Many of the direct vent high efficiency appliances are vented using PVC. Specifically Schedule 40 PVC. Many inspectors and installers are starting to notice discoloration on these pipes. Some appliance manufacturers do not recommend venting their appliances in specific types of PVC pipe. Some manufacturers supply and recommend using their vent kits. Often times these are metal vents. Some of the PVC pipe manufacturers have indicated that their pipe was not produced for this application. Here is a warning from Charlotte Pipe on their website:

Combustion Gas Venting

Failure to properly vent combustion gas may result in serious injury or death from carbon monoxide.

• Always install / use pipe or fittings as specified by the appliance manufacturer’s installation instructions to vent appliances.

• Never use PVC cellular core, ABS cellular core pipe or ConnecTite® fittings for combustion gas venting.

Almost all appliance manufacturers agree that cellular core PVC, cellular core CPVC, and ABS should never be used to vent any appliance, only solid core. The difference between the two is that the inside and outside walls are solid PVC (solid core). Cellular core pipe has an inner core that is cellular (or foamed) PVC, a material that includes tiny bubbles of entrained air. For pipe manufacturers, the main advantage of cellular-core PVC is lower cost, since it requires less resin to make than solid core pipe. Cellular core will also break down with high temperatures faster. When PVC pipe degrades from heat it may produce toxins. Here is a link to a great video from TOH: https://youtu.be/ylX2gfggu_o

 So what should a home inspector be looking for:

• The Plastic vent pipe should be stamped “Gas Vent Type”
• Ipex manufactures an approved CPVC flue gas venting pipe that is rated for 90 C (194 F).
• The plastic vent pipe should not be discolored
• Discolored pipe may also be leaking flue gases at connections
• Many approved plastic vent pipes are actually solid core CPVC which is rated for 90 C or 194 F.
• Schedule 40 PVC is only rated for 60 C or 140F
• Check the pipe; it will be stamped “Cellular Core”
• Solid core may be marked as such or as “Pressure Pipe”
• The PVC pipe and fittings should be from the same manufacturer
• When in doubt, check with the appliance and pipe manufacturer

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