SERVICING
51
6. Start up the unit and record the pressure drop across the
drier.
7. Continue to run the system for a minimum of twelve (12)
hours and recheck the pressure drop across the drier.
Pressure drop should not exceed 6 PSIG.
8. Continue to run the system for several days, repeatedly
checking pressure drop across the suction line drier. If
the pressure drop never exceeds the 6 PSIG, the drier has
trapped the contaminants. Remove the suction line drier
from the system.
9. If the pressure drop becomes greater, then it must be
replaced and steps 5 through 9 repeated until it does not
exceed 6 PSIG.
NOTICE:
Regardless, the cause for burnout must be deter-
mined and corrected before the new compressor is started.
S-200 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 insuffi-
cient 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 draft gauge (inclined manometer) measure the
static pressure of the return duct at the inlet of the unit,
(Negative Pressure).
RETURN
SUPPLY
INCLINED
MANOMETER
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 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.
S-201 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. Check BTUH input to unit do not exceed input rating
stamped on rating plate.
2. Take entering and leaving air temperatures.
3. Select the proper speed tap or dip switch setting for direct
drive units.
4. Take motor amperage draw to determine that the motor is
not overloaded during adjustments.
RETURN
SUPPLY
SUPPLY
T
RETURN
T
RISE = SUPPLY -
T
RETURN
T
Checking Temperature Rise
