Aluminum flue piping is one of the most common code failures we see. But why is it against code?
"Single-wall metal pipe shall be constructed of galvanized sheet steel not less than 0.0304 in. (0.7 mm) thick or of other approved, noncombustible, corrosion-resistant material."- National Fuel Gas Code 12.8.1
The National Fuel Gas Code book states that flue pipe materials must be not less than
0.0304" of thickness. Most aluminum piping is only about a third of the thickness at 0.012".
This material is easy to cut and easy to work with making it popular with homeowners and DIY'ers. This piping was designed for use with bathroom exhaust vents and are dangerous to use as flue piping. This type of pipe is easy to cut but it is also easy to break apart, especially the elbows. If you're not sure what type of material you have for your flue an easy test is using a magnet. Galvanized steel is magnetic and the magnet will stick to it. Aluminum is not.
There are other alternatives that are more DIY friendly such as double-wall flexible venting that are made specifically for venting water heaters and furnaces.
This is an example of a double-wall flexible vent.