S-125 Thermostatic Expansion Valve; S-200 Checking External Static Pressure - Goodman *PH 16 Service Instructions Manual

SERVICING

S-125 THERMOSTATIC EXPANSION VALVE

The expansion valve is designed to control the rateof 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 TSV/check valve
operation in the heating and cooling modes.
COOLING
TXV VALVES
Some TXV valves contain an internal check valve thus eliminat-
ing 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 ofthe su-
perheat spring in the valve.
0% bleed type expansion valves are used on indoor and out-
door coils. The 0% bleed valve will not allow the system pres-
sures (high and Low side) to equalize during the shut down
period. The valve will shut of completely at approximately 100
PSIG.
30% bleed valves used on some other models will continue to
allow some equalization even though the valve has a 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 bult must be mounted so that the capillary
tubing comes out at the top.
THE VALVES PROVIDED ARE DESIGNED TO MEET THE
SPECIFICATION REQUIRMENTS FOR OPTIMUM PRODUCT
OPERATION. DO NOT USE SUBSTITUTES.
40
S-200 CHECKING EXTERNAL STATIC
PRESSURE
The minimum and maximum allowable duct static pressure is
found in the Technical Information Manual.
Too great of an external static pressure will result in insuffi-
cient air that can cause icing of the coil, whereas too much air
can cause poor humidity control, and condensate to be pulled
off the evaporator coil causing condensate leakage. Too much
air can cause motor overloading and in many cases this con-
stitutes a poorly designed system. To determine proper air
movement, proceed as follows:
1. Using a draft gauge (inclined manometer) measure the static
HEATING
2. Measure the static pressure of the supply duct, (Positive
3. Add the two readings together.
NOTE: Both readings may be taken simultaneously and read
directly on the manometer as shown in the illustration above, if
so desired.
4. Consult proper table for quantity of air.
If the external static pressure exceeds the minimum or maxi-
mum allowable statics, check for closed dampers, dirty filters,
undersized or poorly laid out ductwork.
pressure of the return duct at the inlet of the unit, (Negative
Pressure).
Supply
Return
Total External Static
Pressure).