Bulletin 30-20 — Page 23
© Copyright 2010, Unico, Inc.
Figure 26. Measuring Plenum Static Pressure
In the absence of a manometer you can build a simple
but less accurate manometer in one of two ways. One
way is to use a short piece of ruler or yardstick and clear
plastic tubing as shown in Fig. 27.
Figure 27. U-Tube Manometer
Setting the Ventilation Speed. Constant ventilation
is optional. To enable constant ventilation, simply turn
the variable speed control knob clockwise. The lowest
speed that produces some amount of air circulation is
best. This is usually the lowest speed position (as far
clockwise as possible). The controller may also be set at
any other speed but it is not necessary for ventilation.
Charging the System
DO NOT VENT REFRIGERANT TO THE
ATMOSPHERE!! It is a violation of federal law and in
some cases local ordinances also. Always use a refrige-
rant recovery or recycling device.
If charging a heat pump, refer to Bulletin 30-24, Instruc-
tions for "M" Series Modular Heat Pump Systems. The
following procedure is only valid for charging the sys-
tem during the cooling mode.
To check for proper charge record the refrigerant pres-
sures and temperatures. Check the refrigerant charge by
measuring the amount of sub-cooling (or ‘approach’
temperature for some condensing units). If the outdoor
manufacturer does not have sub-cooling or “approach”
temperature charts, then be sure that the sub-cooling is
between 3 and 8 °F (2 to 5°C). For long refrigerant lines
or when the evaporator is above the condenser, the sub-
cooling should be close to 8°F; otherwise, aim for the
low end of the range.
After the refrigerant lines and evaporator have been
carefully leak tested and evacuated, release the R22 op-
erating charge in the condensing unit. The system is now
ready for refrigerant charge adjustment. If the coil is
equipped with a threaded TXV, always verify that the
threads are tight and do not leak.
Start up the system and check line voltage to assure it is
within acceptable limits for the system as dictated by the
condensing unit manufacturer. Run the system for 20 to
30 minutes to get reasonably stabilized conditions. Do
not attempt to adjust charge with outdoor temperature
below 75°F (24°C). An outdoor temperature of 75 to
85°F (24 to 29°C) is preferred.
If the system charge must be checked when the outdoor
temperature is below 80°F (26.7°C), block the condenser
coil until the head pressure is approximately equal to
what its charging chart specifies for an 85°F (29°C) day.
For heat pumps always check the charge in cooling
mode. If this is not possible because of low outdoor tem-
peratures, charge the system in the heating mode, but
return later when the weather is warmer before the sys-
tem is switched to cooling.
Sub-cooling Method Many condensing unit manufac-
turers publish the amount of sub-cooling that the con-
denser will produce. Follow their instructions to charge
the unit. Typical sub-cooling values will be between 3 an
8°F (2 to 5°C). The unit should ALWAYS have some
amount of sub-cooling. To be sure there is enough sub-
cooling, especially if the unit is in a hot attic, check the
liquid line sight glass near the evaporator for bubbles or
measure the refrigerant liquid line pressure and tempera-
ture AT THE EVAPORATOR.
To measure sub-cooling use the following procedure:
1. Measure and record the liquid line pressure using an
accurate refrigerant gauge. Record the corresponding
saturation temperature for this pressure (see Table 8).
2. Measure and record the liquid line temperature using
an accurate metal or glass thermometer, or thermo-
couple. Tape or strap the sensor firmly against the
surface of the liquid line and cover with insulation.
3. Determine the sub-cooling with the following equa-
tion:
Saturated Temperature
— Liquid Line Temperature
= Subcooling
If the sub-cooling temperature at the condenser is low,
the system is undercharged and refrigerant must be add-
ed.
CAUTION
TO PREVENT DAMAGE TO THE
COMPRESSOR, DO NOT ADD LIQUID
CHARGE INTO SUCTION PORT.
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