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External Combustion Air Efficiency Gain

hughbucks Member Posts: 9
Looking to replace my 30+ year old oil boiler with something much more efficient.

My contractor doesn't seem too keen on using external combustion systems. Is there any research done on the net efficiency gain of using external air vs basement air? I get that by using external air for combustion, I'm pulling some cold air into the boiler, but I'd rather pull it into the boiler than into my general living space.

If the furnace uses X cubic feet of air / hour, it has to replace that volume with cold outside air anyway, right? But instead of it being in a closed loop through my furnace, I'm sucking it in via windows, etc, or any other gaps right?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    You got it.
    Bringing in combustion air is the way to go, especially for a tight house.
    I'm still iffy on balanced flue designs (2 pipes-combustion air and direct vent thru the wall), but they are making headway.

    Following manufacturer's suggestions and best practices, a longer duct length will allow the combustion air to temper to room temps.

    There was an error rendering this rich post.

  • Zman
    Zman Member Posts: 7,605
    You are absolutely correct. I crunched these numbers a while back and will post it if I can remember where I saved it. When you compare the reduced efficiency of the lower CA temps in a sealed combustion system to the cost to heat the air passing through the house, it is pretty close to a wash.
    I agree with you that you are better off being in control of how the combustion air gets to the furnace as it will make the house more comfortable. The combustion should also be more consistent as will be unaffected by the building envelope and open vs closed doors and windows.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,887
    Is your contractor the same who's offering the EK? He should know its recommended.
    Usually an easy 2" PVC run with a union so the front can drop for service.
    It really shouldn't even be a question if theres a feasible way.
  • hughbucks
    hughbucks Member Posts: 9
    The original quote is for the Ascent which I don't believe supports external combustion; all the other furnaces do.

  • JoeHNJ
    JoeHNJ Member Posts: 18
    The Ascent can be piped with outside air if it ordered with the silent burner cover.
    Joe Harazim
    Technical Support
  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    While you are considering outside air for combustion and otherwise tightening up your building envelope, a word of caution: you must have adequate air changes per hour in the building -- 2 to 4 is sometimes quoted -- or you will have indoor air quality problems. If the envelope is tight enough, you will need to install sensible heat air to air heat exchangers and the associated fans and ducting to maintain that.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England