I just purchased a home, and I am looking to install a new gas powered boiler. The current boiler vents into 100 year old chimney.

To my surprise, there is an unobstructed view from the bottom of the chimney, viewed through the clean-out door, to the top. Shouldn't there be a chimney liner that attaches to my current boiler that obstructs this view?

Also, the current flue is secured only with masonry (see zoomed in photo). Is this a proper installation? Can I vent the new boiler the same way?

Chimney from check-out door

Zoomed in on current boiler flue

  • 4
    Old boiler, old flue, old rules. Liners are a safer, modern approach. Likely your new boiler should have one, though if you get a really efficient new boiler, it can be PVC pipe with the low-temperature exhaust of a condensing boiler. Or you can do that out the side of the house and delete the old chimney.
    – Ecnerwal
    Commented Aug 1, 2023 at 12:59
  • Or maybe drop a new flue pipe for the boiler down the chimney (assuming the needed size fits)?
    – Huesmann
    Commented Aug 1, 2023 at 13:08
  • 1
    I changed the question title to ask the positive question, as you asked in the body. Just for consistency
    – FreeMan
    Commented Aug 1, 2023 at 13:32
  • The new boiler should have instructions on the size of chimney you need. Good chance yours is too big for new gas boilers and will need a flue pipe to reduce the size.
    – crip659
    Commented Aug 1, 2023 at 14:21

1 Answer 1


Modern high-efficiency boilers produce more acidic exhaust gasses than old ones, and a standard chimney may not be able to handle that well. A lining may indeed be necessary.

Or, for some models, ignore the chimney entirely and "direct vent" out the side of the house; many want to draw outside air for combustion so you're already doing one such connection. Then you get to decide whether, if there isn't something else using the chimney, whether you want to close off the chimney to better seal the house, or even consider renovating to remove it entirely and recapture a few square feet of useful space.

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