Home & Garden Columns

About the House: When Flue Gases Condense Inside Your Furnace

By Matt Cantor
Friday April 18, 2008
Fuzzy Flue Fortells of Furnace Failures?
Matt Cantor
Fuzzy Flue Fortells of Furnace Failures?

As you go for that morning jog ( You are jogging every morning, right? Immediately after that low-fat, lemon, poppy-seed, caramel muffin and the soy latte) you probably note amidst the quiet and still of the neighborhood that there are little puffs of smoke that come from the tops of every house and business. 

If you stop long enough to think about it (as a child might do more than we busy adults) you’ll realize that there are no fires in fireplaces and that, probably, there are no similar fires comprised of oil and smoke being generated in all those buildings. What all that stacks are actually emitting in our California landscape is mostly steam (I mention the locale because there are oil-fired devices that really are smoky in other parts of the globe). 

As though unchanged from the 19th century we are a culture full of steam generating equipment. While we’re not all running little steam engines in our houses, most of us are running incredibly simple heating devices that use burned natural gas to heat our water, cook our food and heat our houses. These devices have some requirements and some problems that are peculiarly endemic to this heating method and while much of this is beyond the technical capacity of the typical homeowner, you might be surprised at how much of this you can understand and how many problems you can begin to diagnose for yourself. 

First, let’s understand a little about what natural gas is. The gas that comes to your house is primarily methane. The same thing our bodies, and many organisms on earth produce as a product of digestion. It’s a small molecule just filled with energy and when combusted with a little oxygen, it produces lots of warmth as well as quite a bit of water vapor. That’s the steam you see above all the roofs. It’s not smoke at all, it’s mostly water vapor and carbon dioxide.  

That last part makes the exhaust a greenhouse gas and one reason we all want to turn off the furnace, turn down the water heater or turn off the dryer as soon as possible. The good news is that natural gas has less carbon output per unit of energy than all of the other fossil fuels by a goodly margin (30 percent less than oil and 45 percent less than coal) so we can feel pretty good about using CNG (compressed natural gas) for our heating. 

Now, back to some basic science and diagnosing your own heating devices. You may notice that when you cook in the kitchen that it steams up the windows. That’s the water vapor that the gas burners are producing. What may be less obvious is that your dryer, water heater and furnace are doing the same thing. If these devices are vented properly, you shouldn’t be seeing the steam (or the paint peeling off the wall, which actually happens sometimes when gas devices are poorly vented). 

We can use this knowledge to see if our gas devices are vented property. Let’s start with the water heater. If you have a water heater inside your living space (and it shouldn’t be reached through the bedroom or bathroom (e.g. bedroom closet)) you can check to see if it’s exhausting properly to the exterior by putting a mirror (I use my glasses) at the top of the water heater where there are air inlets just before the pipe goes upward. If you kick the unit on by running some hot water or by turning its thermostat up just a bit you can check to see if exhaust comes out of the inlets and steams up your mirror. If this is happening (and we call it spillage) you’ve got an exhaust leak and it needs to be fixed. 

By the way, it not just steam that’s coming out of the pipe. This can also include carbon monoxide, an odorless, toxic (and potentially deadly) gas as well as a range of other unpleasant hydrocarbons, so exhaust leaks are serious business. 

If you run your gas dryer empty, you can do the same test and just see if there are signs of moisture (again, use a little mirror or your glasses) around the outside of the device (especially near the vent at the rear). If you can get to your furnace, you can look at that same flue for signs of the same thing. While many furnaces don’t have “draft-diverters” (the inlets we noted on the top of the water heater) you still may see signs of spillage. 

Most furnaces have metal exhaust pipes and, again, these are carrying mostly water vapor and plenty of it. This vapor is hot when it first enters the pipe but if it has a chance to cool off too early, it will rain down inside the pipe creating all sorts of havoc. These gases also contain acidy impurities that like to eat metal and when its happening and you look in the right places, you can actually see it. I see it all the time. Sometimes so much that the exhaust pipes have actually fallen completely apart and the exhaust is just pouring out into the crawlspace below the house. If you can get to where your furnace flue is, look for signs of moisture. One of the clearest signs is a white powdery “precipitate” built up at the joints in the pipe. These are the impurities in the gas crystallized on the surface and they show us that there’s been water cooling and leaking inside. Sometimes they’ll also be lots of corrosion and you might just see a hole or crack or worse.  

All gas heating devices can be subject to these effects so it’s a really good idea to have an expert take a look at these devices every year. Still, looking and learning for yourself is a great idea as long as you remember to rely upon professionals for the final call and any work on a system like this. By the way, remember that flues get very hot. 

When I do see flues that rain inside or seem to spill, its often the result of poor configuration. Steam doesn’t stay hot for long when its asked to take a long trip on a cold day so the best flues go strait to the roof with a minimum of twists and turns. They’re also built of “double-wall” metal that acts like a thermos bottle and keeps the exhaust nice and hot for the whole trip. If you look carefully at metal flues, you can see that the double wall material has dimples where the inner and outer layers meet and locking rings at the ends. More high-tech. 

I don’t intend for grandma to use this article to diagnose her furnace problem and I always think twice before taking you down such a complex path but I still think that a little knowledge is a useful thing. You might actually see a real defect and take action or you might just be able to have a more fruitful discussion with your heating contractor the next time they come to check out your furnace. If you start out by telling your furnace guy or gal that you think that flue gases are condensing inside your furnace flue and that you’re concerned about the configuration of the flue, you can bet that they’re going to take this job very seriously (just as soon as they’ve found their eyeballs and put them back in their head).