Traditional gas-fired, forced-air furnaces produce hot combustion exhaust gasses and therefore need metal vent pipes, or chimneys. In contrast, modern high-efficiency condensing furnaces exhaust much cooler gasses and need only plastic pipe materials—such as PVC, CPVC, or ABS—for their exhaust vents. Some high-efficiency furnaces also include a plastic pipe for intake area, and all types use a third plastic pipe to drain away the corrosive condensation resulting from the combustion process.
Because there are no universal standards for exhaust and intake pipes on high-efficiency furnaces, there is a lack of clarity and accountability over approved construction standards for this piping material. When installing this type of furnace, it's best to follow the pipe specifications of the manufacturer as well as any local building or plumbing code requirements.
Types of High-Efficiency Furnace Venting Systems
There are two types of condensing furnaces: two-pipe, or direct-vent, systems and single-pipe systems, which have non-direct venting.
- Direct-vent (two-pipe) system: The two-pipe system is most common in home heating applications. It provides a direct intake vent that brings outside air to the sealed combustion chamber with one pipe, while a second vent pipe provides sealed venting of exhaust gases back to the outside of your house. In a direct-vent system, you can readily see the two pipes emerge through the side of your house. Vents can also be terminated above the roof.
- Single-pipe system: The single-pipe, non-direct vent system is used where there is no real need for a separate combustion air intake vent. It provides a vent pipe for exhaust gases but uses unconditioned (not cooled or heated) air from the space around the furnace for combustion air. These furnaces are usually installed in unconditioned spaces, such as the garage, crawlspace, basement, or attic, where there is plenty of ambient air to supply combustion.
Why Condensing Furnaces Need a Condensate Pipe
High-efficiency condensing furnaces have two-stage combustion to extract as much heat energy as possible from burning gas. After the first stage, the hot exhaust gasses are cycled through a second combustion stage, resulting in exhaust with very little heat. This process creates condensate, or moisture, from the furnace's heat exchanger. The condensate pipe drains the moisture into a floor drain or household drain pipe.
Why Furnaces Use Plastic Pipe
Condensing furnaces are listed as CATEGORY IV appliances, which require venting systems to be watertight and gas-tight. The furnace uses an exhaust vent motor that pushes the exhaust gas through the vent pipe, creating a positive static pressure in the vent. The condensing furnace produces condensed exhaust gases that contain water and carbon dioxide, which together form carbonic acid that results in a corrosive condensate. Therefore, only special types of plastic are recommended by furnace manufacturers for venting and condensate drainage in a condensing furnace.
Suitable materials for the vent and condensate pipes include PVC (polyvinyl chloride), CPVC (chlorinated polyvinyl chloride), and ABS (acrylonitrile-butadiene-styrene) plastic pipe, depending on the furnace’s specified exhaust gas temperature. These different plastics have different maximum heat service temperatures: PVC has the lowest rating at 140 F., CPVC has the highest at 194 F., and ABS falls in between, at a maximum service temperature of 160 F. Pipe failure, such as sagging or leakage, may occur if sustained temperatures exceed these recommended service temperatures.
Confusion Around Industry Standards
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.” But this lack of specifications can lead to confusion. Although manufacturer instructions will list what types of piping is acceptable for their products, they leave it up to the installing contractor to determine which plastic pipe to use.
Ironically, even though the presence of plastic vent pipes has become synonymous with high-efficiency condensing furnaces, PVC pipe manufacturers do not recommend PVC for this application. Nor are there any official ASTM standards for plastic pipe used as combustion gas venting. Even when a furnace manufacturer does reference a standards agency and standard—such as ASTM D1785 for Schedule 40 PVC pipe—the standard is only for installation of the pipe. The ASTM D1785 standard for Schedule 40 (applying to plumbing drain piping) states “This standard specification for PVC pipe does not include requirements for pipe and fittings intended to be used to vent combustion gases."
Condensing Furnace Codes and Recommendations
Building codes at both the national and local level seem to defer to the furnace manufacturers to specify which plastic pipes can be used as low-temperature vents with their products. Recommendations aside, though, it is the installation contractor who ultimately determines which plastic pipe to use.
Despite the confusion, the safe practice would suggest the use of PVC schedule 40 pipes for the air intake vent on the furnace and CPVC for the exhaust flue vent, given its higher service temperature. That way, should there be a problem with the furnace that causes the exhaust temperature to exceed design, the exhaust vent system has almost 40 percent more capacity to handle the excess heat before it reaches the point where it can fail.