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How To Keep an AC Drain Line Clear (7 Easy Steps)

An AC drain line will clog sooner or later, irrespective of the unit’s size, age, condition, and other variable factors. However, you can be proactive with a bit of preventive maintenance to keep an AC drain line clear. Some of these preemptive measures do not require technical expertise.

Here are 7 easy ways to keep an AC drain line clear:

  1. Review the AC drain line alignment.
  2. Flush and clean the AC drain line.
  3. Clean dirty air filters or replace them.
  4. Install a condensate drain line trap.
  5. Clear the AC drain line regularly.
  6. Use the secondary AC drain port.
  7. Keep the AC drain line outlet clear.

You may consider this easy guide about how to keep an AC drain line clear sequentially. Or, you could choose the part that you think needs immediate attention in your case. However, as I explain in this article, a holistic approach is more likely to be failsafe & sustainable throughout the season.

1. Review the AC Drain Line Alignment

The first step is reviewing the AC drain line installation, especially its alignment. If the drain line or pipe is misaligned, the other steps in this guide are likely to be futile. 

The International Mechanical Code requires an AC condensate drain line to have a minimum diameter of 0.75 inches (19.05 mm) unless the unit is greater than 20 tons (18,143.7 kilograms). Additionally, the entire drain line should have at least a 1% slope throughout the layout. 

  • The minimum slope requirement for an AC drain line in most states is 1.5 inches (38.1 mm) for every 12 inches (305 mm).
  • Some local codes may require a steeper slope, but the regulations can vary for different types of installations.
  • Some properties or installations may require a slope of as much as 3 inches (76.2 mm) for every 12 inches (305 mm) of the drain pipe. 

It is not necessary for you to measure the exact gradient of the AC drain line. However, use a level to ensure the pipe slopes all the way from the AC drain to the outlet outside. 

Confirm whether or not the pipe is connected to the primary drain port of the AC unit. The main AC drain line should not be fitted to the secondary drain port unless your installer made a rare mistake. More about the secondary drain port in the sixth point. 

Now, the AC drain line will not stay clear if there is any point where the pipe is not sloped, or there is an upward gradient. The only exception is the condensate trap, which I talk about in the fourth point.

A common problem with AC drain lines is a lack of adequate sloping throughout the layout. The steep vertical drop at the primary drain port is always evident. However, you must check if the whole pipe slopes as it runs away from the unit to the wall and outside. 

Your AC unit may be in the attic, garage, or utility closet. Nonetheless, some portions of the AC drain pipe may not be readily visible, especially for the parts running under a floor or along the walls. 

The part of a drain line running outside through the wall is not a huge concern, as long as it has a slight slope. However, the portions running parallel to any wall or through rooms should have the minimum gradient for gravity to work. 

An AC drain line should also have adequate supports so that the pipe does not sag anywhere. Normally, securing the pipe with supports every couple of feet is sufficient.

2. Flush and Clean the AC Drain Line

You must start with an impeccably clean AC drain line to keep it clear during the summer. If your AC drain pipe is clogged, even partially, every other proactive step you take will effectively be a half-measure. Thus, flush and clean the AC drain line before any other preventive maintenance.

You can flush and clean the AC drain line using the following tools:

  • Regular garden hose.
  • Wet and dry vacuum.
  • Pressure washer.
  • Compressed air.

You may use any of the four tools subject to what you have handy. However, the effect of a regular garden hose primarily depends on the water pressure. The other pressurized options, whether water or air, are more effective in extracting all the gunk buildup in the drain line.

It is not uncommon for an AC drain line to have a rigid and gunky blockage. So, you can use a small handheld tool like the RedRock Gallo Drain Gun (available on to clear out such clogs. This drain gun comes with 5 CO2 cartridges that are specifically effective for AC condensate drain lines. 

The maximum pressure generated by these drain guns is 800 psi, which is more than sufficient to clean rigid blockages. However, make sure your AC drain line has properly sealed joints if the pipe is PVC. Weak joints may make the assembly vulnerable to high pressures. Copper pipes will fare well under pressure. 

