With all the new technology that has made furnaces more efficient, which in return help reduce our carbon usage, one would think all is great with the future, when it comes to heating your home. Unfortunately, that is not the case. New issues arise when installing the new furnaces into homes that have a chimney built for older, less efficient furnaces.
What if you already have or recently purchased a new efficient furnace? Check the edges and bends of the venting pipes, is a white powdery substance forming around them? If so, this is called “Anhydrous Salt.” The meaning of the word (Anhydrous) is a Greek origin. It means to have no water, or no water of crystallization. It's what you get whenyou mix nitric acid with the zinc in the galvanizing used to coat the steel vents. (Same stuff that builds up on the zinc battery posts of a car). The amount of build up depends on how well the furnace is venting. If it vents well, you'll get almost none; if it's not venting well, you'll get a lot because there's more condensation occurring inside the vent and you've got more acid.
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 and if it has a chance to cool off too early, it will rain down (water leaking inside the vents) creating all sorts of havoc.
When I do see flues that rain inside, it’s often the result of poor configuration. Or the flues are too large in size and cool the steam faster before reaching the top to evaporate. Especially on colder sub zero days. Another sign of this, is if you see icicles forming around the flue cap.
The problem lies when the contractor attaches the smaller vents from the furnace to the larger (in diameter) original clay flues in the chimney. By being larger, it allows colder air to enter and in return cools the vapor before it reaches the top. The proper way to install the newer, smaller vents is to make sure they are inside the larger clay flues.
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 has locking rings at the ends. This is more high-tech.
It is also common now days for the flues to be PVC pipes and lead out of the side of the home, instead of going all the up to exit out of the roof.
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, if you are going to look and learn yourself, remember that flues get very hot.
