Our HVAC unit is in the attic. It has a PVC pipe that goes down the wall to the bathroom sink, draining into the P-trap under the bathroom sink.

Since last year, during the summer when the AC is used heavily, the drain hose under the sink from the HVAC unit is icy cold and causes the black rubbery hose to sweat. This causes a smell of mold and requires a bucket to placed under the hose to prevent further cabinet damage:

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I have had plumbers and HVAC pros come out and they have replaced all the plumbing under the sink, and inspected the HVAC, but the problem persists.

In general, our house is often humid. There is mold around the exhaust vents. Often times it is more humid in the house than out of the house (Dallas, TX area).

If it gets over 90 degrees outside, the A/C never turns off and cannot cool the house below 75 or 76 degrees or so. It is during this time the cold water pours down the pipe and fills the P-trap with icy cold water.

Lastly, I had a general contractor suggest that my evaporator coils may be dirty. Since I've lived in the house (~15 years) I have never had them cleaned. Also, I have had problems with the filters being sucked in because the plenum was so tight, it bent the sides if the filters (even the 3" ones) and after that, they loose all structural integrity. You don't know until you go to change the filter.

What could be causing this sweating and humidity in our HVAC system?

  • 1
    It sounds as though your A/C is pushing a significant amount of cold air down through that pipe and into your drain - only to be vented outside. Does that A/C pipe have a U bend anywhere which would act as a little trap (filling with water condensed from the A/C) which could prevent the cold air blowing straight through?
    – brhans
    Commented Jun 14, 2017 at 17:46
  • Yes, it drains into the P-trap, before the bend, so the water fills the P-trap, and even some of the black rubber hose, which causes the sweating. Commented Jun 14, 2017 at 17:52
  • 2
    Wrap the black rubber hose in insulating tape. This will keep house air from coming in contact with the cold hose. Commented Jun 14, 2017 at 18:11
  • @A.I.Breveleri thanks for the suggestion. I'm interested in solving the root cause of the problem and addressing the moldy vent problems as well. Any ideas? Commented Jun 14, 2017 at 19:26
  • Can you post a picture of the hvac - specifically where the drip line exits? I'm thinking that putting a p-trap in the drip line inside the unconditioned space would stop air flow down the pipe. Commented Jun 15, 2017 at 13:53

5 Answers 5


The problem could be the high humidity in the house. Here, north of Pittsburgh, Pa. I run a dehumidifier any time I am using the A/C unit to help reduce the humidity. Most A/C units are oversized not allowing the unit to run long enough to reduce the humidity. If you live in a high humidity area the A/C unit will not remove enough of the humidity in your house to allow you to feel comfortable. I set my dehumidifier at 40% and allow it to run automatically. I bought a GE unit rated at 70 pints per day from Sam's club and it keeps the humidity in check. One more thing, the drip line should have it's own trap close to the unit to keep cold air from blowing down the pipe causing the pipe to become cold which in turn coupled with a high humidity in the house causes it to swet. I would probably allow the condensate drain to run outside the house if that is legal in your area.

  • Thanks for the tip. Could the problem be related to dirty evaporator coils? Commented Jun 14, 2017 at 22:13
  • 1
    Also, I'm no expert, but I would venture to guess my A/C isn't oversized. In fact, if it gets over 90 degrees outside, it never turns off and cannot cool the house below 75 or 76 or so. It is during this time the cold water pours down the pipe and fills the P-trap with icy cold water. Commented Jun 15, 2017 at 13:11
  • I forgot to address the collapsed filters. This usually happens when the return system at the air handler and the filter is too small for the size of the A/C system. If the size is too small the air velocity and the negative pressure that results can collapse the filter. The air flow increases to way above the filters air speed/flow capacity A 3 ton system will need a minimum 16" X 25"filter and a 5 ton unit will need a 20" X 25" filter and a good filter rack to hold the filter in place. Lastly, the removal and reduction of high humidity greatly reduces the units cooling capacity.
    – d.george
    Commented Jun 17, 2017 at 9:43
  • It is hard to give exact fixes without being there to see just what is happening to your system. If you actually have mold at the registers you must have excessively high humidity in your house. I would wash the area with a bleach solution to kill the mold and look around the house for air leakage into the house from outside or for sources of water/dampness inside the house. Also,, I have a roof fan to reduce the air temp in the roof area, just replaced my windows, serviced my A/C unit, added blinds to some of the windows, run my system fan 24/7. So please go buy a big dehumidifier or 2.
    – d.george
    Commented Jun 17, 2017 at 10:01

Drip line catches moisture from the condenser and routes it away from the HVAC. When I've seen HVAC installed in the attic the drip line has always been routed outside the house, not to an interior drain line.

Suggestion 1: Reroute the drip line to the outside of the house.

Suggestion 2: Put a P-Trap in the attic between the hvac and the insulated space in order to create an air barrier from the condenser to the sink so that cold air from the hvac can't push down the pipe and exit out your sink's drain into the bathroom.

