Goodman [A/G]PG, Service Instructions Manual

SERVICING

41

S-105 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 refriger-
ant to the compressor.

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 the indoor coils.
The 0% 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 Pressure.

Good thermal contact between the feeler bulb and the suction
line is essential to satisfactory valve control and performance.

The bulb must be securely fastened 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 should be mounted so that the capillary tubing comes out
at the top.

THE VALVES PROVIDED ARE DESIGNED TO MEET THE
SPECIFICATION REQUIREMENTS FOR OPTIMUM PROD-
UCT OPERATION.  

DO NOT USE SUBSTITUTES.

S-106 OVERFEEDING

Overfeeding by the expansion valve results in high suction pres-
sure, cold suction line, and possible liquid slugging of the com-
pressor.

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 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 whenever its tem-
perature is higher than the saturation temperature correspond-
ing to its pressure.  The degree of superheat equals the de-
grees of temperature increase above the saturation tempera-
ture at existing pressure.  See Temperature - Pressure Chart.

1. Attach an accurate thermometer or preferably a thermo-

couple type temperature tester to the suction line at a point
at least 6" from the compressor.

2. Install a low side pressure gauge on the suction line ser-

vice valve at the outdoor unit.

3. Record the gauge pressure and the temperature of the line.

4. Convert the suction pressure gauge reading to temperature

by finding the gauge reading in Temperature - Pressure
Chart and reading to the left, find the temperature in the °F.
Column.

5. The difference between the thermometer reading and pres-

sure to temperature conversion is the amount of super-
heat.

EXAMPLE:

a.  Suction Pressure = 84

b.  Corresponding Temp. °F. = 50

c.  Thermometer on Suction Line = 63°F.

To obtain the degrees temperature of superheat subtract 50.0
from 63.0°F.

The difference is 13° Superheat.  The 13° Superheat would fall
in the ± range of allowable superheat.

SUPERHEAT ADJUSTMENT

The expansion valves used are factory set and are not field
adjustable.  If the superheat setting becomes disturbed, re-
place the valve.

S-109 CHECKING SUBCOOLING

Refrigerant liquid is considered subcooled whenever its tem-
perature is lower than the saturation temperature correspond-
ing to its pressure.  The degree of subcooling equals the de-
grees of temperature decrease below the saturation tempera-
ture at the existing pressure.

1. Attach an accurate thermometer or preferably a thermo-

couple type temperature tester to the liquid line as it leaves
the condensing unit.