Smaller is better, maybe

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.

12 thoughts on “Smaller is better, maybe

  1. Power venting the exhaust, they put an inline fan into the vent, our hot water heater had this. I’d use this as a reminder to check you CO alarm(s), those rosey red cheeks folks may see you with may not be from the brisk autumn air.

    • Maybe – I’ve seen a couple of comments on posts about this problem that imply that might help, or it might make the problem worse: if it pulls a bunch of extra cold air through, it may condense faster and cause even more of a problem. A powered vet fan sounds right intuitively, but intuitively natural gas shouldn’t have anything solid condensing out of it, either. I honestly don’t know. I’ve not found a really good HVAC-type-guy saying “this is the solution, and here’s why,” just a lot of generalities. If that really is the best solution, it’s a pretty straight-forward one, and likely not particularly expensive in the global scheme of things.

  2. I have both a high efficiency condensing gas furnace and a high efficiency condensing tankless gas water heater; each has an internal fan to boost exhaust velocity, and because they extract so much heat from the combustion process they vent through (separate) schedule 40 PVC pipes (the water heater uses “pipe within a pipe” venting, bringing in outside air via the outer circumference and exhausting through the inner pipe, which means the fan is doing double duty). The furnace has a second condensate outlet – the first is condensate from the “A” coils used in the air conditioning mode – which connects to the main condensate line. The tankless water heater has a condensate outlet on the flange connecting the heater body to the PVC “chimney”. I Have seen condensate from the furnace dripping from the main outlet, but never from the water heater; I suspect that may be because the fan in the water heater audibly changes speed to match the amount of gas burned – one faucet on, the fan runs very noticeably slower than with multiple hot water draws operating. It could also be from the very much smaller amount of gas the water heater uses – the summer monthly bills rarely show even one hundred cubic feet of gas used.

    I didn’t think about it at the time of installation (both the furnace and water heater were new installations after I was able to have a gas line run to the house) but the HVAC industry touting the ability to vent high efficiency condensing natural gas and propane appliances through PVC may be more than “look how much money we’re saving you by not requiring an expensive brick chimney” and closer to “we use PVC for venting because: 1) we can due to the low temperature of the exhaust gases, and; 2) PVC resists combustion by-products better than metal or masonry.”

    • Hmmm. Good to know.
      The furnace is nearly 19 years old. I should get a proper inspection done, and if there are impending problems, they maybe going with the more expensive high-efficiency furnace (that could use a PVC vent run through the existing vent) might be a not-quite-so-much-more cost proposition. Only potential downside there is the water-heater vent is hot… More to investigate, but more to think about too.

      • Just thought of something else RE: PVC venting – instead of a 8″ chimney (or larger) in which hot gases rise by convection, the PVC vent pipe for my furnace is 2.5″ so gas velocity in the pipe has to be higher, requiring a fan. Could be the higher velocity reduces the time available for internal condensation. Not to mention the PVC pipe is much shorter than a chimney that tops out several feet above the roof. I’m curious enough now that I’ll research the velocity.

        As for PVC pipe inside your existing chimney, given the temperatures involved I’d guess there might be a lot of different routing options for a PVC vent; since it’s fan-forced, could be that even horizontal or near-horizontal would work.

        As for water heaters, I can heartily recommend gas tankless – smaller (they’re wall mounted), more efficient (no gas use without water demand), and important to me, 47 watts at 115 volts for normal usage, 88 watts at maximum usage, meaning I can run it off a 1500 VAC UPS – that translates to 2X 6 min showers/day for 12 days without needing external electricity. Only downside is a tank provides hot water instantly to the pipe upon demand, a tankless requires 15-25 seconds to deliver it. The “quick solution” is a small (2-4 gallon) electric tank water heater between the tankless and the hot water line. I elected to tough it out for the 15-25 seconds instead.

        I don’t know what electrical demands your present gas furnace has, but I replaced 60 amp/240 volt backup resistance heat (and 48 amps at 240 for the heat pump) with 13.3 amps at 115 V for the gas furnace (still have the heat pump, but HP-to-gas switchover occurs automatically at 40F outdoor temp, and I can select gas-only at the thermostat). That’s the difference between civilized heat and hot water from a Honda EU2000 during a power outage and portable kerosene heaters (the wood stove and a chimney for it is in next year’s budget….).

  3. I had the same type of problem on a furnace in a rental house I owned. The furnace would only run for short periods of time then shut off. I called my brother who had an HVAC license and he told me to press the reset button on the safety switch. I did but it still kept going off. He came over and tested the switch which was OK. Then he took the flue pipe apart and found a dead bird in it which had apparently fallen down the chimney and worked its way into the flue pipe before it died. As he explained to me the safety switch was there to make sure the heat and gasses were rising safely up the flue and out the chimney. He taught me a lot over the years.

  4. Using a smaller vent pipe should increase the exhaust velocity. It sounds like you’ve got time to figure it out, since I bet no one else cleaned that pipe in 19yrs.

    • No, it’s likely not a serious DO IT RIGHT NOW!!! problem, but it’s clearly been corroding away the pipe for the last 19 years, and will in all likelihood continue to do so. So, I need to insulate it well if I can (attic crawling next weekend), and see about replacing or mitigating the problem so it doesn’t turn into a “furnace venting into attic FIX IT RIGHT NOW!!!! problem.

  5. (ahem) Take your “high efficiency” furnace and add an electric heating element in the flue to keep the exhaust hotter (ahem).

    My solution, after detecting CO and later being threatened with some dire action such as eviction from my own house, was to rip the gas system out, trash it, and install an electric furnace. Problems solved.

    I liked having the gas furnace, but holy shit; some things are not worth the grief. Your gas furnace won’t operate during a power outage anyway, so it’s a matter of the capacity of the generator you’d need as backup. Now I’m thinking of a pellet stove to serve as a standby system. Some of them have battery backup as part of their design, and will run on a 12V automotive system.

    • The generator is hooked up to all necessary systems, so when we lose power I roll it out, start it up, and we still have hot water (gas), cooking facilities (gas range), heat (gas furnace), etc. Septic pumps are on the gen-set box, too, in case power’s out for a long while. The downside to gas, compared to electric, it pretty modest, and the price is good.

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