Air Conditioning Condensate Drains

Air Conditioning system drains can be categorized into tow main types.  There are Blow-Through and Draw-Through systems.  There are many common requirements for both types of systems, but they do have some special differences.  The main difference in the two is that the Draw-Through type system requires an external trap in the drain line to allow it to drain.

Draw-Through Drains

Usually seen on most electric air handlers.  A system with the blower in the air stream after the evaporator coil is considered a Draw-Through type system.  This means the air is being drawn over the evaporator coil which causes a negative pressure on the coil and drain pan.  With this negative pressure air will be drawn into the drain pan through the drain hole unless there is a water filled trap to stop the flow of air.  A draw through drain without a proper trap will create a turbulence at the drain port and hold the water in the drain pan.  This can cause the water not to drain and can cause overflow problems.  A standard residential air conditioning system needs to have a trap of at least 1 1/2 inches to drain properly.  There should not be an air vent between the drain pan and the trap.  There should normally be a vent after the trap.

Blow-Through Drains

Usually seen on most gas heat systems.  A system with the blower in the air stream before the evaporator coil is considered a Blow-Through type system.  In this system the blower is pushing on the evaporator coil and creates a positive pressure on the drain line.  This actually helps push the water down the drain.  In this system you do not need a trap other than in the main sewer line.  In most cases you do need an air vent in the line to keep the air from blowing up through your sink, tub, or shower drain that it is connected to.

Drain Systems in General

A drain system for a central air conditioning system needs to be installed properly to ensure proper drainage and minimize the risk of overflow.  In the Corpus Christi area a standard 3-Ton air conditioning system that is working properly will remove 20 to 35 gallons of water a day from the air.  All this water has to flow down the drain system.   One of the safety devices that we highly recommend and can be added to most any drain system is called a Safety-T.  This safety device is installed in the drain line close to the unit and will shut the air conditioning system off if the drain line fills with water.

It is common sense that all drain lines need to be sloped away from the unit and toward their final connection to the sewer or outside.   I can’t tell you how many drain systems I have seen through the years that have either no slope or are actually running slightly up hill.   The minimum slope required by code is 1/8 inch per foot.  That means in an eight foot horizontal run the drain must drop at least one inch.  This is the bare minimum slope and I would highly recommend much more whenever possible.

What causes algae Growth in a drain line

One of the main causes of stopped up drains in our area is algae growth in the drain line.  Algae is formed because dirt and dust that accumulates on the evaporator coil gets washed down the drain line.  This combined with the cold water standing in areas of the drain can cause algae to grow.  This is especially prevalent in the trap of a drain system.  Two things to do to minimize the potential for algae growth in your drain line are to use a good high efficiency filter to keep the dirt and dust off the coil and to treat your drain line periodically.  To treat you drain we used to recommend pouring  diluted bleach water through the drain system to kill the algae.  We now recommend using vinegar because it it much less corrosive and will kill the algae just as well.  This treatment should normally be done two or three times per cooling season.  To treat your drain line you should first turn your system off so that the blower is not running.  Then you should have an access fitting to be able to pour about 1/4 cup of vinegar down the drain line.  If you have a trap in the drain make sure it goes into the trap for best results.  Let the vinegar sit in the drain line for about 5 minutes then flush the drain line by pouring about a gallon of water through it.  Doing this at least once per cooling season should minimize the chance of an algae build up clogging your drain line.  It will also allow you to make sure that the gallon of water flows relatively unobstructed through the drain line.

Horizontal units in Attic Spaces

If your inside unit is located in a horizontal orientation in your attic space then you must have an Auxiliary Drain Pan installed under the coil of the unit.  This is in case the main drain pan fills up and overflows the auxiliary pan will catch the overflow and save your ceiling.  This auxiliary pan should extend out beyond the size of your coil or blower coil unit and should have a float safety switch on it to shut the system off if it fills up.  It can also have a drain line that goes to a visible spot outside so that you will see it draining and know that you have a problem.