SERVICING
23
Final Charge Adjustment
The outdoor temperature must be 60°F or higher. Set the
room thermostat to COOL, fan switch to AUTO, and set the
temperature control well below room temperature.
After system has stabilized per startup instructions,
compare the operating pressures and outdoor unit amp
draw to the numbers listed in the technical manual. If
pressures and amp draw are too low, add charge. If
pressures and amp draw are too high, remove charge.
Check subcooling and superheat as detailed in the
following section.
5. With the system still running, remove hose and
reinstall both valve caps.
6. Check system for leaks.
Due to their design, Scroll compressors are inherently
more tolerant of liquid refrigerant.
NOTE: Even though the compressor section of a
Scroll compressor is more tolerant of liquid refrigerant,
continued floodback or flooded start conditions may
wash oil from the bearing surfaces causing premature
bearing failure.
Checking Compressor Efficiency
The reason for compressor inefficiency is broken or
damaged scroll flanks on Scroll compressors, reducing the
ability of the compressor to pump refrigerant vapor.
The condition of the scroll flanks is checked in the following
manner.
1. Attach gauges to the high and low side of the
system.
2. Start the system and run a “Cooling Performance
Test.
If the test shows:
A. Below normal high side pressure.
B. Above normal low side pressure.
C. Low
temperature difference across coil.
D. Low amp draw at compressor.
And the charge is correct. The compressor is faulty -
replace the compressor.
Thermostatic Expansion Valve
The expansion valve is designed to control the rate of liquid
refrigerant flow into an evaporator coil in exact proportion
to the rate of evaporation of the refrigerant in the coil. The
amount of refrigerant entering the coil is regulated since
the valve responds to temperature of the refrigerant gas
leaving the coil (feeler bulb contact) and the pressure of the
refrigerant in the coil. This regulation of the flow prevents
the return of liquid refrigerant to the compressor.
The illustration below shows typical heatpump TXV/check
valve operation in the heating and cooling modes.
COOLING
HEATING
TXV Valves
Some TXV valves contain an internal check valve thus
eliminating the need for an external check valve and
bypass loop. The three forces which govern the operation
of the valve are: (1) the pressure created in the power
assembly by the feeler bulb, (2) evaporator pressure, and
(3) the equivalent pressure of the superheat spring in the
valve.
0% bleed type expansion valves are used on indoor and
outdoor coils. The 0% bleed valve will not allow the system
pressures (High and Low side) to equalize during the
shut down period. The valve will shut off completely at
approximately 100 PSIG.
30% bleed valves used on some other models will continue
to allow some equalization even though the valve has shut-
off completely because of the bleed holes within the valve.
This type of valve should not be used as a replacement for
a 0% bleed valve, due to the resulting drop in performance.
The bulb must be securely fastened with two straps to a
clean straight section of the suction line. Application of
the bulb to a horizontal run of line is preferred. If a vertical
installation cannot be avoided, the bulb must be mounted
so that the capillary tubing comes out at the top.
THE VALVES PROVIDED BY GOODMAN ARE
DESIGNED TO MEET THE SPECIFICATION
REQUIREMENTS FOR OPTIMUM PRODUCT
OPERATION. DO NOT USE SUBSTITUTES.
Overfeeding
Overfeeding by the expansion valve results in high suction
pressure, cold suction line, and possible liquid slugging of
the compressor.
If these symptoms are observed:
1. Check for an overcharged unit by referring to the
cooling performance charts in the servicing section.
2. Check the operation of the power element in the
valve as explained in Checking Expansion Valve
Operation.
3. Check for restricted or plugged equalizer tube.
Underfeeding
Underfeeding by the expansion valve results in low system
capacity and low suction pressures.
