When you burn natural gas perfectly you get carbon dioxide, water vapor, and heat. Here’s the chemical equation:
Of course few chemical reactions are perfect, and because we’re using air instead of pure oxygen and air is mostly nitrogen there will be some nitrous compounds created also, and potentially some carbon monoxide CO which is deadly. All of these flue gases need to get out of the house.
For a standard efficiency (80%) furnace with a metal flue it’s important that enough heat is going out the flue to keep the water in its vapor form – steam. Because if that water cools too much and condenses into liquid water then that water will drain back down the flue, and the flue and furnace are not designed to handle this liquid water. Plus the heat is what’s causing the flue gases to rise up and out of the house. Without enough heat the flue gases might not go up but instead could backdraft and spill into the house. That’s not safe.
This water will be slightly acidic (because of the carbon dioxide) and so it will cause some corrosion of the metal flue, which is probably made of galvanized steel or aluminum. Galvanized means that it has a protective zinc coating. So the slightly acidic condensed water will corrode the metal flue and leave zinc oxide or aluminum oxide, both a white powdery substance. Brownish staining means that the steel is rusting. You’re likely to see these stains at the joints of the flue where it can leak out.
The problems with this situation are that (1) the acidic water is corroding the flue and might eventually create a hole that lets flue gases escape into your house; and (2) if the flue gases can’t get up and out of the flue in the first place then they might already be backdrafting and spilling flue gases into the house; and (3) if this water drains all the way back into the furnace it will damage the inducer fan and maybe other things.
Fixing this problem can be hard because the cause isn’t always easy to pinpoint. It’s often caused by improper installation of the flue system – either too many bends or too far of a horizontal run or the wrong diameter size of flue pipe. The codes for combustion appliance venting include page after page of charts describing the proper flue set-up depending on all sorts of different variables. And it’s likely that whoever installed your furnace didn’t look at these charts, or if they did they might have misread the numbers. So the place to start when trying to fix this problem is simply to confirm that the flue system conforms to the code.
The codes consider such factors as:
- how many BTU’s of heat input is being vented
- how many appliances are being vented
- what type of draft is being used, natural or fan-assisted
- how far horizontally the vent goes before it turns up and runs vertically
- whether the flue pipe is single-wall or double-wall (B-vent)
- whether the vent runs through the house or through a chimney on the outside of the house
If this problem is happening just above the appliance then the best place to start is probably replacing the single-wall pipe with double-wall B-vent pipe. This is fairly easy to do and will probably help quite a bit. B-vent has better insulation properties so it holds in the heat better, preventing the water vapor from cooling and condensing.
The problem might be with the way that the flue vent is terminated above the roof. It might be installed in such a way that wind tends to push down on the top of the vent causing the flue gases to be trapped and unable to escape.
The problem might be that the gas appliance is short-cycling, meaning that it fires up only very briefly and then shuts down. This doesn’t allow enough heat to be generated to create a proper flue draft.
A little bit of this condensation problem is pretty common and not usually a significant issue. But you should have a good HVAC contractor evaluate your gas appliances and their flue systems and make the required repairs to help stop this problem.
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