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.