Irrespective of the method you choose, test if the drain line is completely clear of gunk, mold, and algae. Older AC units with significant rust can have bits and pieces of corroded metal in the drain pan and pipe. Such buildup should be cleaned, too.

3. Clean Dirty Air Filters or Replace Them

When you flush and clear the AC drain line, check the air filters and clean them if necessary. An old and uncleanable air filter needs to be replaced. 

You should clean all the air filters on the supply and return vents. Make sure you check the filters on the air handler unit and the main return duct to the AC. 

Dirty air filters worsen the dust and debris buildup inside the AC’s evaporator unit. Thus, the water in the condensate drain line will be dirtier and gunkier, effectively increasing the chances of a blockage in the pipe. 

Air filters may need frequent cleaning depending on their condition and the indoor environment. Routine cleaning is exponentially simpler and more manageable than dealing with a clogged AC drain line. 

4. Install a Condensate Drain Line Trap

Your AC drain line may or may not have a condensate trap. Consider installing a condensate trap if your AC drain line clogs repeatedly. 

The International Mechanical Code states that AC drain pipes should ideally have a condensate trap. However, the local codes in many states do not make such a fixture mandatory. Additionally, most manufacturers are somewhat ambiguous about their stand.

Some manufacturers mention in their manuals that a condensate trap is not necessary, but they recommend it nevertheless. This unclear policy is probably more due to the issue of negative air pressure than AC drain line problems.

Many AC manufacturers say that their evaporator units do not create a significant negative air pressure that can lead to drawing air in through the drain port, thus the line or pipe. However, any negative air pressure in the AC evaporator unit is theoretically and practically quite likely.

Suppose the indoor evaporator unit of your AC with the coils does not get sufficient air to cool and circulate. In that case, negative air pressure will form inside this housing. The blower will continue to circulate air. So, the negative pressure will forcibly draw air through the drain port. 

Now, the lack of sufficient airflow inside the evaporator housing may be due to different reasons. A clogged filter on the main return duct can limit the airflow into the indoor AC unit. Additionally, the air filters on the supply vents and air handler may be dirty, thus inhibiting an efficient circulation.

A negative pressure drawing air into the evaporator housing through the drain port will restrict the condensate flow through the line or pipe. In some cases, AC drain lines may have an airlock due to the same problem. An airlock can effectively block the water flow in the AC drain line.

Therefore, you should have a condensate trap for your AC drain line. 

Furthermore, you must have a tee with a vent atop the condensate trap. The vent cap will be handy if you have to get rid of an airlock. Besides, this access point is quintessential to keeping an AC drain line clear, as I explain in the next step. 

There are a few facts to note about a condensate trap, especially if you do not have one or are unfamiliar with this fixture.

  • A condensate trap should always have sufficient depth based on the AC unit’s capacity and thus drain pipe diameter.
  • The condensate trap or its curved pipe must have water. The water blocks the airflow to the evaporator unit when there is negative air pressure due to any reason. 
  • The condensate trap’s inlet pipe connected to the AC’s drain port should be higher than the outlet line of the fixture. 
  • A condensate trap does not necessarily have an extra tee with a vent. However, you will benefit from such a feature. 
  • Like the entire AC drain line, the condensate trap should be sloped from the unit and towards the rest of the pipe running outward. 

5. Clear the AC Drain Line Regularly

Following all the steps until now will prepare you for an easy remedy to keep the AC drain line clear throughout the season. 

An AC drain line clogs due to:

  • Dust and dirt mixed with water.
  • Mold and mildew growing inside the pipe.
  • Algae bloom anywhere inside the pipe.
  • Specks of rusted and corroded metal.

You must clear this gunk from your AC drain line regularly. Some opt for a monthly routine. You may choose to clear the pipe fortnightly or whenever you deem fit. The rule of thumb is to clear an AC drain line before it clogs severely, which prevents the natural water flow to the exterior outlet.