  • "always been routed outside the house", agreed.
    – Mazura
    Commented Jun 16, 2017 at 23:27
  • Also, wow: nail on the head. @OP, is there a trap on the condensate line...?
    – Mazura
    Commented Jun 17, 2017 at 0:03

Based on your description I am guessing you do not have enough air flow and your filter is too small. If your coil was clogged the filter would not be pulled in since there would be limited air movement. If the filter is too restrictive it could be pulled in. Excessively cold condensate is a sign of insufficient air flow as is lack of cooling. I am just guessing since I don’t have enough info. You could try and remove the filter for a while and see if the condensate warms up a bit. If it does you probably need a larger or less restrictive filter. If the pipe is sweating it should be insulated but I am guessing based on what you said the sweating is a symptom of low air flow.


Your HVAC unit is in unconditioned space. Whatever is dripping needs to be insulated. It's either that or you can incorporate your attic onto your HVAC system - yeah, I didn't think so :)

That pic was probably there when I posted this, but for some reason I pictured it dripping in the attic. It's not where it's supposed to be. As to the system not being able to keep up, you should contract an energy auditor; someone who can tell you why your system isn't working right. Not a general contractor; not an HVAC guy: an energy auditor.

Assuming they're competent, those are people who can tell you what the appropriately sized system for your given house is, or any other reason why your system doesn't work well (like too many windows, low R-value house, overall heat gain/loss, improper duct work, ect.). And probably most importantly: save you money in the long run. Which might seem like an outrageous up-front cost (energy audits can be upwards of $500, and new systems thousands) but IMO economics and efficiency are secondary to a system being able to provide comfort.

  • I had a general contractor suggest that my evaporator coils may be dirty. They might, but that's not the problem - it's certainly not any problem if your AC works just fine. Clogged E-coils are extremely rare IME. You have to run it for at least a few months w/o a filter, depending on your air quality. If the system works well it'll be sopping wet in there. Most old coils I see are clean as a whistle.
    – Mazura
    Commented Jun 15, 2017 at 0:11
  • Oh, the contractor was talking about the mold by the vents. Uh, maybe. But only if he thinks it's because your AC can't dehumidify your house - but a little dirt on the coil shouldn't make-or-break your system. It's oversized like d.george said. If so, reduce the speed of the blower, or restrict the air flow.
    – Mazura
    Commented Jun 15, 2017 at 0:24
  • Reducing the blower speed and/or restricting air flow, can be tricky business. If you don't heat up the coil enough, you could turn it into a block of ice.
    – Tester101
    Commented Jun 15, 2017 at 11:13
  • @Tester101 - If the system has the proper amount of refrigerant, the coil can never go below 32f. Unless I'm missing something... ? During freak weather (tepid and humid)?
    – Mazura
    Commented Jun 16, 2017 at 23:19
  • I've never measured the temperature of a frozen coil, so I'm not sure how low it can go. If you don't add enough heat to the refrigerant, it's possible that it could not all boil. If the refrigerant is still a liquid when it reaches the compressor, it's not so good for the compressor.
    – Tester101
    Commented Jun 17, 2017 at 11:56

I had the same issue in Corpus Christi. Problem is your AC drain line coming in from the until through the attic and or walls/crawl space. The ice cold water in the non or partially non insulated PVC drain pipe (pipe probably not insulated in the wall) - hot air in the attic and cooler but still warm air in the walls of your home (most codes don’t require any insulation in interior walls. In addition free air flow though cracks, and every perforation in your homes envelope allows the hot moist air to interact with the 33-40 degrees water in the drain pipe causing excessive sweating at the dew point which is easily reached between these temperature extremes. If it’s bad enough you will even get significant moisture in the walls that will eventually leak into the lower levels of the home- mold, mildew, not to mention thirsty mice, rats, and roaches in the walls. Unfortunately for me the way I discovered was that the moisture in the walls went undetected and caused the plywood floor to warp around the toilet flange causing a leak from the toilet. Caused significant damage to my home- so it’s a bigger deal then most realize.

Fix: make sure drain pipe is clear- bleach once a month during the hottest months to kill cold water fungus that can clog these pipes and cause a slower than normal draining of the condensate produced by your AC, additionally avoiding an overflow situation in the pan- and eventually into the home causing additional damage. Put pipe insulation where you can get to easily- where you cannot- fill with Great Stuff foam insulation. Fill the space in the wall with foam also- this will keep everything insulated and dry. Forever fix- you need to control the flow of air from inside the home to the outside of the home. Closed cell foam insulation will seal the homes airspace and conditioned air from the convection and vacuum created by hot humid air in your attic rising and escaping from roof vents. It’s also a much easier environment for your AC equipment to work in making it more efficient lowering your electric bill. You will cure your condensation problems forever, your home will be more comfortable, and your electric bill will dramatically go down. You can do it yourself and save a lot of money, even do it in stages- do the floor of the attic first, then do the underside of the roof down the road to obtain the maximum benefit. There are sites that sell everything you need at better than box store prices. They are around $200 at your local HD or Lowe’s.

Hope this helps

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