In the “learn something new every day” category.

The furnace doesn’t kick on much during the summer here in the PNW, and for 4-5 months of the year we just heat with the waste heat from appliances and electronics, controlling the temperature mostly by opening windows. Locals know the drill. Well, with fall rolling around, eventually it was time for the furnace to kick on and move a little warm air around. But the spousal unit pointed out that it was still a tad chilly in the house, even after turning the thermostat up.

Hmmmm… A quick investigation and experimenting showed that when we turned the thermostat up, after a while the furnace would turn on for a short while, then back off. If we just turned the blower on to circulate air it appeared to run fine. I didn’t get any whiff of gas in the garage like when the igniter broke. So I sat at watched it for a while. Ah, after it cycled, a “status” LED would flash, indicating it… had a problem with venting? But the gas water heater, right next to it, shared the same vent and IT was working fine; that would seem to rule out a vent blocked by a bird’s nest over the summer or things like that. Sooo… I figured it must be something inside the furnace, or perhaps a faulty sensor. So I called a furnace-repair guy. The weather was fine, so there wasn’t any hurry.

He showed up, hit the re-set switch for the venting sensor, and it appeared to kick on just fine. Wow, if that is all it was, it was going to be an expensive button-push. “Hmmm,” said the furnace guy, holding his hand next to the top of the gas hot water tank right next to the furnace. He stuck his “mirror on a stick” into the water-heater’s top venting gap. It fogged up slightly. “Hmmmm,” he said again. He takes a minute to pull off a bit of the venting duct-work from the furnace (after turning it off, of course), and eyeballs the tube.

The basic configuration of the flue vents are a “Y” shape on a modest angle that hook together at a larger pipe, then go up to an elbow and turn up inside the house to rise to the roof vent. After pulling off the pipe going to the furnace we could look directly to the elbow… which was literally looking like the slope of a sand dune, half full of a white powdery stuff on a perfectly even slope, blocking about half the pipe. Wow! Where on earth does THAT come from? Were some of the neighborhood pigeons hiding their stash in my flue?

So the furnace guy vacuums it out, and everything works fine. Wonderful. Great. Marvelous. A house-call by a furnace guy that requires buying no new parts, minimal labor, and not a lot of time – a best-case scenario in every possible way. Except for leaving the questions of what happened, would it happen again, and what long-term problems was this a symptom of?

His brief explanation: stuff condensing from the burning natural gas built up over time. The water-heater still worked fine because it was a lower BTU rating and a hotter exhaust gas temperature, thus needing less venting capacity, but it would have eventually done the same thing. When he reset it, it worked OK because it was only the furnace on at that time, but only marginally. It would have tripped again the next time both water heater and furnace were on at the same time.

Again, what WAS that stuff? Pure natural gas is just methane (carbon and hydrogen), burning oxygen to make water and CO2 – not a lot of white powder in that formulation. Saying “condensation” leaves me scratching my head – water condensing is a liquid (which galvanized pipe is OK with), and CO2 condensing would be, er, rather surprising.

So a bit of web research turned up this: when you have incomplete combustion (and it’s never perfectly complete, of course), then the products include a bit if nitric acid (from atmospheric nitrogen) and sulfuric acid (from the sulfur present or added to make it smell bad so you can detect leaks), which are, of course, acidic. It can react with the galvanized metal of the exhaust vent IF the rising gas is cooling too fast (and therefore loses buoyancy too fast), allowing it to condense out inside the vent. It means there is a venting problem, though it took be a while to find out what exactly “venting problem” meant. If venting is really bad, the combustion water vapor can condense out, meaning you get water running back into the system causing rust in the furnace. If it’s next to a masonry wall, the acid gasses condensing on them can also react with the cement and bricks, and weaken/degrade that as well. Interestingly, it appears that it may be more a problem if you put a high-efficiency furnace in where and older, less efficient furnace is, because its exhaust gasses are cooler and will condense out more easily. (But don’t take that as definitive – I’ve found a lot of posts saying “it’s a venting problem, call a qualified HVAC guy!” without specifying what exactly the venting problem is; I’d have never guessed that the solution to a venting problem might be that you need a SMALLER vent pipe so the exhaust gasses rise faster and don’t have a chance to condense.)

So, the flue vent is losing is galvanization, slowly but surely. It needs to be better insulated, or eventually it will need to be replaced (that sounds expensive). Or perhaps there is some other option someone out there in the magical land of the internets can offer up to mitigate the problem.