I am having a 199 kBTU direct vented gas-fired combination boiler installed. Because of how the house is arranged, it's difficult to find a good place to vent the exhaust. The easiest spot is underneath a bay window. It meets code (3 feet above grade, 1 foot underneath the bottom of the window) but it worries me. Specific things i can think of are

  • Exhaust might seep in through bottom of bay window bump-out,
  • Exhaust will create condensation and mildew on bottom of bump-out, and
  • Billowing steam in front of the window when the boiler is on in cold weather.

The bottom of bay window comes out 17 inches from the wall, is just over 4 feet from the ground, and is 80 inches wide. Nothing is installed yet. All pipes will have to come out of the siding a foot and half above ground and snorkel up.

I don't have any experience with direct vent natural gas boilers so I don't know how much moisture is in boiler exhaust or how far it gets kicked out of the vent. Are these unwarranted concerns?

  • 3
    When our house was built 2 years ago they had to vent the powered propane vent at least 6 feet from any window according to code. Not sure if this is apples to apples though.
    – RetiredATC
    Mar 13, 2023 at 21:11
  • 2
    1 foot underneath the bottom of the window) but it worries me. it would worry me as well. What if the gas heater has problems with burning and creates carbon monoxide which is dangerous
    – Traveler
    Mar 13, 2023 at 22:17
  • Do the windows in the bay open or are the fixed? If they don't open, you're only going to get exhaust seeping through if you're also getting cold winter air seeping through.
    – FreeMan
    Aug 11, 2023 at 14:01

1 Answer 1


Generally speaking...

  • The exhaust from a power-vented appliance usually exits at high enough velocity to clear that distance.
  • The exit termination is probably nearly the 17" anyway.
  • Moisture is unlikely to be a serious problem due to the above, unless you have prevailing wind constantly blowing it back to the house.
  • Exhaust gas ingress is also not likely to be a problem unless you have a negative pressure situation in the home and there's a substantial leak at in that area. You'll have to evaluate other appliances and exhausts to assess that.

You'll have to look at this holistically and see what you think. It's hard to say from here. Talk it over with your installer and make the best exit plan for mitigation.

I will add that many a furnace vent exits under or near windows in general. We're not all keeling over or growing extra limbs.

  • 1
    The age of the home might matter for ingress. Older homes were not as well sealed as more recent builds.
    – crip659
    Mar 13, 2023 at 21:39

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