Twin wall flue system design, flue exit size

Flue exit size of the stove for the twin wall flue pipe system The flue exit size of a stove is the inner diameter of the flue spiggot that comes off the stove. The flue pipe goes inside this spiggot. There are standard flue exit sizes of 100mm, 125mm, 150mm, 200mm, 225mm. The inner diameter will tend to be a little over this to allow for sealing with fire cement.

Some old stoves and a very few new stoves are designed so that the flue pipe fits outside the spiggot. The spiggot will be slightly conically shaped so that when the flue pipe is pushed over it it makes a tight seal all around. In this case we will need the external diameter of the spiggot and again it should conform to one of the standard sizes as above.