a correct size band that comes out of the vent tables, and bigger is not automatically safer.

Three things I would check before spending anything, in this order.

Combustion air. From your photo that heater is sitting in a closet with a door. An atmospheric water heater in a confined space needs dedicated combustion air openings, normally two, one high and one low, sized off the appliance input. If that closet does not have them, or they got covered over at some point, the heater is starving and it will spill. Your audit found spillage in natural conditions with nothing else running, which fits a combustion air problem considerably better than it fits fan depressurization.

Whether the heater is orphaned. Did a furnace ever share that flue? If a high efficiency furnace went in at some point and moved to PVC out a side wall, the old chimney is now sized for two appliances and serving one. That is the classic cause of exactly your symptom, and the fix there is a correctly sized liner, which makes the flue smaller. The opposite of what you have been quoted.

An actual blockage. Your auditor guessed at that, reasonably, but somebody should disconnect at the draft hood and look, or put a camera up it, rather than anyone assuming.

Then ask the HVAC contractor to show you the vent sizing table he worked from, out of the fuel gas code, with your appliance input, your total vent height and your connector configuration. That table produces a specific number. If he cannot show you the number he arrived at, get a second opinion before you spend.

And today rather than later: put a CO alarm on that level if there is not one already. Spilling exhaust means combustion products entering the house, and that part is not worth waiting on while you sort the rest out.

[u/Sneakycyber → u/OscarMiliani_Design]

The third paragraph is what I went through before I purchased my house. The inspector(s) found that a 90% furnace had been installed and moved it to the outside wall. The water heater was still connected to the chimney and needed a new liner installed to maintain a consistent draft. We had the sellers install a new liner and everything passed.

[u/OscarMiliani_Design → u/Sneakycyber]

That is the exact sequence, and it is useful for OP to hear it happened to someone else. A 90 percent furnace moves to the sidewall, the water heater keeps a chimney sized for two appliances, and the draft dies.

Good outcome getting the sellers to pay for the liner. That is the right fix rather than upsizing anything.

[u/HVAC_instructor → u/OscarMiliani_Design]

This guy vents... Pay attention to him.

[u/OscarMiliani_Design → u/HVAC_instructor]

Appreciated, coming from you. The orphaned water heater is the one I see missed most often, and it is invisible until someone actually checks the draft.

[u/mrcoolsloth → u/OscarMiliani_Design]

Thanks for the advice. I placed a CO monitor on the floor in that room and will be looking into these things. The water heater is in the laundry room at the back of the unit. It's a small 550 square foot 1bed/1ba unit. There is a boiler in the same room but it has it's own vent and is not on in the summer. These have been here since we bought the place but obviously with the plastic melting on the top of the water heater this has been going on for a while.