I am trying to figure out the purpose of a couple of pipes that are currently coming out of the house.

First, a couple of notes:

  • One-story house.
  • HVAC system is in the attic.
  • It is winter here. Running the heat. Using the water heater (which is in the garage). Not running the A/C.

The first pipe, shown here, I am pretty sure is the emergency drain line for the HVAC system. It is coming out from under an eave near the top of the house. Can someone verify that I am correct? It is not currently dripping.

Emergency Drain Line???

The second pipe is the one that I am not very sure about. It is actually dripping slowly, but steadily. I would say one drip every 3 seconds or so. It has been raining here, but I can't imagine this pipe having to do with draining rain water. Like I said, we do run the gas heater daily. And the hot water is, of course, used daily as well. The water heater is on the other side of the house in the garage. Any thoughts as to where the water from this pipe is coming from?

Not sure what this pipe is for

  • 2
    is it a high efficiency furnace?
    – Tester101
    Jan 23, 2012 at 19:38
  • Yes it is. (95% AFUE) Jan 23, 2012 at 20:14
  • Does warm air come out of the upper pipe when the furnace is on? If I had to take a wild guess, I would say the upper pipe is furnace exhaust, and the lower pipe is furnace drain line.
    – Tester101
    Jan 23, 2012 at 20:15
  • Can a furnace use this type of exhaust?
    – lqlarry
    Jan 23, 2012 at 20:20
  • 2
    @lqlarry High efficiency furnaces do indeed exhaust through PVC. Blew my mind the first time I heard it too.
    – Tester101
    Jan 23, 2012 at 20:51

2 Answers 2


High efficiency furnaces will make condensate. They run the air back through the hot exhaust to get more heat out. This also cools your exhaust enough to use PVC - that's what your upper line is.

In the process of cooling the exhaust, you might get some condensation. Typically that's collected in a little sump tank. When the sump reaches a certain level, a little bitty pump pumps it out - that's your lower line.

If it freezes where you are, you might want to insulate that lower line. If it freezes shut, your sump could overflow in the mean time. It's just a dribble of water, but still. It's a leak of water in your attic.


They both look to be condensation drip lines. A/C'S will create condensation, especially in humid times. The one coming out from to the ground could be from a waterheater, if it is close to the drain.

  • I could be showing my ignorance here, but, since it is winter, I am not running the A/C - only the heater, which would only turn on the furnace, right? Jan 23, 2012 at 20:28
  • I live in a desert and the temperature has been in the 90's last week, I didn't go there with my brain. But the upper pipe A/C is not dripping and the lower one is. That's why I think it is the water heater
    – lqlarry
    Jan 23, 2012 at 22:16
  • The overflow drain pipe from our hot water system runs about 5m through the wall (it was quickest exit along a wall), so "close to the drain" could mean "most convenient exit point at the time it was built" and not be all that close at all. Jan 26, 2012 at 18:07
  • If your hot water system is like ours, there might be a lever that releases water through the overflow valve for testing. There's no harm in doing that (in fact, it was recommended to do occasionally to help keep it clear). Jan 26, 2012 at 18:10

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