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Furnace Exhaust and Fireplace Opening Sealing

jj1981 Member Posts: 1
So, I purchased a 100 year old home. I've had a chimney specialist inspect and clean the fireplaces and chimneys and was told that they are not usable. The previous owners had vented the oil furnace exhaust with no liner (furnace located in the basement) into an old chimney which runs through the middle interior of the house (at one point in the past was used as a coal burning fireplace). My question is, I have no desire to actively ever use the fireplace or have it rebuilt, but have so much cold air coming in and there is NO damper, so can I seal off the opening to that fireplace/wall area safely although the oil furnace exhaust is being vented into it? And should I be worried about carbon monoxide from the exhaust?


  • Jamie Hall
    Jamie Hall Member Posts: 21,099
    If the fireplace and the boiler share the flue... that's a no-no. You can have either the fireplace into the flue or the boiler. And yes, if they share the flue you should indeed be worried about carbon monoxide -- and fire, should anyone be so misguided as to try to use the fireplace.

    However. Are you sure there aren't two flues (or more!) in the chimney? There often are.

    In any event, the flue serving the boiler should be lined various boilers and fuels have different requirements for that, and I would suggest a good chimney sweep should be able to determine what is needed. If the fireplace goes into the same flue, it should be sealed off -- and again, a good sweep (who does masonry -- most do) can do that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,131
    I’ll assume the oil furnace is venting into its own flue and each fireplace had their own individual flues.
    If that is the case, then yes you can seal off all the fireplace flues.
    However, you can’t fire any fuel burning appliance into an unlined chimney per code, and most likely that flue/chimney is probably way too large. That flue for the oil fired furnace would require a properly sized/installed stainless steel chimney liner.