Thread: Excessive condensation from A/C set too low

Hey guys, got something I never ran into before and having a hard time trying to figure out how to verbilize, or explain the result. I've already had a list of questions from the buyer before the inspection, and he was not there so I know when he gets the report I will be asking to explain this in great detail. I already mentioned this to someone in idle conversation, and they could not understand this phenomenon, the more I tried to explain it, the more I started getting confused.

Anyway, this is South Florida, still hot mid 80s, and still humid. Got to this inspection at 1:00 PM, I noticed while walking around outside all the windows had condensation on them (inside). When the Realtor got there, we went inside, and I immediately noticed it was cold....It was 68 inside, thats cold here as far as keeping your house at that temp via A/C.

House is 2004, 4000 sq ft. 2 story, both air handlers on second floor in closet, in conditioned space. Both A/C units operating as intended. The condensation was only on the windows,the air handlers, and a few A/C grills, all smooth surfaces except the air handler room ceiling which was saturated.

I think it just came to me, I will put my new thought at the end.

The air handler closet had no return air flow (louvered doors), the returns were nearby, but now that I think about it there was no real air flow in the closet, it was not sealed, but the return air was pulled from elsewhere.

The part im struggling with, and have already been asked is......if the A/C is excessively running, the humidity should be lower, so why the condensation? I can understand the windows, single pane, with the outside heat load.

So, when the lightbulb just came on, my theory is that if some of the return air was being pulled through the air handler room, there would have been air movement, and the air handler would not have been sweating, thus the ceiling would not be soaked.

I have heard of something similar a few years ago. Brand new house, vacant, A/C guys came to start A/C, either wired T stat wrong or whatever, but the A/C never shut off. Weeks later a salesman went to show the house to someone, opened the door and about fell over. The whole house was covered in mold, drywall was so wet it was falling off the studs. I heard all the windows had consensation also. The ended up gutting the house and redoing it.

Any thoughts? Can having a house too cold via A/C be harmful to the house?

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Perhaps the A/C systems capacity is way over sized. In a case like this, the system would drop the temperature too quickly while not running long enough to dehumidify the air.

The colder humid air is then condensing on the warmer wall surfaces. Cold air sinks so that might explain the condensing at the lower areas of the walls.

Check the evap coil drain pan and drain lines.
If evap pan is full of water the fan can blow water droplets through the ducts.
Since water is not being removed from the house, relative humidity goes go up as the temp goes down.

I agree with the others, you have a moisture problem.
Just because it is cold does not mean the unit is removing moisture.
They are correct in their premise, that a properly sized and functioning unit does remove humidity, but it is possible to cool without removing humidity.
This is the principle of meat lockers and produce coolers, move lots of air over large coils in a relatively small space while keeping the temperature drop across the coils low so as to maintain a high humidity.

It sounds like you have a meat locker!

The condensation on cold surfaces is the give away that the humidity is too high.

My first guess would be a massively over sized unit like John suggested. You could also have leaks or other moisture sources dumping water inside the house.

Another thing to consider is the amount of air movement.
If the air is moving to fast over the coil the moisture will not condense out correctly.

Originally Posted by John Dirks Jr

Perhaps the A/C systems capacity is way over sized. In a case like this, the system would drop the temperature too quickly while not running long enough to dehumidify the air.

The colder humid air is then condensing on the warmer wall surfaces. Cold air sinks so that might explain the condensing at the lower areas of the walls.

The system is 6.5 tons total, for 3900 sq ft puts it at 600 sq ft per ton which is exactly in the middle of rule of thumb sizing here 500-700 sq ft per ton. Both units were draining freely. I had to take both covers off air handlers for Chinese Drywall check, no water in the pans.

Also the wet drywall was the ceiling of the air handler room, not the side or lower walls. Also, the drywall outside this small air handler room was dry.

Let me take a SWAG.
Vacant house has been without any A/C for extended period of time and the house is full of moisture. Sellers agent stops by the evening before the inspection and sets the t'stat to 40 deg. The temperature drops over night and the windows and minimally insulated ceiling above the air-handlers become cold enough to hit the dew point and condensation develops on the cold surfaces. The t'stat is still at 40 so when the sun rises it does not bring the temp in the house up above dew point and the moisture on the windows and ceiling does not evaporate. My guess is that after the house has stabilized the problem will have gone away.

Originally Posted by Vern Heiler

Let me take a SWAG.
Vacant house has been without any A/C for extended period of time and the house is full of moisture. Sellers agent stops by the evening before the inspection and sets the t'stat to 40 deg. The temperature drops over night and the windows and minimally insulated ceiling above the air-handlers become cold enough to hit the dew point and condensation develops on the cold surfaces. The t'stat is still at 40 so when the sun rises it does not bring the temp in the house up above dew point and the moisture on the windows and ceiling does not evaporate. My guess is that after the house has stabilized the problem will have gone away.

That makes some sense. I am quite sure the problem is temporary.