My apologizes if I do not know the correct terminology for some of the parts. I will try my best though.

We have a Trane A/C system that is causing us a headache. It is causing water damage in our finished basement.

With our HVAC system the airflow is going downward through the unit, with the furnace on top, and the evaporator at the bottom of the unit. Our problem is that condensed water that is supposed to work its way down to the drain pan, instead fly off as random droplets of water and collect at the bottom where there is a large duct that goes from one side of the basement to the other where vents shoot off of that. So at the bottom of this duct the water slowly collects and eventually turns into a puddle and eventually it gets large enough that it will leak through holes in the ducting onto the drywall below it which covers the entire area (you don't see this duct). After time the water works it way through the drywall and then we have a mess.

The HVAC system was installed when the house was built in 2006 by a local company in the Seattle area that does HVAC systems. I believe this problem has actually occurred since the day that we moved in, but since in the Seattle area we do not get hot weather very often, and due to the fact the water had to work its way through the drywall, it took 2-3 years before the water worked its way through to even let me know the problem existed. I have since taken down the drywall around the leak and when looking at the drywall you can see numerous elliptical stains that look like they had been wet and then dried, that is my main reason I think the problem has been happening for a long time.

Unfortunately after 2 years the builder's warranty is no longer in affect, which I feel is unfair due to the fact this probably was most likely happening since the day I moved in. With that said we called the original installer to come out around two years ago to try and fix the problem. Initially they thought it was the drain tube that was clogged up, which it was a bit. So he cleaned that all out and I thought the problem was fixed. Sadly it was not, the problem immediately started happening shortly after their visit. I called them again to come out and this time he cut through on the bottom ducting to look up underside the evaporator and you could see that randomly every now and then a droplet of water seemed to have been flung off of the evaporator down to the hole where he cut. His solution was to cut a piece of sheet metal and put it underneath the sides of the evaporator a bit to hopefully collect the water and force it down into the pan where it would collect and then go out the drain tube. His solution did not work. His next statement I found hard to believe, but he said he thinks that dust is on the evaporator which is causing the water to not flow down properly into the dust pan. I looked up pictures of evaporators and to me this evaporator is about as clean as it can get. He basically said the next step would be to take the evaporator out and clean it for around $1000, or buy a new evaporator.

My question to you guys, do you think he is on the right track? I feel he has absolutely zero clues which is why I did not call him back after this visit because I did not want to waste my money. When I look up diagrams of evaporators it seems like every single one has the air going upward not downward with gravity. That made me wonder if the problem might actually be how they designed this with the airflow going down which combined with gravity might be enough to throw droplets of water off of the evaporator to collect below on the ducting. I have an image of a diagram which shows this, but need to wait until I get some more reputation to include that.

If the airflow was going upward, I could see it making it very hard for droplets to want to fall down since the airflow would be pushing it to stay on the evaporator where it slides down off on to the pan. Do you think this is part of the problem?

I have an image of how ours is designed and will include that later.

  • 1
    It sounds like your question is more about dealing with the contractor then asking "how do I fix this myself". I don't think it's necessary to single out the contractor on this site. If you don't like what they are saying, why don't get you an opinion from another HVAC contractor?
    – Steven
    Jan 24, 2013 at 2:57
  • Many evaporators can be installed in upflow or downflow configurations. I think the "A" needs to be upright for both. Is yours? Jan 24, 2013 at 3:30
  • "do you think he is on the right track?" is a pretty vague question to answer. "Do you think this is part of the problem?" makes me wonder what, exactly, you mean by "this". I'm pretty sure somebody here can give you help, but it will be much easier for everyone if you could focus your question(s).
    – alx9r
    Jan 24, 2013 at 4:27
  • Sorry for the long delay in replying, it is summer now and as usual on the more muggy days it will leak and our bucket will catch the water. The "A" is upright and is just like that with water trickling down the side with 95% of it going outside like it is supposed to. It is the other 5% of random drops that randomly come off of the evaporator going down with the airflow and collecting in the ducts. When enough builds up in the ducts, it then comes out into the basement. When I say "do I think he is on the right track", I am referring to what I mentioned before with light dust as the cause.
    – Brian
    Aug 15, 2013 at 19:12

3 Answers 3


The condensate collection works by allowing the water to run down along the fins and collect in pans on either side of the 'A'. Upflow systems work a bit better because the air flow supplements the water's tendency to adhere to the fins, whereas downflow systems tend to encourage release. Typically, even downflow systems collect most of the condensate. One problem that can occur is dirt collects on the fins, causing the water's surface tension to break. Surface tension is what causes the water to run down along the fins instead of straight down.

Another problem, which is probably what you are seeing here, though the others may be contributory factors, is simply just too much humidity. With high humidity, so much water develops of the fins that there is too much mass to be supported by surface tension and the water simply drops straight down instead of running along the fins.

