5 Reasons Why Your Window AC is Leaking Water

window air conditioner

If you find water dripping from your window air conditioning unit, it’s essential to identify and address the underlying causes to prevent further damage and maintain the unit’s efficiency. Here are five common reasons why your window AC might be leaking water:

  • Condensation Buildup: Window AC units work by cooling warm air and removing moisture. When warm indoor air passes over the cold evaporator coils, moisture in the air condenses into water droplets. This water is usually collected in a drain pan and then drained away. If the drain pan is damaged or improperly angled, it can cause water to leak.
  • Clogged Drain Line: Over time, dust, dirt, and algae can accumulate in the drain line, obstructing proper water drainage. This blockage can cause water to back up and leak from the unit.
  • Improper Installation: Incorrect installation of the window AC unit can lead to issues with water drainage. If the unit isn’t tilted slightly downward on the outside to allow proper water runoff, water can accumulate and seep into your home.
  • Dirty or Clogged Filters: Dirty or clogged air filters can restrict airflow across the evaporator coils. Reduced airflow can cause the coils to become excessively cold and freeze. When the coils thaw, excess water can overflow the drain pan, leading to leakage.
  • Refrigerant Leaks: A refrigerant leak can cause the evaporator coils to become too cold and freeze. When the coils thaw, excess water may leak from the unit. Refrigerant leaks are a serious issue that requires professional attention.

What to Do

  • If you notice water leaking from your window AC unit, turn it off immediately to prevent further damage.
  • Check the drain pan and drain line for blockages and clear them if necessary.
  • Ensure that the unit is correctly installed with a slight downward tilt toward the outside.
  • Regularly clean or replace air filters to maintain proper airflow.
  • If you suspect a refrigerant leak or if the problem persists after addressing these issues, contact a professional HVAC technician for a thorough inspection and repair.

Where the Water in a Leaking Window AC Comes From

To understand where the water leaking from your window air conditioner comes from, I believe you should first understand how a window AC works, which is just like all the other types of air conditioners.

Apart from cooling your indoor air, the other function of an air conditioner is dehumidification. That is simply the removal of water vapor (humidity) from the air.

What you should know that is that warm air carries more moisture than cold air. Therefore, as the air is being cooled, it loses this moisture, which is called a condensate.

Just like other AC types, a window air conditioner has an evaporator coil, compressor and condenser coil. A refrigerant/coolant (commonly known as Freon) is circulated between these 3 components to effect heat exchange.

The evaporator coil is the component in charge of cooling the air and is therefore located on the front side of the unit, which is usually inside the house. When the refrigerant absorbs heat from indoor air, it is compressed by the compressor to increase its pressure and temperature, so that the heat can be lost to the outside air.

The heat is lost inside the condenser coil, and that is why it is located outside the house. If you stand close to the outside part of the window AC (which houses the condenser coil), you can feel hot air being blown your way.

During cooling, the refrigerant enters the evaporator coil as a very cold liquid. The unit’s fan pulls warm and humid air from the house and passes it across the evaporator coil.

The refrigerant absorbs heat from the indoor air, which is how cooling happens. When the humidity in the air comes into contact with the cold evaporator coil, it condenses (turns to water) and drips on a drip pan located under the unit.

Usually, the window AC has a medium separating the evaporator coil and the condenser coil. Since the water drips directly under the evaporator coil (how gravity works), there is a hole called a condensate drain hole which drains this water to the back of the AC where it is then drained out.

And that is how a window air conditioner works.

Why Your Window Air Conditioner is Leaking Water

window air conditioner

In summary, the following are the causes of a leaking window air conditioner:

  • Clogged condensate drain
  • Hole/crack in AC drip pan
  • Frozen evaporator coil
  • Faulty condensate drain pump
  • Improper installation

Let us look at each one of them separately and in more details:

1. Clogged Condensate Drain

Although the air conditioner is fitted with an air filter, some dust particles and other impurities like lint, hair, dander and pollen among others will manage to pass through it and embed on the evaporator coil.

With time, these dirt will fall off as the condensate drips and settle inside the drip pan and eventually clog the drain hole which is unfortunately quite small hence clogs easily.

Apart from dirt particles, the humid nature of the window AC drip pan makes it a perfect bleeding spot for algae, mildew, mold and other such organisms. These organism will therefore multiply within a short period of time and completely block off the condensate drain opening.

When the drain hole is completely blocked, the unit cannot drain off the water and hence the drip pan will overflow. This is what usually happens when a window air conditioner is leaking water from the front.

When you have a leaking window air conditioner, 50% or more of the time the problem is a clogged condensate drain hole. Luckily, unclogging the drain hole is quite easy.

Here is how to unclog a window air conditioner condensate drain hole:

  • Start by unplugging the window AC unit from the power supply to prevent the risk of electrical shock.
  • Go outside the house and look for the drain hole at the back or bottom of the unit. Depending on the model you have, you may need to remove the back cover to access the drain hole.
  • Use a sharp and thin object like a needle, wire or even small screwdriver to probe the drain hole. Wiggle the object about to remove out all of the gunk.
  • Put the cover back on and turn on the AC.

