Condensate drains are one of the simplest parts of an air conditioning system, unfortunately they can be one of the most frustrating also.
The concept is pretty straight forward; water from the evaporator coil drips into the condensate pan, then it drains from the pan through a pipe to a building drain, or outside. Easy! Until it stops working...
How many times have you gone out on a condensate leak call, blown out the line and cleaned the trap, only to be called back again for the same leak?
You blow out the line again, certain you “got it this time” only to receive another call back later the same day! The problem continues until, out of desperation, you run a whole new condensate line. What was wrong? Was there something lodged in the piping? Not likely.
Chances are you ran into a double trap situation.
All condensate drains have (or should have) a “P” trap installed at the air handler drain connection. Its purpose is to form a liquid seal that prevents air being drawn into the condensate drain that would prevent water from draining out. I know, pretty basic so far.
The outlet of the trap is connected to a drain line that routes the water to wherever it’s being dumped. The trap and the drain line are usually made of PVC pipe, and normally run a good distance to the final drain point.
The problems begin if the drain line develops a sag in it that forms a second trap.
To make this easier to imagine, picture holding your thumb over the end of a straw filled with water; can the water drain from the straw? No, not until you remove your thumb. Why? Because the blockage (your thumb) caused a vacuum to form. Remove your thumb, and the water drains from the straw.
The dip or sag in the condensate line is like your thumb over the straw opening. Remove the dip in the line and the condensate drains.
You'll normally run across this in attic installations. The extreme temperatures softens the PVC pipe allowing it to sag between the pipe supports.
There are two possible solutions to the problem. The first is to mount the pipe supports close enough together to eliminate the potential for sagging.
The second is to install a vent tee at the outlet of the trap, and for longer pipe runs, at strategic locations along the run. These vents will prevent a vacuum from forming should the drain line sag.
When installing vents, keep the branch of the tee pointing up, and always install a stand pipe that extends a few inches higher than the topmost point of the drain pan lip. The stand pipe will prevent water from overflowing from the vent tees should the drain line become obstructed at the outlet.