Allied Air A97USMV Installation Instructions Manual page 26

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 36
Figure 35
Evaporator Coil using a Common Drain
(Unit shown in Horizontal Left-Hand Discharge Position)
Figure 37
Page 26 of 74 Issue 1622 507028-03