Traditional gas-fired, forced-air furnaces used metal venting, usually routed into a chimney stack, to exhaust the combustion gases created in the fire chamber. But in the modern high-efficiency condensing furnaces, special plastic pipe material (most often PVC, ABS, or CPVC) is used for air intake, exhaust gas, and to carry the corrosive condensation resulting from the combustion process. However, there has been a lack of clarity and accountability over approved construction standards for this piping material.
First, let's look at the different types of furnace systems that fall into this category.
Types of Vented Condensing Furnace Systems
There are two types of condensing furnaces—two-pipe systems the vent directly, and single-pipe systems that have non-direct venting.
Direct vent (two-pipe) system: The two-pipe direct vent system is most common in home heating applications. It provides a direct 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.
Non-direct vent (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 air infiltration is high enough to provide adequate volumes of unconditioned combustion air.
Why Does a Condensing Furnace Require a Condensate Pipe?
The electronic ignition high-efficiency condensing furnace has special venting and condensate drainage requirements. Because of the special heat exchanger technology used by a condensing furnace, heat is extracted from the fuel combustion process for a longer period of time, to the point where the combustion exhaust gases have "cooled" and condensed. The exhaust gases are depleted of heat until the water condensate drips out of the furnace's heat exchanger, and the low-temperature flue gases escape from a special plastic pipe, instead of being delivered to a chimney. The pipe for condensate often runs to a floor drain or other catch basin.
Providing correct venting is critical to the proper, efficient operation of the furnace. It provides correct drainage for condensate and helps maintain the integrity of the entire air intake and exhaust gas vent system. Improper installation of the furnace venting or condensate system can lead to a furnace malfunction.
Why Plastic Pipe?
Condensing furnaces are listed as CATEGORY IV appliances according to industry standards, identified in official documentation as ANSI Z21.47 (CGA-2.3), which require venting systems to be water-tight 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. As such, only special types of plastic are recommended by furnace manufacturers for venting and condensate drainage in a condensing furnace.
It is recommended that vent piping should be made from certain types of PVC (Poly-Vinyl Chloride), CPVC (Chlorinated Poly-Vinyl 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.
Industry Confusion Regarding 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.” Herein lies the problem. 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. In fact, 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."
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% more capacity to handle the excess heat before it reaches the point where it can fail.