Furnace Flue Pipe | Furnace efficiency requirements

Furnace flue pipes may not be something you’ve thought about before, but they’re an important part of your gas furnace system. It’s good to have an understanding of what they are and why they’re important … and how they match to your furnace efficiency. A furnace flue pipe (also known as a vent pipe or simply as a flue), vents the exhaust from your furnace to the outside of your home. Furnace exhaust is dangerous and so having the right furnace exhaust pipe is important.

Different types of furnaces require different types of flues.

Standard Efficiency furnaces produce heat at 80% efficiency while high efficiency gas furnaces produce heat at 92% - 96% efficiency. (Furnace efficiency is also known as AFUE. For more on this see our video: What is AFUE?

If you have an 80% efficiency furnace, then you need a metal flue pipe. If you have a 92% or higher efficiency furnace than you should have a plastic flue pipe. This is important because an 80% furnace will put out exhaust at about 500 degrees Fahrenheit through the flue pipe. A metal flue pipe is built to withstand that level of heat and won’t melt, but the plastic PVC pipe is not. Plastic pipe is only rated to about 212 – 250 degrees Fahrenheit. A 92% or higher furnace produces condensation as a by-product and would rust the metal pipe, creating holes and then letting the exhaust. escape into the house. So, understandably, these flue types are NOT interchangeable.

Check out the video where we run a test to show you the importance of matching your furnace exhaust pipe to your furnace type. We’ve put a plastic flue pipe onto a gas furnace and run 500 degrees into it – simulating the heat of a low-efficiency furnace. What do you think will happen? Check it out! (This is not something to try at home, the furnace exhaust contains dangerous carbon monoxide. This is a very serious issue and a furnace MUST be installed by a licensed HVAC technician. This article and video are strictly for informational and demonstration purposes.)

Posted 12/9/2019