Why Your AC Drain Pan is Full of Water & What to Do

air conditioner drain pan full of water

A full AC drain pan is a common issue that can lead to water leakage and potential damage to your HVAC system and home. Understanding the reasons behind a full drain pan and knowing how to address it is crucial for maintaining the efficiency of your air conditioning system. Here’s a summary of why your AC drain pan might be full of water and what to do about it:


  • Clogged Drain Line: One of the primary reasons for a full AC drain pan is a clogged or obstructed drain line. Over time, dirt, debris, and algae can accumulate in the drain line, preventing proper water drainage. This blockage causes the water to back up and fill the drain pan.
  • Frozen Evaporator Coils: If your AC system runs continuously, has a refrigerant leak, or encounters airflow problems, the evaporator coils can freeze. When they thaw, excess water may overflow the drain pan, leading to a full pan.
  • Damaged Drain Pan: A cracked or damaged drain pan can also result in water accumulation. If the pan is not effectively collecting condensate, it can allow water to leak into your home.

What to Do

  • Turn Off the AC: If you notice a full drain pan, the first step is to turn off the AC system to prevent further water leakage and potential damage.
  • Check for Obstructions: Inspect the drain line and drain pan for clogs, debris, or algae buildup. Clear any obstructions you find using a wet/dry vacuum or a pipe cleaner.
  • Clean or Replace Air Filters: Dirty or clogged air filters can restrict airflow, leading to increased condensation. Regularly clean or replace your air filters to ensure proper airflow and reduce the risk of a full drain pan.
  • Thaw Frozen Coils: If you suspect frozen evaporator coils, allow them to thaw completely before turning the AC back on. Address the underlying issue, such as poor airflow or refrigerant leaks, to prevent future freezing.
  • Inspect the Drain Pan: Check the drain pan for cracks or damage. If you find any, replace the pan to ensure it functions correctly.
  • Flush the Drain Line: After clearing clogs, flush the drain line with a mixture of water and bleach (about one cup of bleach per gallon of water) to prevent algae growth and maintain a clean line.
  • Schedule Professional Maintenance: Consider scheduling routine maintenance with an HVAC technician. They can inspect your AC system, clean coils, and ensure all components are in good working order. Regular maintenance can prevent a full drain pan and extend the life of your AC unit.

How an Air Conditioner Condensate Drain Pan Works

evaporator coil on top of a condensate drain pan

Apart from cooling your indoor air, an air conditioner is also designed to remove/lower its humidity. Humidity is simply the water vapor in the air.

Air conditioners have an inside and outside unit. The inside unit is the evaporator coil while the outside unit contains the compressor and condenser coil.

Heat from indoor air is absorbed by a refrigerant circulating inside the evaporator coil and released to the outside in the condenser coil.

The evaporator coil is equipped with a fan/blower which pulls warm air from the house through the supply air ducts. Cooled air is then forced out of the coil and back to the house through the supply air ducts.

The refrigerant (popularly known as Freon) enters the evaporator coil as a very cold refrigerant. When the warm indoor air comes into contact with the coil, the refrigerant absorbs heat from the air, and that is how cooling happens.

As you already know, when water vapor is subjected to low temperatures, it condenses. And that is exactly what happens on the surface of the evaporator coil.

The condensate drain pan is therefore placed directly below the evaporator coil to catch the condensate and therefore prevent water damage.  Condensate drain pans are however not reservoirs.

The water should continuously be removed from the pan by the condensate drain line, which runs from the pan to the outside of the house. Usually, the condensate drain pan will catch anywhere between 5 and 20 gallons in a day depending on the size of the AC unit.

Primary vs Secondary Condensate Drain Pans

Depending on the type of HVAC unit you have, you could have one or two condensate drain pans. There are two types of HVAC units:

  • Vertical units
  • Horizontal units

In vertical HVAC units, the evaporator coil is located directly above the furnace or air handler and air moves upwards/vertically. Return air duct comes in from the bottom of the unit while supply air duct exits from the top.

Horizontal HVAC units are installed in places like attics or crawlspaces where there is no enough vertical space. Air flows from side to side. Return air duct comes in from one side while supply air ducts exits from the other side.

In vertical units, the condensate can only drip downwards. As such, these units have only one condensate drain pan located below the evaporator coil.

Things are however different for horizontal units. You cannot always be sure that the condensate will drip downwards. It has the potential to flow sideways as well.

For that reason, horizontal units have a primary drip pan (placed under the evaporator coil) and a secondary large drip pan (located below the entire unit). The secondary AC drain pan is a backup for the primary drain pan.

If you have therefore have a horizontal HVAC unit and the secondary drain pan is full of water, you should also check the primary drain pan as well. Chances are that it is full of water as well.

What Do You Do When Your AC Drain Pan is Full of Water?

The solution for an air conditioner condensate drain pan full of water will depend on what the problem is in the first place.

