Your right but what happens during the winter? The P-trap dries out and now
you can suck in sewer gases. Some times you can get away with running your
condensate line into a vent pipe when you have a small pressure valve that
keeps your P-trap full of water.
That doesn't apply if the condensate drips into free air space before
trickling into the sewer drain. In fact, it can help ensure that a
seldom used drain trap doesn't run dry.
I know of an instance in an office building where a return air plenum
access panel was inadvertently left open, creating negative air pressure
in a fan room where the floor drain trap was dry. The result was sewer
gas being distributed throughout the building. Phew! That might not
have happened if condensate was going down that drain (and if the HVAC
tech was more conscientious).
Huh. The condensation drain--the visible part---starts as a couple-inch
long piece of pipe projecting horizontally out of the AC unit (which itself
is on top of the furnace). But very close nearby there _is_ a second,
similar projection, but that's capped off.
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