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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

9 Answers 9

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

My top DIY that I've come up with:

  1. I sectioned the outside drain pipe and insulation so I can simply detach the sections if they clog with ice and take them inside to thaw in the tub/sink. Usually just the bottom piece. If the build up looks bad then I do #2.

  2. I also extended the vent pipe and use a funnel to pour hot water directly in to the drain from inside the attic. This is more of a prevention flush that clears out the ice build up before a total blockage. Works well with #1…

  3. Unhook the hot water from the washing machine, attach a hose and simply spray he outside drain with hot water until it melts. My least favorite because the garden hose needs to be thawed first too and detaching the washer takes some work. I use a ladder to hold the hose in place so I don't have to stand outside too long.

  4. As an emergency only, I split the PVC right out of the bottom of the furnace then attached control valve and a screw on hose attachment. When the outside drain pipe freezes, I extend the hose through the ceiling access into a nearby toilet tank. It's not pretty but it works instantly and it beats going outside in the freezing dark.

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Condensing furnaces are designed to be installed ONLY IN A CONDITIONED SPACE. Whoever installed one in an attic might need to get qualified to do heating work. Get the specific manual for the unit and see if there was something special that was supposed to be done to allow for that unit to be installed where it was subject to freezing temps. Again, the answers are always specific to the unit in question. If you provided this info you could get an ACCURATE answer.

Also, you CANNOT vent furnaces through the building sewer vent.

The venting and condensate draining is very specific to the unit and manufacturer. Check with the manual and see if the EXHAUST is installed properly. All condensate normally should drain inside the dwelling. You may need to insulate the EXHAUST pipe inside the building. You should have a separate condensate DRAIN about 3/4" diameter (could be larger or smaller, but probably not by much). Again, specific requirements apply to each manufacturer and to each model. There is a reason these come with DETAILED DRAWINGS of what can and cannot be done.

When condensate from a furnace or air conditioner cannot be drained where it is, it is possible to use a condensate pump to remove the condensate via a pressurized hose. This could dump into a p-trap installed to a drain pipe, but may not be pushed inside the trap, as an air gap must be maintained. Direct connection without a trap would cause sewer gas to corrode your heat exchanger prematurely, with resulting CO poisoning.

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I also have this problem. So far, the best solution has been to shine a desklamp with an incandescant bulb on the last couple of inches of the tube before it exits the house, and on the hole the tube goes through. There is only a very short length outside of the house. This has worked all winter -- until this weekend, when we have had unprecedented cold weather -- much below normal! I had to thaw the outside part with hot water and the inside part with a hair dryer on high heat.

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I have had similar problem, but my 3/4" condensate drain (not the 2" exhaust pipe) is the problem. My unit was installed with a heat pump & the condensate drain runs on the outside of the house from the attic inside of a large piece of downspout w/ the Freon lines. I have considered heat tape, but the manufacturers say that it is not to be used on drain lines & could overheat the plastic pipe.

One solution told me by an HVAC guys (if you have a crawlspace was to run the drain down an interior wall & then out horizontally, BUT, instead of putting an elbow under the house, put a T with an elbow on the back side of it. That way if/when the drain freezes on the outside, the condensate will drip under house until it thaws & then resume dripping outdoors.

For the time being, I have just put a bulb right at the bottom & MOST of the time this works, but it has frozen a couple of times each year the last 2 years. I have found that blowing the water out of the a/c portion of the trap allows a small amount of warmer air to come out the drain when the unit runs & this also is a help.

My biggest problem has been when it was cold AND windy. Single digits on a night without wind followed by a sunny day & no problem. But let it get near 10°F with a stiff westerly wind & it will freeze even with the bulb. Then I had to get the hairdryer & blow air up the downspout until the ice slid out the bottom.

I cam here looking solutions & haven't found anything great yet. I think I will try the pipe down interior wall trick this spring, so tired of this on the very coldest nights. BTW - I image the furnace IS your emergency heat so switching to that would not help in that case. If another source is the emergency heat, it should help.

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I just had this very issue happen to me. It's been a cold few weeks and the drain going to the outside of the house must have started backing up slowly. The drain pan ended up freezing with a top layer of ice and then water spilled into the ceiling.

Anyway, the answer to your heat cable issue for the drain pipe is the "easy heat freeze cable". You will need to get the 10802 connecting piece as well (there's a guy on ebay called mobile homes that sells you the whole unit crimped out at different lengths -- doesn't come with the thermostat). Couple that with a thermocube which turns on at 35 and off at 45 (f) -- should solve the problem. The thermocube also has one rated to go on at 20 and off at 30 (f) but I think that is risking it a bit. For 25ft of cable, it's taking about 60 watts when on.

As a added measure, I installed the diversitech float switch.

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The system in our new house is setup so that in the winter the condensate from the heater drains through the washing machine drain.

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Had same problem this winter, heat man installed vent in plentium to blow on drain pipe. Installed T joint above drain end so if freeze occurs it would drain in crawl space.seems to have worked well !!Open or close vent as needed.

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Simply put, moist air from anything can build up condensate. If this moist air or vented furnace vapor is exposed to freezing temperatures, eventually it will form a build up. The temperature inside has to be greater than freezing air around it, or it will build up a frosty ice cube.

Pay close attention to insulating heat exchange if the furnace is not running to full duty cycle. For example, if the furnace runs 2 times a day for 20 minute cycles, the entire PVC pipe will be frozen from the outside of the house in. However if the furnace cycles for 15 minutes 24 times a day, frozen condensate build up will be less of a problem.

The solution is to remove as much condensate while it is still in vapor and liquid form inside the warm house, and inside the warm parts of the pipe by providing a condensate drain inside the house as close to heater as possible and slope all pipe in a up direction from that point, until the pipe reaches outside. This will allow vapors to condensate inside the pipe where it is warm and drain to the condensate drain point where it is also warm and not frozen. A simple coil or loop of the actual condensate drain tube will prevent forced gasses from the power vent to force burnt gasses into the house. Water will condensate in this loop or coil and block the low pressure of the power vent.

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