Allied Air A96US2V Installation Instructions Manual page 37

Evaporator Coil using a Common Drain
IMPORTANT
When combining the furnace and evaporator coil
drains together, the A/C condensate drain outlet must
be vented to relieve pressure in order for the furnace
pressure switch to operate properly.
Condensate Trap with Optional Overflow Switch
Figure 54
Figure 53
Evaporator Coil using a Common Drain
*Piping from furnace must slope down a minimum of 1/4" per ft. toward trap.
Figure 55
507404-04 Issue 1621 Page 37 of 80