There are three solutions to choose from:

  • Liquid soap or detergent.
  • Distilled vinegar.
  • Chlorine bleach.

Liquid soap or detergent mixed with warm water is a safe option. However, it is not as effective as distilled vinegar or chlorine bleach at killing algae, mold, and mildew. 

Distilled vinegar is a gentler option than bleach. However, bleach eliminates mold, mildew, and algae more effectively than vinegar. 

On the flip side, you must not use bleach if your AC drain line or pipe is copper. Bleach corrodes copper, despite the metal’s resistance to corrosion. Bleach may also be unsuitable for all glues and primers used for a PVC drain line’s joints and connectors.

Choose among liquid soap or detergent, distilled vinegar, and chlorine bleach, subject to the AC drain pipe material you have. You must also consider where the drain line leads outside. If you have some plants around that outlet, you should not use bleach to clear the AC drain line.

Here are the steps to keep an AC drain line clear using liquid soap, vinegar, or bleach:

  1. Open the vent cap atop the condensate trap.
  2. Pour the cleansing agent into the AC drain line. 
  3. Allow the solution to work for half an hour.
  4. Flush the drain pipe using warm water.
  5. Refill the condensate trap with clean water.
  6. Close the tee vent with the threaded cap.

This preventive routine will prevent gunk buildup, be it dust, grime, mold, mildew, or algae. 

However, you must use a pressurized cleaning method if the AC drain line is already clogged. Pouring a bit of vinegar, bleach, or any other cleaning agent may not get rid of a rigid blockage. 

In this case, you have to use compressed air, a pressure washer, a wet and dry vacuum, or a drain gun to clear the pipe. 

The quantity of liquid soap, distilled vinegar, or chlorine bleach depends on your AC unit’s size, the total length of the drain line, and the kind of gunk buildup you anticipate.

As a preventive measure, half a cup of chlorine bleach may be more than sufficient. If you want to use distilled vinegar, the required quantity may be more than a standard cup. Liquid soap or detergent with warm water doesn’t pose a threat in regards to its quantity, so use as necessary.

6. Use the Secondary AC Drain Port

As I mentioned earlier, an indoor AC unit has at least two drain ports, primary and secondary. A large AC unit may have more. While the primary drain port connects to the condensate line, the secondary outlet remains unused. You can use this secondary drain port in different ways.

Here are two ways to use the secondary AC drain port:

  • Install a float switch to turn off the unit when the primary drain line is clogged.
  • Use an overflow pan with a condensate pump and a second drain line as a contingency. 

The secondary drain port on your AC unit is slightly higher than the primary connection. Or, the secondary connection has an internal dam so that the water flows out of the drain pan through the primary port before overflowing through the backup port. 

A float switch on the secondary drain port is a safety measure. This switch can turn off the AC thermostat when the water level rises in the secondary port. Thus, you can prevent a crisis due to water flowing out of the drain pan and flooding the space. 

An overflow pan connected to the secondary drain connection is a backup tactic. You can have a separate drain line for this port, either gravity-enabled or powered by a condensate pump. The contingency tactic allows you to continue using the AC even if the primary drain line is clogged. 

7. Keep the AC Drain Line Outlet Clear

Last but not least, keep the outdoor outlet of the AC drain line clear. This requisite applies to the primary and secondary drain ports, whether gravity-enabled or powered by a condensate pump. 

The outdoor outlet of an AC drain line may be at ground level or higher. Irrespective of these variables, the exterior outlet should not be clogged by anything.

Key Takeaways

Here are the seven key takeaways to keep an AC drain line clear:

  • Ensure the pipework is sloped to allow the condensate to flow out.
  • Start with a clean drain pipe as the temperature increases.
  • Clean the filters on the air handler, supply vents, and return ducts.
  • Install a condensate trap for the AC drain line if you don’t have one. 
  • Use vinegar or bleach to eliminate algae, mold, and mildew from the pipe.
  • Install a float switch at the second drain port or use an overflow pan. 
  • Keep the exterior AC drain line or pipe outlet clear of obstructions. 


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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