How to Size a Flue Pipe

Written by y.t. lin
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When speaking of piping connections, the sizes on today's furnaces are fairly standard. However, the manufacturers can make small differences. The standard furnace vent opening is 4 inches in diameter but can be as small as 3 inches or as large as 8 inches depending on the furnace's size and the rate at which it burns natural gas. Here are steps to help you find out the size of your flue vent opening to determine the pipe diameter needed.

Skill level:

Things you need

  • Tape measure

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  1. 1

    Measure with a tape measure the vent opening, or flue hole, on the top of your furnace. The outside diameter of the pipe you buy should match the inside diameter of the vent opening.

  2. 2

    Measure the distance from the top of your furnace's vent opening to the exhaust hole outside your home. This is the pipe length you will need.

  3. 3

    Take these measurements to a hardware store or home improvement centre and request pipes that are made from galvanised steel. You also might need to buy pipe elbows or caps. It's easier to buy all the pipes in one location to make sure that all connections fit.

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