Backdrafting Water Heater Has Buildup

15 Year old Draft Induced Furnace and Natural Draft gas water heater tied to B-vent chimney. All done by builder 15 years ago and inspected by county. Occasional backdraft which was caused by unsealed return air ducts in basement causing negative pressure. Sealed returns and backdraft appears to be gone.

So today, HVAC replaced with 95% high efficiency unit no longer using B-vent chimney. Pumber comes out to sub-contract Nat gas connection to furnace. Goes to re-light standing pilot of HW and say WOAH... Refuses to re-light as HW burner has about a 1 inch build-up of debris, combo of grey matter and rust like. Removes draft hood on top and sees same build-up on internal baffles. Leaves WH gas off and suggests chimney sweep to inspect B-vent. I understand he doesn't want to risk re-lighting pilot.

Plan on removing single-wall vent pipe between WH and B-vent and brushing out, cleaning, and inspecting. Then plan on cleaning out baffles from top and burner from bottom. Once clean I would think safe to relight and test for proper draft using smoke candles or equivalent.

At this point suspect backdrafting caused build-up. Opinions??

P.S. I have CO sensor in basement 5 feet from HW and one each floor all conneected together. Never had an alarm....

CO2 isn't the problem, the detectors are CO (carbon MONoxide), which is the one that typically kills you! (course, too high CO2 can as well).

Once you clean everything, you may uncover holes. On a WH that old, it may be best to replace. Could be worth the try, but don't hold your breath.

I would replace it the tank could rupture in a week or 6 months and you
would be replacing after all that work.I suggest Rheem power vent.

heater

If I understand the original situation, you had forced draft furnace and a natural draft water heater connected to the same "B" vent. If that is true, then the forced draft unit would ALWAYS look for the easiest path, and that would have been down the water heater vent. The water heater could NEVER vent properly when the furnace was operating, regardless of any open registers. However, logic says that there was CO in the basement, and if there were return air vents down there it was being sent to the rest of the house.

Agreed

HJ: I agree with all you said. Plumber said should never have passed inspection, even 15 years ago. The HW was tied in 1.5 feet ABOVE the furnace. HVAC guy was pretty sure that for it to be close to legal, the HW should have tied in at the bottom with the induced draft furnace tied in 1.5 foot above. Now, only water heater is using B-vent so will will watch carefully. Plumber also said that water heater should have used barometric damper instead of natural draft hood for a combined application.

I had problem with backdrafting due to unsealed return air vents creating negative pressure in basement. When I opened door to basement you could feel rush of air into basement when forced-air furnace running and backdraft would go away since large source of air was available with open door. After sealing return air vents (only did this 3 months ago), backdrafting appeared to go away.....

As for CO in basement, probably there, but was usually just standing pilot which isn't enough to kill or even trigger alarm... Without furnace draft running, HW would draft properly. When furnace draft inducer running I'm sure most went up chimney and remember draft inducer draws air from room so any backdraft thru small HW pipe was probably drawn back in... So, alarm never triggered....

Can B-Vent Go Bad?

The "chimney" in this house is double-wall B-Vent. Have you pros ever seen B-vent corrode and break down??? Still trying to figure out the source of all the residue/debris in bottom of water heater....

Asked HVAC guy if furnace's heat exchanger could have been corroding/peeling away from the inside and he did not think so... Before old furnace was carted off, we had look at outside of heat exchanger and it appeared fine (no holes and no cracks)

vent

A barometric damper would have made no difference, nor would the location of the water heater connection. The induced draft furnace would create a positive pressure while it was running and would have taken the easiest route to an exhaust point. And the point would have been the nearby water heater flue. The CO would not have been from the pilot, but from the heater burner while the furnace was operating.