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Air Handling Unit
Installation, Operation, and Maintenance Manual
April 2021
Figure 7-2. Negative Internal Static Pressure in Coil Section-
Fan On (Draw-Through Application)
Condensate drain pan
H/2
minimum
Sea level when
condensate
begins to drain off
H = Fan inlet pressure
(in. W.C.) + 1 in.
Fan inlet pressure
(negative)
H
Trap for draw-thru unit, condensate
beginning to drain away.
H = Negative Internal Static Pressure in Drain Pan
Section (in w.g.)+1”
H/2 = Negative Internal Static Pressure in Drain Pan
Section (in. wg) divided by 2.
Example:
Neg SP in Drain Pan Section=3.6 in/w.g.
H = 3.6+1 = 4.6”
H/2 = (3.6 / 2) = 1.8”
Figure 7-3. Positive Internal Static Pressure in Coil Section-
Fan Off (Blow-Through Application)
Condensate drain pan
Recommendation:
1/2-in. minimum
H = Fan outlet pressure
(in. W.C.) + 1/2 in.
(minimum)
Maximum sea level
with fan off
Trap for blow-thru unit, fan off.
H = Positive Internal Static Pressure in Drain Pan
Section (in W.G.)+.5”
Example:
Pos SP in Drain Pan Section=3.6 in/w.g.
H = 3.6+.5”= 4.1”
Total Trap Dimension =4.1+.5” = 4.6”
