What Type of Venting Material is Required?
Condensing furnaces are listed as CATEGORY IV appliances according to ANSI Z21.47 (CGA-2.3) standards 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 of water and carbon dioxide which form carbonic acid and result 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.
Vent piping is recommended to 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 designed exhaust gas temperature. These different plastics have different maximum heat service temperatures. PVC has the lowest rating at 140 degrees F, CPVC has the highest at 194 degrees F and ABS is in between at a maximum service temperature of 160 degrees F. Pipe failure may occur such as sagging or leakage at sustained temperatures exceeding these recommended service temperatures.