Choosing the Right Flue Vents

June 21, 2019

Boilers and furnaces are engineered to efficiently extract the most possible energy from fuel combustion. After the energy is extracted, the waste product—flue gasses—must be exhausted outdoors. Several products and systems exist to perform this function, but how do we know which products provide the best performance and which to stay away from?

Boiler and furnace manufacturers dictate the size and length limitations of the flue pipe in their certifications. These are listed in the product’s installation manual and must be complied with completely.

For years, the gold standard material for high-efficiency appliances has been AL29-4C Stainless Steel. It carries the highest temperature rating of all the common materials and is proven in the real world. Its only drawback is a higher cost. As a short-term, cost-saving measure, some manufacturers certified their units to be vented with non-metallic (plastic) materials. The units pass laboratory tests, but the real world isn’t a controlled environment.

At present, a boiler manufacturer can list a material for use as a flue pipe even if the pipe manufacturer never intended it to be used in that way. Therefore, the pipe manufacturer holds no blame if problems arise, as they never state it’s a suitable use of their product.

PVC and CPVS pipe are an example of this. Manufacturers generally don’t list tables, charts, or limitations for their products as a hot gas transport material, but somehow boiler and furnace manufacturers are able to list it as an allowable flue vent material.

This is where independent certification becomes important. Underwriters Laboratory (UL) and other independent entities provide various standards to dictate performance requirements for flue gas venting applications. For example, if a pipe manufacturer references that its product is built to UL-1738, you know it is intended to be used as a flue pipe for a Category VI boiler or furnace. Because the product is intended for use as a hot gas transport, the manufacturer will be there to help if problems arise.

Polypropylene is a certified non-metallic product. It handles temperatures upward of 110°C—much hotter gasses than PVC—and carries the UL rating to prove it was built with the intention of being used in flue vent applications.

Want to know more about boilers, furnaces, and flue venting? Mulcahy Company has some exciting educational opportunities coming up that will prepare you to make informed decisions.