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Last night, my heat (forced air) suddenly stopped. Today, a plumber figured out that my heater is 90% efficient, produces lots of condensate, and the drain line had frozen. We went outside and saw the drain line was about 2 feet long of 2" PVC, mounted vertically about 6 inches off the ground. He sawed off the bottom 9 inches which had frozen, the water trickled out, and the heater started working again.

Problem solved for today... but it may freeze again, soon. So the question is: How to fix it?

The plumber recommended rerouting the drain to use the sewer vent, despite it being against code. But I don't want to do that. To me, it would make more sense to insulate, heat, shorten, or widen the drain pipe so that it doesn't freeze. It's only a trickle of water.

I'm a beginner here. Here are my ideas. What do you think of them?

  • Shorten the pipe outside, so that there's less area exposed
  • Widen the pipe at the end - if the ice starts at the end, from the drip, expanding it to 4" might work
  • Insulate or heat the pipe using heat tape or something like that (which I don't know anything about)
  • Something else?


  1. Do I need to do anything to the part of the drain that's inside? The system is is my (unconditioned, tiny) attic
  2. Is there a way to run the system in emergency mode, so that we're not all shivering, until we fix it? I read about this online but don't know how to do it


  • I live in Central New Jersey. The winter can get its cold spell, but nothing beyond that.
  • We ended up cutting off the entire run of the outside pipe. Now there's only a spout facing directly down. The spout is about 3 feet off the ground.
  • Do I have to worry about the water damaging things? The water trickles down my siding now. It's near an external AC unit and a window well for the basement window, but it doesn't hit either one of those. If I make it a bit longer, it will come closer to those, unless I make it much longer
  • Someone told me that the condensate isn't regular water, but is corrosive. Is that true?
  • I have an in-house humidifier. I shut it off for now, because I thought it would make the condensate worse. Is that true? Can I turn it back on?

Finally I'm really a real beginner at DIY. If I should a section of (wide?) pipe (with insulation) to it, can you recommend specific products and give me links?

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Do you have any other drains inside that you could tie into, e.g. washing machine drain, floor drain, or a utility sink? –  BMitch Jan 24 '13 at 22:22
@BMitch The only thing in the attic is the sewer vent. –  S. Robert James Jan 25 '13 at 19:19
Don't forget to think three dimensionally. Consider what's inside the walls below the attic. –  BMitch Jan 25 '13 at 20:36
This doesn't sound right. The two inch pvc should be the vent and should be sloped so that all condensate comes back into the unit to be drained from inside the house. The vent should not have a trap of any kind. I'm probably missing something, but I just wanted to make sure that we're talking about a drain and not a vent. –  Edwin Jan 26 '13 at 2:21
When your furnace burns, the exhaust gas has to go somewhere. In an old furnace, it goes out a metal pipe. In high efficiency furnaces, the gas temperature is low enough that it can go through pvc, which is good, because the low temp gas produces lots of condensate, which would corrode a metal pipe. As the condensate forms in the exhaust pipe, it should flow back into the unit, where go through tubes in the unit and then out the unit from a smaller pipe (3/4" pvc)to be drained. I don't understand why you would have a 2" drain pipe and want to make sure it's not your exhaust vent. –  Edwin Jan 29 '13 at 16:54

1 Answer 1

Any or all of your ideas will help, it largely depends on your climate and how long of a below freezing stretch you can expect. The drain outlet is supposed to be within some distance of the ground, 18" I believe, which is why it was configured that way. In areas subject to freezing, all piping carrying water should be run as far as practical inside the heated envelope, so the drop to within some distance of ground should have been done inside. If possible, rerouting the run inside, with only a very short run outside should solve the problem.

As BMitch suggests, routing the drain through trap would comply with plumbing codes and is the best solution.

Shortening the pipe could fix the problem entirely since the water may not have time to freeze. In frigid climates though, any water outside will freeze. The high outlet could damage you outside wall finish and be an annoyance to passers by.

Widening the pipe will require more build up before blockage, so if the freeze periods are not too long, this could work. In frigid climates, it will still block up eventually.

Insulating the pipe would probably be all that's needed, it has the same effect as shortening the pipe, without wall damage. Actively heating the pipe would certainly work, but is probably only needed in extreme cases. Heat tape that wraps around the pipe and is plugged in to an outlet, with a thermostat to turn off the current above freezing is a common hardware store item in cold climates.

You only have to worry about the attic if it gets below freezing in there. Attics, though ventilated with outside air, tend to stay a fair bit warmer than the outside. I would guess that unless you have good runs of outside temperatures consistently below 0F(-18C), you shouldn't have a freezing problem in your attic.

I'm not sure, but I suspect there is not an override. A blocked drain can cause significant problems if the furnace continues to run. By shutting off the heat, it is good incentive for the owner to solve the problem.

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Thanks bcworkz. I updated the question to address your questions. Basically, I cut off the pipe and only have a spout, which trickles down my siding. What's a good way to protect my siding without risking freezing? –  S. Robert James Jan 25 '13 at 19:27
NJ eh? I don't think your attic will be an issue, you could toss a min/max thermometer up there during the next cold snap to be sure. What kind of siding? Would not be good for clapboards. No matter the material, you could get mold and algae growing there too:( I think it may be corrosive, I'm not super familiar with high efficiency furnaces, but it wouldn't surprise me from what I do know. I understand this to be condensate from the combustion process, which uses outside air, so what you do inside has no effect at all. I think you should replace the pipe in the near future, (cont.) –  bcworkz Jan 26 '13 at 0:25
(damn character limits) no big rush, insulating it with whatever is readily available for the 2" size. Bigger shouldn't be necessary, but hedge by not gluing the connection to the existing. If it still freezes, pull it off and take it indoors. Carefully detail the insulation fit to the siding and around the ell for maximum effect. –  bcworkz Jan 26 '13 at 0:35

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