Recently had a new furnace/ac/ductwork installed in our home. The furnace/air handler are located in the attic on the second floor of the home. A drain line is run from the unit to the utility room on the first floor where it feeds into a floor drain.

About a month after the system was turned on I noticed that the drain line (clear plastic flex tubing) was not draining properly. This tubing runs through a joist bay between my first and second floors (currently open due to renovation) in order to get from the attic on one side of the house to the utility room on the other side. The tubing is not run at a pitch so the water is pooling and not draining.

I asked the HVAC installer if that was a problem and he said it wasn't. I assume it IS a problem, is that correct? If the line were to be adjusted to run at a pitch, would that solve the problem? Or should it be draining currently due to negative pressure?


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    I’m voting to close this question because it's been abandoned. – FreeMan Dec 26 '20 at 16:46

If it's draining from the second floor to the first floor it will drain. You are correct though it should be pitched so there's no standing water. Standing water will lead to the build up of scale, mold etc. and those things will eventually clog it.


For attic installations I always (if possible) run a full vented and trapped drain up to the attic. I always install a secondary drain pan under all the equipment. A small leak on a furnace in the basement would result in an "oh maybe I should have a look at this" response. A small leak in an attic results in having to replace the drywall on ceilings and walls as well as repainting. I'd get it in writing from your contractor that they're responsible for any damages. You may have to agree to a yearly maintenance for for that to be valid. You won't get warranty on your car if you neglected to maintain it.

The sanding water in the pipe is not likely to be of concern but some funk does build up hence the yearly maintenance.


It isn't a problem. This is all condensed humidity, or basically distilled water, and none of it is evaporating inside the pipe, so scale won't happen. Nor is mold likely, as there isn't much for it to grow on (although I wouldn't drink the water).

Many people (myself included) have condensing furnaces, where the condensate is pumped up, across, and down into some sort of drain. In that case there's various things dissolved in the water from the burning process (e.g. making the condensate acidic), but it still isn't a problem.

If you really want to, take a look in a year or two and see if anything's built up. My guess is it won't. You'll more likely have leakage/blockage problems at the A/C air handler itself; you should probably think about what happens if the drain gets blocked there.

  • I forgot to mention in the initial post, but there is already some buildup (you can see on the left side of the picture if you zoom in). This is after only 3-4 months. That came as quite a shock to me, as I assumed that the likelihood of scale/mold inside of the tubing was unlikely as you suggested. I just noticed the buildup when I took the picture this morning, it was not there in January when I first noticed the standing water. I have not yet gotten a chance to get up close and inspect the buildup to determine what it is. – Scheer Apr 2 '19 at 19:29

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