Excessive AC Condensation Problem

This morning’s Warranty Inspection had a situation I have not encountered before and I would like some input on it.

The weather this morning was low 80s with scattered rain. It has been a particularly wet and humid summer here.

When I walked in I noticed that it was pretty cold in the house. My wife likes to keep it pretty cool at home so I am used to 74 or so temps. This was uncomfortably COLD to me. When I got to the thermostat it was holding steady at 65.

When I got to the attic to look at the unit, I noticed a lot of moisture on the decking all along the primary condensate trap and line. Then I saw a pool of water on the top of the furnace housing. Looking further there was water in the secondary/emergency drain pan. Then I noticed moisture at the supply plenum. There was moisture on the top at some of the duct connections (they were pretty well installed with mastic) and some moisture on the decking under the plenum. As I bent down to look closer under the plenum I noticed that the bottom panel of the plenum was bowed out. I touched it and my finger went straight through and a stream of water flowed out. The material was completely saturated.

I brought the homeowner up to the attic and explained the moisture problems to him and put a bucket under the plenum. I asked if they always kept the thermostat set at 65. The answer was yes.

I would love some input on this problem/situation. I told him that due to the low temperature they are maintaining, they are creating excessive condensation, as well as apparently creating condensation inside the plenum and perhaps even the ductwork. That he should have everything checked by a licensed HVAC guy and discuss with him whether there are any modifications they can add or incorporate that will allow them to cool the house that much, without these problems developing. I also suggested that he have a mold sampling done. As it seems that this is a perfect environment and has been present for months.

Are there any methods that can be incorporated to remove moisture and allow them to run the system that low? It seems to me that even if you get the moisture out of the conditioned air, you cant do anything about the 140 degree high humidity air of the attic surrounding all the ductwork and plenum’s.

I have attached some pictures, but it is hard to see the moisture and problems.

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I see the vent on the wrong side of the trap which can cause the drain pan to overflow. I am suprised that they have not had more problems. Was the unit recently serviced?

You’re right! That trap isn’t to prevent sewer gasses like sewervent, but to prevent air blowing out. So, this is a guess, the condensate is moving down the pipe, and blown out and up into the attic.

Not really a vent, but looks like a clean out. Needs a cap. Might be in the pan.



Nope. That is fairly typical around here. Not the cause of the problems described.

Is this a draw thru or a blow thru setup, can’t tell from the photos.

The blower is before the evaporater coil, so blow through. Don’t think I have ever seen a draw through around here.

I would think that running the system at a constant 65 degrees would cause all kinds of condensation on the equipment. 65 degrees is a huge drop from the outside temp so the system would be running almost constantly. All of the components connected to or close to the coil would get very cold and then condensation would form on them. I’m surprised they haven’t frozen the coil by running it that low. Maybe that’s part of the problem too, the coil is partially freezing and the condensation can’t get out through the drain.


That is what I thought too. The primary drain was drain fine, you could hear it and I could confirm it with my IR. I din’t even ask what their electric bill was.

But, as you said the system was just so cold, that everything around the evaporator, located up in the hot, humid attic was condensing out water.

They are going to get it checked out. I told them, and think they understand, that I believe it was more of a usage outside of normal parameters problem than a installation problem. I willcheck back with them later and see what the HVAC guy says.

Yes, that vent pipe is on the wrong side of the trap, and not necessary. Plus that running trap is probably not deep enough to block the air flow even if it ever was full of water. This enables the cold air to blow thru the drain line making it colder than the dew point, causing condensation. Also, all the ductwork should be insulated and have a moisture barrier on the outside of the insulation to prevent condensation thru the insulation onto the cold metal.
With clean filters and sufficient air flow, the coil shouldn’t freeze as the coil temp. will only go below 40*F if the unit is undercharged.