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What Causes Condensation on an AC Vent?


condensation on AC vent

If you’ve noticed condensation on an AC vent (air conditioner vents “sweating”) or worse, an AC vent leaking water, you’re right to be concerned and start investigating what might be going on. If you let it go, you could end up with property damage along with a potential air conditioner repair bill.

Let’s go over how moisture can get on your AC vent, and the potential damage it can do. Then we’ll explain the possible causes. It’s important to check them out, because the water on your AC vent might be condensation, but it also might be coming from your ducts or your air conditioner itself.

Where does condensation on an AC vent come from?

The first thing you should understand is that condensation often happens during hot and humid weather. Condensation is water vapor that turns into liquid form when warm and humid air comes into contact with a cold surface. When you leave a cold drink sitting out on a hot and humid day, you’ll see condensation collect on the cold surface of the glass. The condensation you see on the AC vent gets there the same way. In this case, the cold surface is your AC vent or register.

It’s also possible that the condensation on your AC vent (especially if the moisture is actually dripping) might be coming from inside your ducts. In that case, you might have condensation in the ducts as well as the AC vent. Or, the moisture may be coming from your AC unit and dripping through your ducts until it comes out of the AC vent.

There’s also a chance that the moisture you’re seeing on the AC vent might be coming from a roof leak above the duct, or from a plumbing leak in the ceiling or wall. If you have access, you might start by ruling out those possibilities.

Condensation can cause expensive damage

The problem with condensation (or any moisture coming out of your AC system) is the water damage that can happen over time if you don’t take care of the underlying cause.

Depending on the source of the moisture and and how long it accumulates, you can end up with damaged ceilings and walls, as well as damage to the insulation lining your HVAC ducts. In the worst case, you could end up with mold growth. And that’s nothing to fool around with because it can make people sick.

So, here are some tips for troubleshooting the cause of condensation on your AC vent, and what steps you’ll need to take care of the problem.

Common causes & fixes for condensation on an AC vent

Lack of duct insulation or damaged insulation

If you have HVAC ducts made of sheet metal with no insulation, or ducts with damaged insulation, you’re much more likely to develop condensation inside the ducts and also condensation on the AC vents. It happens because the warm, humid air outside the ducts leaks inside, where it comes into contact with the cold duct walls. It’s obviously cold inside the ducts because your air conditioner blows cold air through them and into your space.

Sometimes this problem is compounded by a lack of ventilation in attics, crawl spaces, or closets where AC ducts are sometimes placed. (That’s a big issue here in NYC because spaces are so tight.) That can cause a buildup of humidity and moisture.

To find out if this is the cause of your condensation, take a look at your ducts. Better yet get an expert to do an inspection, because it may be hard for you to see the inside of the ducts yourself and if there is water in there, there might be mold.

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To fix the condensation problem, you might need to install new insulated ducts, or add insulation on the outside of your existing ducts.

Leaky boot

Your AC duct connects to the vent grille via a metal tube called a “boot.” If there is insufficient insulation around the boot, or if there is a leak where the boot connects to the duct, warm air can get in and cause condensation inside the boot that drips down to the AC vent grille.

This problem is not too difficult to fix, assuming the ducts are in good condition. You can simply patch the leak, or replace the boot, or add insulation as needed to take care of the condensation on your AC vent.

Clogged condensate drain line or faulty condensate pump

As I mentioned earlier, condensation is not the only cause of moisture in your ducts or dripping from your AC vent. The water might be condensate coming from your air conditioning system. This is probably what you were worried about when you first noticed the moisture.

Condensate is liquified water vapor, just like the condensation we described earlier. Condensate is a byproduct of your cooling system: it’s the moisture that your AC system removes from the air and “condenses” into liquid form in the process of cooling the air in your space. Normally, condensate accumulates on the evaporator coil and drains away through a tube called a drain line, sometimes emptying into a drain pan, and sometimes pumped away by a condensate pump (depending on the design of your system).

So there are several things that could go wrong here. The drain line could get clogged, causing water to leak out. The drain pan could get clogged, causing it to overflow. Or, if you have a pump, the pump could stop working. Any of those things could cause water to end up in your duct, and eventually drip out through your AC vent.

If you have access to the system, you could take a look to see if you can clear a clogged drain pan. But you’re better off calling in a professional to get to the root of the problem.

Frozen evaporator coil

frozen evaporator coil is another possible cause of water coming from your AC vent. In this case, you will probably also notice that the system is not cooling well and there’s very little air coming from your vents. If you turned off your system to investigate, and then noticed the water, a frozen coil could be your problem. The ice on the coil can begin to melt, dripping through the ducts and your AC vent.

Frozen coils are often caused by airflow problems (like a clogged filter) or possibly a refrigerant leak. In this case, it’s definitely best to get your system checked by a professional. The strain on the system from frozen coils can lead to compressor failure, which is definitely something you want to avoid! Also, be smart and don’t try to deal with refrigerant chemicals without training.

If you’re in the NYC area, Arista is here to help, so there’s no need to risk your safety! Give us a call now.