So part of the solution may be to mitigate the contributing factors. If you can cool with a lower air flow, that will help. Change filters frequently, which it sounds like you are doing. Reduce the humidity in the house. This is easier said than done. But short of installing whole house dehumidifiers, be conscious of lifestyle habits that generate moisture and try to minimize it, such as always using exhaust fans when showering and cooking. Clean surfaces with liquids during the cooler parts of the day. Try not to breathe too much :)

I don't think you will fully mitigate the dripping with such measures. Moisture happens, and it will end up on the ductwork. You should consider fashioning the portion of the duct on which the water falls into a wide shallow funnel to collect stray condensate droplets and pipe it off somewhere where it can be safely removed.

  • Thank you for your answer, you do seem to understand the problem and you describe exactly what is going on. So even with just a bit of dust your are saying this can contribute to the problem. I would think a slight dust would build up every month and the problem would reoccur right? Wouldn't that mean I would need this cleaned all the time if that is the cause, I don't think that would be economical. I did use Air Conditioner Coil Foaming Cleaner and a fin comb kit, and when through each fin with no luck. They look clean to me.
    – Brian
    Aug 15, 2013 at 19:27
  • As I mentioned in other comments, it does typically only happen on really humid days in the Summer. I can usually predict the type of day that it will leak on. The airflow in the system does seem very high, so maybe that is something work tweaking. Our home is 4000+ square feet, could be one reason they have the airflow so high?
    – Brian
    Aug 15, 2013 at 19:31
  • I have thought about doing something where it collects on the ductwork. Unfortunately this is in the basement and below all the concrete walls, so the only way out would be to go up against gravity. I am wondering if doing something so that it collects in a pool instead of running off through seams via the ductwork into the basement would be a good resolution? The amount of water that actually exists is probably fairly low, and any water collected would likely evaporate on the other 320 days of the year the problem doesn't exist. Would be against any sort of code?
    – Brian
    Aug 15, 2013 at 19:37
  • Maybe sealing the seams with Mastic Duct Sealant so that the water stays in the ducting? While not an ideal solution, is this something you think would be safe to do?
    – Brian
    Aug 15, 2013 at 19:37
  • Do not seal the duct, you are just hiding the problem. Dirt is a minor contributing factor, the real problem is too much moisture. I like the collecting it out of the ducts idea, but it's likely against code as condensate is supposed to be discharged outside. I'd rather it collected in a container which can be emptied rather than rely on evaporation. You could even setup a sort of small sump pump to remove any collected water automatically, in which case the setup would be code compliant.
    – bcworkz
    Aug 15, 2013 at 20:51

My office is below one of these units mounted in downflow configuration. The first seam in the ducting is just over the desk extension about 4 inches from the wall. For a long while, we had a permanent waste basket installation to catch water.

The shuttle valve failed twice locking the system into AC mode during winter which freezes a big block of ice causing major flooding. When that happens, you have a couple gallons of water you will get out of the system as the ice block melts off the evaporator coils. It can't run along the fins to the condensation drain.

In between these two instances, we had the condensate drain block from some sort of construction debris that had gotten into the system. Not so much water but quite a bit.

The system always had a tendency to leak water during the extremely humid days of summer. You can always tell when this happens as the water flow is much less, probably more like what is being described. It started about 3-4 years after we had the unit installed. Finally I got to talking to our HVAC tech with the new company we hired and asked him about the situation. He explained that the unit is a flooded evaporator system and if you're low on freon, the evaporator coils start running at too low a temperature and you get a minor icing problem that messes up the water flow on the fins.

Basically, we have them do a service call before summer hits to do maintenance, check freon levels and the problem went away. No more waste basket sitting on my desk.

Don't know if this applies to your system, but maybe it will be applicable for others.

  • I don't think my problem is related to the lack of airflow or ice. I have crawled all the way up in there while its running and can see exactly what is going on. The water trickles down like it is supposed to, but random droplets shoot off with the airflow instead of collecting in the pan and going outside. Also the condensate drain is not blocked, he did clean that out and I can verify a nice stream is coming outside via that drain. Were you able to see any ice in your system? Or is that something that is so small it is not noticeable, but causes problems still?
    – Brian
    Aug 15, 2013 at 19:14
  • Like you though, it typically occurs on the more humid days. I can usually predict the type of day it will actually leak on. If it is hot, but not humid, it will not typically have this problem.
    – Brian
    Aug 15, 2013 at 19:18

He installed an A coil on a down flow unit? That's part of the problem there. Usually there's a special cased coil for down flow applications. The problem is that the air is hitting the flat part of the coil and its deflecting off it and creating turbulence where the evap pan is. You can have an "A" coil on the return side on a down flow but it doesn't work as good. If a down flow application is desired and the house is a new construction, a horizontal right or left configuration is usually best. Every contractor has their preferred method but I try to steer people away from down flow unless it's a retrofit application.

  • Yup that is what they did, I had a feeling there had to be a better way. I am by far not an expert on this matter, but logically it makes sense to me why this might be happening. Thanks for the feedback.
    – Brian
    Apr 17, 2016 at 22:47

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