Again, resist the urge to drill a hole at the bottom of your unit. It may work for some units but there are some units that need a bit of water in the pan to work effectively.

2. Hole/Crack in the AC Drip Pan

Is your window air conditioner leaking from the bottom? It is very likely that your unit is at least 10 years old.

A window air conditioner leaking from the bottom is usually caused by a hole in the condensate drain pan. After years of usage, the drip pan will corrode, develop a hole or holes and it will start to leak from the bottom.

Since the drip pan is made from metal, all conditions needed for corrosion to occur are present: Water, oxygen and the material (iron) itself. When the drip pan corrodes, it becomes weak leading to the formation of hole or crack.

When that happens, as soon as the condensate drips from the evaporator coil, it will leak from the bottom of the unit (through the hole).

Fixing this problem is not as easy as fixing a clogged drain hole. You first need to examine the drip pan and decide if you want to repair it or replace it.

If apart from the hole the other parts of the drip pan still looks good, you can easily fix the leak by plugging the hole using a water-resistant sealant. On the other hand, if the entire drip pan is in bad shape, you may want to replace it.

Replacing a window AC drip pan is not something most homeowners can do. You will most likely need to bring a technician over to do it for you.

While the AC drip pans are quite cheap, it is the labor charge that will make the overall cost to be high.

3. Frozen Evaporator Coil

It is not unusual for air conditions to freeze, especially when they are not every well maintained. A frozen air conditioner can leak water inside your house when the ice starts to thaw.

The 2 main causes of frozen air conditioners are:

  • Dirty air filters
  • Low refrigerant level

Before indoor air gets to the evaporator coil, it has to first go through an AC filter. The filter removes dust, lint, pollen and other impurities (basically cleaning the air).

AC filters need to be cleaned or replaced regularly (especially during summer) to keep the unit working at optimum conditions.

Failing to clean/change the AC filter restricts the flow of air to the evaporator. What then happens then is that the refrigerant temperature fall below freezing point and instead of the condensate dripping on the pan, it ices over the coil.

When the ice to starts to melt, the water will leak out of the unit and that could be the reason why your window AC unit is leaking.

Another reason why your window AC evaporator coil could be frozen is as a result of a low refrigerant level in the system. Since the AC system is a close-looped one, you don’t need to top up the refrigerant unless there is a leak somewhere.

When the refrigerant level is low, its means that its pressure is low. A drop in the refrigerant pressure causes the refrigerant temperature to drop as well.

When the refrigerant’s temperature drops below freezing point, the condensate ices over the evaporator coil and eventually when it starts to melt, water will be seen leaking from the window unit.

It is easy to fix a leaking window AC unit if the frozen evaporator coil is caused by a dirty filter. You will only need to remove the front access panel, clean the filter (or replace it) then turn on the unit.

Note: If you have a frozen air conditioner, turn it off until the ice has completely melted.

If your AC refrigerant level is low, you will need to bring a technician over to first locate the source of the leak and fix it, then they can recharge the system with more of the refrigerant.

4. Faulty Condensate Drain Pump

Is your leaking window air conditioner installed below the ground like in the basement? The condensate drain pump could have malfunctioned.

You see, if your window AC is in the basement, the water from the drain pan cannot flow out via gravity. And that is where the condensate drain pump comes in

The condensate drain pump is installed in the drip pan to pump out the condensate as soon as it reaches a certain level inside the pan. If the pump malfunctions, the drip pan will overfill with water and leak.

If upon investigation, you find that the drip pan is full of water, the pump is the culprit. It will need to be fixed or replaced.

Unfortunately, this is not a task you can do on your own. You will need to hire an HVAC technician to do it for you.

5. Improper Installation

Is your leaking window air conditioner newly installed? And was it installed by a professional HVAC technician or by yourself?

All window air conditioners are installed tilting/sloping backwards. That allows the condensate to flow from the front of the unit where the evaporator coil is to the back of the unit where the drain is located.

Different brands have different tilt angle specifications and that is why you should have the unit installed by a professional. Sometimes even the professionals get it wrong.

Anyway, if the tilt on your unit is not good, the condensate will not flow out of the drip pan as designed and instead it will flow back and leak out.

A badly installed window air conditioner can also cause it to leak when it rains. Again, that can be caused by lack of a good tile/slope towards the back of the unit.

It could also be as a result of poor sealing around the window itself, which also makes it energy-inefficient. Just inspect the seal around the entire unit for gaps which could be allowing rain water to get in the house.

If you get any, you can use a piece of foam tape of aluminum tape to fix the gap. On the other hand, a badly installed unit will need to be reinstalled by a professional.

Wrap Up

And basically those are the reasons why your window air conditioner is leaking water. If it is something you can fix on your own then turn off the unit and do it. If not, you will need the services of a professional HVAC technician.