Newly purchased home (December, northeast U.S.) has a high-efficiency furnace that vents to exterior of the house. When the furnace is on, the vent drips every few seconds, creating a puddle in the dirt next to the house. About a quart of water collects over the course of 12 hours. Had routine furnace service/inspection and technician said the dripping is normal. I am concerned about water pooling near the house.

Is this condensation due to normal operation or is something wrong?
If it is normal, what action, if any, should be taken to alleviate the water pooling?

Edit: Thank you all for the responses! This is all very helpful information.

  • 2
    The primary byproduct of natural gas combustion is water. However, the vent should be sloped upward such that condensation runs back into the collection point at the furnace. Do you have a photo of the vent exit?
    – isherwood
    Dec 19, 2018 at 17:17
  • 1
    And a quart of water is insignificant except from an aesthetic standpoint. Doesn't it rain where you are? Could you landscape the "dirt" with mulch or washed rock?
    – isherwood
    Dec 19, 2018 at 17:18

2 Answers 2


As isherwood notes, burning natural gas creates carbon dioxide and water vapor. Since high-efficiency furnaces try to draw as much heat as possible from the burned gasses, you're going to get water condensation, and that has to be dealt with explicitly.

As isherwood also notes, the combustion vent pipe should be sloped back to the furnace so that any condensate drains back into the furnace, and then into the furnace's condensate drain system (generally a pump into the sanitary sewer system). It sounds like the vent slope is wrong in your case, so that the condensate either accumulates in the pipe or drains outside. If it just drains outside then that seems like an annoyance at most; if it actually accumulates in the pipe then your furnace exhaust could be blocked.

Since you're in the northeast (as am I), with freezing winters, then this could get more interesting; you could get some serious furnace-cicles from your vent pipe, which may even block it in very cold weather.

In summary, none of this seems urgent or dire, but you should keep an eye on the vent outlet to see what the consequences are.

Edit: I checked condensing boiler installation manuals from Lochinvar, Bosch, Weil-McLean, Munchkin, Rinnai, Burnham, and Williamson-Thermoflo, and every manufacturer specified that the exhaust pipe must be pitched a minimum of 1/4 inch per foot back towards the boiler to allow drainage of condensate.

  • 1
    To me it sounds like they intentionally vented it to the outside this is regularly done in my area both on furnaces and hvac systems depending on the code for that area it may be a normal drain. The amount sounds within normal operation.
    – Ed Beal
    Dec 19, 2018 at 19:47
  • Manufacturers recommend sloping the vent back toward the boiler; see my edit. Dec 19, 2018 at 20:58
  • I am not talking about the exhaust vent but the condensate drain , I agree the exhaust needs a slope but then the collected water may drain on the ground from a tube in the bottom of the pan, the water has to go some place.
    – Ed Beal
    Dec 19, 2018 at 23:15
  • @EdBeal Sorry if I reacted strongly, but the OP was specifically talking about the exhaust vent, and your comment said "vent", so I assumed you were also discussing the exhaust vent. On the condensate drain, though, in freezing weather I don't see how you'd get it to drain outside without being blocked by ice. Dec 20, 2018 at 14:26
  • To me it was more about the puddle most folks don't have a clue on condensate what it is or where it comes from I was trying to use similar wording as the op
    – Ed Beal
    Dec 20, 2018 at 14:30

Other responders have correctly stated that the amount of water dripping out is completely normal. To handle the ground and to avoid mud, just use a downspout splash block to guide the drip away from the foundation a bit, or just put down some decorative gravel to let the water drip onto and gently soak into the ground so as to not make mud. You will get ice buildup in the wintertime, but the amount of water coming out should not impact any sort of foundation concerns.

Just make sure ice doesn't block the outlet, but with the warm exhaust coming from the furnace, ice blockage shouldn't be an issue as the warm exhaust should melt any ice that might prevent proper exhausting. Just keep an eye on things and make sure the air inlet and exhaust are clear of ice and snow, and you should be good.

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