SERVICING
37
S-202 DUCT STATIC PRESSURES AND/OR STATIC PRESSURE
DROP ACROSS COILS
This minimum and maximum allowable duct static pressure for
the indoor sections are found in the specifications section.
Tables are also provided for each coil, listing quantity of air
(CFM) versus static pressure drop across the coil.
Too great an external static pressure will result in insufficient air
that can cause icing of the coil. Too much air can cause poor
humidity control and condensate to be pulled off the indoor coil
causing condensate leakage. Too much air can also cause motor
overloading and in many cases this constitutes a poorly designed
system.
S-203 AIR HANDLER EXTERNAL STATIC
To determine proper air movement, proceed as follows:
1. Using a draft gauge (inclined manometer), measure the static
pressure of the return duct at the inlet of the unit, (Negative
Pressure).
2. Measure the static pressure of the supply duct, (Positive
Pressure).
3. Add the two (2) readings together for total absolute value of
external static pressure.
(for example, -0.30"wc +0.20"wc=0.50"wc total static
pressure)
TOTAL EXTERNAL STATIC
NOTE: Both readings may be taken simultaneously and read di-
rectly on the manometer if so desired.
4. Consult proper table for quantity of air.
If external static pressure is being measured on a furnace to
determine airflow, supply static must be taken between the "A"
coil and the furnace.
TOTAL EXTERNAL STATIC
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