The evaporator condensate drain is often one of the last things run when installing an air conditioner and sometimes is not given a lot of thought. The trouble is it can demand your attention if it does not work. A check list for constructing a working condensate drain that will continue to work and be easily maintained would be
1. The drain should have cleanouts near the unit and the trap to allow for routine maintenance.
2. Draw through coils should always have a trap after the drain pan.
3. The trap should be 4” deep and hold 2” of water.
4. The leaving side of the trap should be at least 2 inches lower than the drain pan outlet.
5. After leaving the trap, the condensate line should slope at least 1/8” per foot.
6. Except for the one trap at the unit, the condensate line should NEVER run up.
7. Long condensate lines may require a vent. If you install a vent, you MUST install a trap and the vent must be AFTER the trap, never before the trap.
8. In humid areas the parts of the condensate line closest to the unit may need to be insulated to prevent sweating.
Drains are getting a little more attention these days because the newer mechanical codes require cleanouts. In the past condensate drains were often cleaned by cutting out the old trap, blowing out the line, and installing a new trap. Now you are required to leave a way to access and clean the drain without cutting into it. Some people use tees or crosses at places where elbows would normally go. You can also purchase some fittings to install in the drain line that make accessing and cleaning the drain line easy. One is called “ALL-Access” Link http://www.allaccessdevice.com/
Another is offered by Rectorseal http://www.rectorseal.com/index.php/easy-klear/
Rectorseal also makes a manufactured trap which is clear, allowing you to see when the trap is full of “condensot.” It has service access built in as well. http://www.rectorseal.com/index.php/ez-trap/
A trap is always required with a draw through coil, otherwise, water will be held in by the negative pressure and the air flowing in through the drain. Many people don’t trap positive pressure coils, such as a coil installed on a furnace. Usually, they will drain because the pressure is pushing the water out. One argument for leaving the trap off on positive pressure coils is that traps tend to get clogged more quickly with “condensnot,” the slimy brown algae that grows in drain pans and drain lines. However, if the air velocity on a blow through coil is high enough, it may still have a negative pressure at the drain outlet. Adding a trap on a blow through coil also provides an air seal between the coil and the outside Manufacturers often specify traps even on blow through coils. Packaged units often specifically warn NOT to trap because they sometimes have a built in trap, and adding another would amount to double trapping. The trap needs to hold enough water to offset the negative pressure of the coil. A common specification calls for a 4” deep trap with a 2” riser so that the trap holds 2” of water and the outlet is 2 “ below the drain pan outlet.
Ever notice that plumbing systems have vents? The vent keeps the draining water and effluent from creating a suction which can slow or even stop drainage. On longer condensate lines, a vent may be required to insure drainage. Vents can also help on lines with minimal slope. The vent is just a tee with a riser which is open to the air. When used, there MUST be a trap and the vent should be AFTER the trap. Putting a vent before the trap pretty much undoes any good the trap would do by allowing air to pass through the vent. The vent should be at the start of the condensate line (but after the trap) and the riser should go above the rim of the drain pan.
One of the laws of plumbing is that waste products run down hill. After the line leaves the trap at the unit it should never go back up. Running the line up after dropping down creates a second trap. Remember that the line is not under pressure, it is gravity flow. It is easy for “condensnot” to accumulate at this second trap and cause the drain to overflow at the unit. I am embarrassed to admit that I have personal knowledge of this. An unintended second trap is a messy service call waiting to happen.
If your unit runs a lot and the coil is located in a humid area, the condensate line can sweat. Usually, the PVC is enough insulation to prevent this, but in some areas people have had problems with dripping condensate lines from sweating. Insulating the trap and all the drain between the trap and the unit with armaflex takes care of this.
Finally, always follow the manufacturer’s instructions. They usually are pretty specific about how the drain is to be run. Trapped or not trapped, vented or not vented – those things are all typically in the instructions. Here is a link to another online posting that shows one manufacturer’s instructions and a drain run according to those instructions.