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.
Cleanout
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/
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.
Vent
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.
Slope
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.
Insulation
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.
Instructions
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.
