"Curmudgeon" wrote in message
...
That I know...

So why do horizontal condensate drain lines from AC units have a trap
bend?
They connect to nothing but air.
The only thing I can figure is to keep insects and the like from crawling
into the unit.
But again, a trap bend in a vertical line connected to nothing would make
no
sense...for either gas or insects.


Its because there is negative pressure inside the air handler as opposed to
atmospheric, due to air pressure drop through the cooling coil.

Without a trap, the drain simply sucks air into the unit, and so then the
condensate collects all over inside the unit, overflows the drain pan and
generally makes one hell of a mess whenever the fan shuts down.

The trap seals against this occurance and allows draining because it traps a
water column of sufficient mass and at a low enough elevation such that this
negative pressure cannot overcome and lift water out of the trap--remember
vaccuum is often measured as "Water column".......together, this all allows
the condensate to drain to the outside of the cabinet under
gravity..........

--

SVL