SERVICING
45
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.
NOTE:
THIS TEST CANNOT BE DONE IN
THE HEATING MODE
Verification of proper rotation of Scroll Compressors is made
as follows.
NOTE:
The compressor may run backwards (noisy opera-
tion) for 1 or 2 seconds at shutdown. This is normal and
does not harm the compressor.
1. Install gauges and verify that the suction pressure drops
while the discharge pressure increases.
2. Listen for normal compressor sound levels. Reverse rota-
tion results in elevated or unusual sound levels.
3. Reverse rotation will result in substantially reduced amp
draw from tabulated values.
To correct improper rotation, switch any two power supply
leads at the outdoor unit contactor.
The 3 phase Scroll Compressors are direction of rotation sen-
sitive. They will rotate in either direction depending on the
phasing of the power. There is no negative impact on durabil-
ity caused by operating 3 phase compressors in reversed
rotation. The compressors internal protector will trip, de-en-
ergizing the compressor. Continued operation of 3 phase scroll
compressors with the rotation reversed will contribute to com-
pressor failure. All 3 phase scroll compressors should be
checked for correct phase rotation.
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 heat pump TXV/check
valve operation in the heating and cooling modes.
COOLING
HEATING
THERMOSTATIC EXPANSION VALVES
Some TXV valves contain an internal check valve thus elimi-
nating 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 12 to
18 degrees superheat of the suction gas. Before checking
the superheat or replacing the valve, perform all the proce-
dures outlined under Air Flow, Refrigerant Charge, Expan-
sion Valve - Overfeeding, Underfeeding. These are the most
common causes for evaporator malfunction.
