Primary and Secondary Drain Connections

Have you ever wondered why evaporator coils often have several drain connections? It is not because the manufacturer had extra PVC drain plugs they needed to use. Most coils have both primary and secondary drain connections. Sometimes they have more than one set for convenience, and sometimes they have multiple sets because the coil can be positioned more than one way. It is important to recognize the difference between a primary and secondary drain connection and pipe them correctly.

The secondary drain provides an outlet for the water in case the primary gets plugged up. Since the secondary connection is a backup drain, it is located slightly higher than the primary connection. Sometimes this is done using an internal dam which forces water to go through the primary drain unless it is plugged up.

Primary drain on the right and secondary drain on the left.
Notice the internal dam on the secondary drain connection.

Most manufacturers recommend that the primary drain be trapped, but the secondary is typically not trapped. The secondary and primary should not run to the same drain line. If the drain line gets plugged up and both drain connections run to it, there is really no point in having a secondary drain connection. Often, the secondary drain line runs  very short distance to an overflow drain pan or a condensate pump.

Another way to utilize the secondary drain connection is to install a condensate overflow switch on the secondary drain connection so that the system shuts down if water builds up to that level. I like that because the customer knows there is a problem and calls for a correction. If the secondary drain handles the water without incident it might not be noticed until it stops up as well.

Overflow switch connected to secondary drain connection on the left.

I saw a system this summer which was several years old and had never been piped correctly. The primary and secondary drain connections had been swapped. The primary drain was piped to an overflow drain pan under the unit  and the secondary drain was connected to the actual condensate drain line which was completely dry and clean. The overflow pan was full of brown slime, and the drain to that overflow pan had stopped up and the water was now overflowing into the ceiling. The difference in the two drain connections was obvious – the secondary drain connection was located higher. But maybe not so obvious to the installer. Just know that there is a reason for the two connections: one is for your primary drain and the other is to avert disaster. If you get them confused you are creating a future problem.

Verify

How do you know if your students have actually learned the material? Most teachers verify the results using tests. You check the student’s knowledge against a standard. If the student falls short, you provide some remediation and then test again until they meet the standard. In education, you would not assume that someone has mastered a subject simply because they read a book that had all the material in it or attended all the right lectures. You verify that they have learned the material using written tests, lab exercises, or some combination of both. How do you know that an air conditioning system performs efficiently as designed by the manufacturer? For many people this has involved using design using tools like ACCA Manual J and D, and installation using skilled personnel. These two steps are certainly important, but they can be compared to reading the book and attending the lecture. Without testing to verify your results, you really cannot be sure that the system is performing correctly. After installation, system performance should be verified by testing crucial system operating characteristics. The 2010 ACCA Quality Installation Standard provides a good format for keeping track of all aspects of a quality installation that need verification. Verification is becoming a very big deal in our industry. Increasingly, energy codes, utility rebate programs, and government programs such as Energy Star are insisting on performance verification. It is no longer enough to install a high SEER unit, you must now test and seal the ductwork and test system performance against manufacturer specifications. You can download the ACCA Quality installation Standard from their website https://www.acca.org/industry/quality/quality-installation