GSX14 Service Manual

SERVICING
155
S-105B 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 approxi-
mately 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 SUB-
STITUTES.
S-106 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 S-110 Checking Expansion Valve Operation.
3. Check for restricted or plugged equalizer tube.
S-107 UNDERFEEDING
Underfeeding by the expansion valve results in low system
capacity and low suction pressures.
If these symptoms are observed:
1. Check for a restricted liquid line or drier. A restriction will
be indicated by a temperature drop across the drier.
2. Check the operation of the power element of the valve as
described in S-110 Checking Expansion Valve Operation.
S-108 SUPERHEAT
The expansion valves are factory adjusted to maintain 8 to 12
degrees superheat of the suction gas. Before checking the
superheat or replacing the valve, perform all the procedures
outlined under Air Flow, Refrigerant Charge, Expansion Valve
- Overfeeding, Underfeeding. These are the most common
causes for evaporator malfunction.
CHECKING SUPERHEAT
Refrigerant gas is considered superheated when its tempera-
ture is higher than the saturation temperature corresponding
to its pressure. The degree of superheat equals the degrees
of temperature increase above the saturation temperature at
existing pressure. See Temperature - Pressure Chart on
following pages.
CAUTION
To prevent personal injury, carefully connect and
disconnect manifold gauge hoses. Escaping liquid
refrigerant can cause burns. Do not vent refrigerant
to atmosphere. Recover during system repair
or final unit disposal.
1. Run system at least 10 minutes to allow pressure to
stabilize.
2. For best results, temporarily install a thermometer on the
liquid line at the liquid line service valve and 4-6" from the
compressor on the suction line. Ensure the thermometer
makes adequate contact and is insulated for best pos-
sible readings. Use liquid line temperature to determine
sub-cooling and vapor temperature to determine super-
heat.
NOTE: An optional method is to locate the thermometer
at the suction line service valve. Ensure the thermometer
makes adequate contact and is insulated for best
possible readings.
3. Refer to the superheat table provided for proper system
superheat. Add charge to lower superheat or recover
charge to raise superheat.
Superheat Formula = Suct. Line Temp. - Sat. Suct. Temp.
EXAMPLE:
a. Suction Pressure = 143
b. Corresponding Temp. °F. = 50
c. Thermometer on Suction Line = 61°F.