PVC Vents for Condensing Furnaces

Since the advent of condensing furnaces, furnace manufacturers have been specifying  Schedule 40 PVC as the venting material they recommend. A good discussion of condensing furnace venting by Bob Formisano can be found at the HomeRepair site. Traditional metal vent material generally will not work because the vent gas on a 90%+ furnace is very wet and at a positive pressure. Other than stainless steel, metal vents would not last very long. Because the vent operates at a positive pressure, it must be sealed air tight. Traditional Type B vents are not air tight. With a negative pressure vent, the very small leaks in type B vents do not pose any problem. But if they were carrying pressurized vent gasses, even small leaks are a safety hazard. Since the flue gas temperature of a condensing furnace is much lower than a traditional furnace, plastic materials are feasible. So PVC seems like a good choice. It is definitely water proof and sealing it air tight is quite easy. The fact that PVC is inexpensive also makes it attractive. However, the standard referenced by many furnace manufacturers, ASTM D1785 for Schedule 40 PVC pipe, does not actually test PVC piping as a flue material. In fact, it specifically states that the standard does NOT cover its use as a vent for combustion appliances.

There have been concerns about PVC vent systems failing. Two sites I found that discuss failures and show pictures are  HeatingHelp, and Plumbing Engineering  Both sites discuss PVC venting systems on condensing water heaters and boilers where the PVC material turned brown, became brittle, and cracked or broke. In Canada, plastic vent materials must conform to ULC 636 – meaning no more “standard” PVC venting. It appears we may follow in the US. Some plastic materials made specifically for venting include UL 636 PVC, UL 636 CPVC, and a special polypropyylene pipe.

Here are a few links.
UL 636 PVC, CPVC
Polypropylene