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Is Pvc Acceptable for Gas Furnace Flue?

Is PVC an acceptable vent material for flue gases? Installing a new water heater or furnace can be tricky for the novice HVAC technician. There are many checks that need to be done and codes that need to be followed. While not being complete with the internals can lead to a harmful situation, one of the most important decisions that you will have to make is what material you should make the flue out of.

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Most furnace flues are made out of a metal material which is a proper decision do the high temperatures released through the flue during the use of the furnace.

Due to the cost of metal, some technicians have chosen to use PVC pipe as material for furnace flues as a less expensive alternative. Polyvinyl Chloride, otherwise known as PVC is a hard plastic pipe typically used in plumbing applications. Polyvinyl Chloride Pipe is the third most widely produced plastic in the world behind only polyethylene and polypropylene. Wikipedia quotes that PVC is “cheaper and stronger than more traditional alternatives such as copper or ductile iron.

It can be made softer and more flexible by the addition of plasticizes, the widely used Phthlates. In this form, it is used in clothing , upholstery, and electrical insulation. ” After inspections of commercial and residential installations, Inspectors have discovered that using PVC pipe can lead to destructive and possibly harmful conditions. It was noted that if the water heater scales up due to minerals in the water supply, it can cause the flue gas temperatures to rise which can lead to the fittings and pipe melting.

Melting of the PVC pipe can be very dangerous. If behind a wall or in the ceiling, a melted pipe can leak condensation and create mold growth behind drywall. Even more dangerous is the release of carbon monoxide gas out of a melted pipe. Carbon Monoxide is a lethal gas and can lead to a fatal situation. Manufacturers sometimes recommend using PVC for their installations. Research has found that although they recommend using PVC pipe, it is not necessarily safe and I would not recommend using it due to the melting hazard.

The funny part I find is that although there is not a standard in any of the codes in the United States for PVC or plastic combustion flue vents, most manufactures ask you to reference the U. S. code. There is a Canadian standard which allows temperatures of the flue to exceed the temperatures limits of the manufacturer of the PVC material. The Canadian standard (ULC S636) covers the venting of gas fired appliances. It classifies the temperatures of the venting systems in three different classes.

The fist class is a class one venting system for gas fired appliances producing flue temperatures of 135 degrees C (275 F) but not more than 245 C (473 F). A class two venting system produces temperatures of 135 C (275 F). A class three venting system is classified into 4 temperatures. 1) up to and including 65 C. 2) up to and including 90 C. 3) up to and including 110 C. 4) up to and including 135 C. I did a little research on a major manufacture of PVC and other plastics, JM Eagle, and could not find any PVC manufactures that can reach those limitations as gas flue temperatures can reach above 250 F.

In closing and after my research, I find that using Stainless steel for your gas furnace or boiler flue is not only essential but necessary as a safety precaution. Although it is a little more expensive, you will not have the melting hazard and the risk of the release of Carbon Monoxide. ———————————————————————————————————- References: http://www. plumbingengineer. com/may_11/code. php Is PVC an acceptable vent material for flue gases? By Ron George CPD http://en. wikipedia. org/wiki/Polyvinyl_chloride www. jmeagle. com