Let us look at each problem independently and the solution to it.

1. Clogged Condensate Drain Line

The humid conditions around the AC’s evaporator coil makes it a perfect bleeding ground for mold, algae and other such organisms. Rapid multiplication of these organisms can completely clog the drain line and therefore restrict the flow of the condensate from the drip pan.

Air conditioners are also equipped with air filters which are installed just before the return air gets to the evaporator coil. These filters should be changed after every 30-90 days.

Failure to change AC filters (sometimes called furnace filters) will result in dirt being deposited around the evaporator coil and condensate drain line and clog it. Using the wrong size of air filter and/or using a poor quality filter can also let in dirt particles which will end up clogging up the drain line.

So how do you fix a clogged condensate drain line and drain water from the drip pan? Here is how to do it:

  • Start by turning off the AC unit.
  • Inspect the condensate drain pan and line for cracks or visible clogs. Removes any visible clogs and/or replaced cracked parts.
  • Use a wet/dry vac to drain the water from the drip pan. You can get the vac from your local hardware store.
  • Remove the cap at the top of the drain line and inspect inside for visible clogs. You can even insert a wire brush inside just to be sure you have removed everything.
  • Unclog the condensate drain line using a shop vac. You can connect the shop vac on the drain line outside the house where it terminates or where you removed the cap. Run the shop vac for about a minute to suck out anything that is inside the line.
air conditioner drainpipe

Tip: The shop vac’s hose will most likely not fit around the AC drain line. you can use a piece or rag or tape to create a seal prior to starting the vac. Don’t forget to dispose of the dirty water after clearing the line.

The good thing about condensate drain line clogs is that they are not stubborn. Shop vacs have been used countless times to unclog them.

How to Prevent Your AC Condensate Drain Line from Clogging

You don’t have to wait for your drain line to clog. As they say, prevention is always better than cure.

The following are some of the ways to prevent your AC drain line from clogging:

  • Change your AC filter at least after every 3 months and use the correct size.
  • Regularly clean your evaporator coils
  • Use a shop vac to suck waste from the drain line. You can do this after every 3 months.
  • Regularly open the cap on the drain line and pour down some bleach. The bleach will kill the mold and algae inside the line, preventing them from multiplying and clogging the line.

2. Frozen Evaporator Coil

An air conditioner freezes when the flow of warm air to the evaporator coil is restricted or when the level of refrigerant is too low. Airflow restriction to the evaporator coil is caused by:

  • Blocked ducts
  • Leaking/collapsed ducts
  • Dirty evaporator coil
  • Clogged air filter
  • Faulty fan/blower

When the refrigerant levels are low or when the flow of warm air to the evaporator coil is restricted, the pressure of the refrigerant drops, which also causes its temperature to drop below freezing point (32 degrees).

When that happens, instead of the condensate dripping on the pan, it freezes on the evaporator coil forming ice.  Freezing can also happen inside the pan, meaning that the condensate will be unable to drain out.

If you inspect your evaporator coil and notice that it is frozen, turn your AC off and call in an HVAC technician. The technician will check the refrigerant level, clean the ducts and coils, inspect the ducts for leaks and also change the filter and a faulty fan.

3. The Unit Wasn’t Properly Installed

It is always recommended that HVAC systems are installed and maintained by professionals. DIYs are installations by unqualified persons can turn out to be very expensive in the long run.

If for instance your unit was installed at an angle (slanting to one side), you can expect it to fill and overflow on one side while the other side remains dry. Also, the drain line outlet should be on the lower side so that the condensate can freely flow out via gravity.

The drain pan also needs to rest on a strong support otherwise it can easily sag and cause water to pool in the middle away from the drain outlet.

If you inspect your AC unit and find the above to be the problem, you will need to bring a technician to install it properly.

4. Faulty Condensate Pump

In large HVAC systems, we cannot rely on gravity to drain out all the water from the AC drip pan. We need something more forceful. And that is where condensate pumps come in.

Condensate pumps are electrically powered centrifugal pumps that pump water out of the condensate pan through the drain line. When this pumps fail, the drain pump will be full of water as there is no way of draining it.

A condensate pump is not something the average homeowner can fix or replace. It will need to be fixed/replaced by a professional HVAC technician.

5. Faulty Float Switch

An AC float switch works like a toilet float. It moves up and down in accordance with the level of water. A toilet float shuts off the toilet fill valve when the water level in the tank is okay.

An AC float switch is electrically powered. There are 2 wires connected to it. The float moves up as the water in the pan increases and when it reaches the critical level it forces the 2 wires to make contact and turn off the system.

When the system is turned off, it will stop producing more condensate. This system prevents the drain pan from overflowing.

If your AC condensate drain pan is overflowing, you clearly have a defective float switch. Call in an HVAC technician to replace it.

Wrap Up

And basically those are the reasons why your air conditioner condensate drain line is full of water. I hope that these guide was helpful.