SERVICING
30
Checking External Static Pressure
The minimum and maximum allowable duct static pressure
is found in the Technical Information Manual.
Too great of an external static pressure will result in
insufficient air that can cause icing of the coil, whereas too
much air can cause poor humidity control, and condensate
to be pulled off the evaporator coil causing condensate
leakage. Too much air can cause motor overloading and in
many cases this constitutes a poorly designed system. To
determine proper air movement, proceed as follows:
1. Using a manometer measure the static pressure
of the return duct at the inlet of the unit, (Negative
Pressure).
Total External Static
2. Measure the static pressure of the supply duct,
(Positive Pressure).
3. Add the two readings together.
NOTE: Both readings may be taken simultaneously
and read directly on the manometer as shown in the
illustration above, if so desired.
4. Consult proper table for quantity of air.
If the external static pressure exceeds the minimum or
maximum allowable statics, check for closed dampers, dirty
filters, undersized or poorly laid out ductwork.
Checking Temperature Rise
Temperature rise is related to the BTUH output of the
unit and the amount of air (CFM) circulated over the heat
exchanger.
All units are designed for a given range of temperature
increase. This is the temperature of the air leaving the unit
minus the temperature of the air entering the unit.
The more air (CFM) being delivered through a given
unit the less the rise will be; so the less air (CFM) being
delivered, the greater the rise. The temperature rise should
be adjusted in accordance to a given unit specifications
and its external static pressure.
1. Take entering and leaving air temperatures.
2. Select the proper speed tap from the unit’s blower
performance data in the Technical Manual for the
specific unit.
3. Take motor amperage draw to determine that the
motor is not overloaded during adjustments.
T
T
RISE = SUPPLY -
T
T
Checking Temperature Rise
