Lack of Accountability in the Industry
The International Fuel Gas Code states in section 503.4.1.1 (IFGS) “Plastic pipe and fittings used to vent appliances shall be installed in accordance with the appliance manufacturer's installation instructions.” Although manufacturer instructions will list which types of piping is acceptable for their products they leave it up to the installing contractor to determine which plastic pipe to use!
Ironically, as much as plastic vent pipes are synonymous with high efficiency condensing furnaces, PVC pipe manufacturers do not recommend PVC pipe use for venting combustion exhaust flue gases and there are no ASTM standards for plastic pipe used as combustion gas venting. Even when a furnace manufacturer references a standards agency and standard such as ASTM D1785 for Schedule 40 PVC pipe, the standard is only for installation of the pipe. In fact the ASTM D1785 standard for Schedule 40 (applications such as plumbing drain, waste and vent for sinks) states “This standard specification for PVC pipe does not include requirements for pipe and fittings intended to be used to vent combustion gases".
So the building codes seem to defer to the furnace manufacturers to determine which plastic pipes can be used as low temperature vents with their products. As I mentioned earlier, although manufacturer instructions list which types of piping are acceptable for their products, it is the installation contractor who ultimately determines which plastic pipe to use!
If it were me I would insist on using PVC schedule 40 for the air intake vent on the furnace and CPVC for the exhaust flue vent given the higher service temperature of CPVC pipe. That way if there was a problem with the furnace and the exhaust temperature exceeded design, the exhaust vent system has almost 40% more capacity to handle the excess heat before it reaches the point where it can fail. I look at it as relatively cheap insurance.