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CONDENSER AIR PLENUM
The CAP, Condenser Air Plenum, is used to eliminate negative air pressure in the conditioned space caused by exhausting
the condenser air. The plenum attaches to the condenser inlet air opening on the side of the unit. They connect by removing
the grille assembly and dropping in the plenum in its place. 12 inch round duct can then be routed from an exterior source
and attached to the plenum flange. Additional duct may be attached to the condenser discharge flange for a totally ducted
condenser section. Condenser air plenums can substantially reduce air noise and increase options for solving difficult and
unusual cooling applications. All NorthWind units use a dedicated, automatic, 2 speed condenser blower controlled by the
“load” or pressure in the high side of the refrigerant system.
Another important topic to consider is the amount of duct that can be attached to a unit. Adding duct adds resistance to air-
flow. Too much duct will prevent the unit from operating properly. For proper application, “equivalent feet” must be calcu-
lated. To calculate equivalent feet, add the number of feet of duct on the inlet side of the blower (inlet air plenum) plus the
number of feet on the discharge air side (duct connected to discharge air flange).
Then, for every 90° elbow in the ducting, add 6 feet to the calculation. Here is an example of an equivalent feet calculation:
Number of feet of duct attached to discharge flange 15
Number of feet of duct attached to inlet plenum 20
Number of 90° elbows in discharge run, 1 (= 6 feet) 6
Number of 90° elbows in inlet run, 2 (= 12 feet) 12
Total equivalent feet 53
The following quick reference chart illustrates maximum equivalent feet and maximum E.S.P. (External Static Pressure) al-
lowed for the condenser section of NorthWind units.
Condenser
Plenum Model
Round Duct
Size, Inches
Maximum
Equivalent Feet
Maximum
E.S.P.
CAP12
12 50 .50
CAP18
12 50 .50
CAP36
12 80 .60
CAP60
2-12 110 .70
CAP90
2, 16 120 .80
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