Thermostatic Expansion Valves (Txv); Problems Affecting Txv; Piston Body Cleaning Or Replacement - Carrier FA4ANF Service Manual

humidistat contacts opening on increasing humidity and reduce its
airflow to approximately 80 percent of nominal cooling mode
airflow. This reduction will increase the system latent capacity
until the humidity falls to a level which causes the humidistat to
close its contacts. When the contacts close, airflow will return to
100 percent of the selected cooling airflow. To activate this mode,
remove jumper J1 and wire in a standard humidistat. (See Fig. 29.)
Carefully consult product airflow data for cooling and dehumidi-
fication modes.
EASY SELECT
BOARD TERMINAL
BLOCK
D
J1
REMOVE
JUMPER
Fig. 29—Humidistat Wiring for De-Humidify Mode

THERMOSTATIC EXPANSION VALVES (TXV)

The FC4B, FK4C, FX4A, FV4A, and 40FKA Fan Coils are
factory equipped with a hard shutoff (HSO) TXV.
The hard shutoff TXV has no bleed port and allows no bleed-
through after system is shutdown. No pressure equalization occurs.
A start capacitor and relay must be installed on single-phase
reciprocating compressors to boost torque to compressor motor so
it may overcome the unequalized system pressures.
The standard TXV is a bi-flow metering device that is used in
condensing and heat pump systems to adjust to changing load
conditions by maintaining a preset superheat temperature at outlet
of evaporator coil. The volume of refrigerant metered through
valve seat is dependent upon the following:
1. Superheat temperature sensed by sensing bulb on suction tube
at outlet of evaporator coil. As long as this bulb contains some
liquid refrigerant, this temperature is converted into pressure
pushing downward on the diaphragm, which opens the valve
via push rods.
2. The suction pressure at outlet of evaporator coil is transferred
via the external equalizer tube to underside of diaphragm.
3. The needle valve on pin carrier is spring-loaded, which also
exerts pressure on underside of diaphragm via push rods,
which closes valve. Therefore, bulb pressure equals evapora-
tor pressure at outlet of coil plus spring pressure. If load
increases, temperature increases at bulb, which increases
pressure on topside of diaphragm, which pushes pin carrier
away from seal, opening valve and increasing flow of refrig-
erant. The increased refrigerant flow causes increased leaving
evaporator pressure which is transferred via the equalizer tube
to underside of diaphragm, with which the pin carrier spring
pressure closes valve. The refrigerant flow is effectively
stabilized to load demand with negligible change in superheat.
The bi-flow TXV is used on split system heat pumps. In cooling
mode, TXV operates the same as a standard TXV previously
explained. However, when system is switched to heating mode of
operation, refrigerant flow is reversed. The bi-flow TXV has an
additional internal check valve and tubing. These additions allow
refrigerant to bypass TXV when refrigerant flow is reversed with
only a 1- to 2-psig pressure drop through device. When heat pump
switches to defrost mode, refrigerant flows through a completely
H
HUMIDISTAT
R
A95316
open (not throttled) TXV. The bulb senses the residual heat of
outlet tube of coil that had been operating in heating mode (about
85°F and 155 psig). This temporary, not throttled valve, decreases
indoor pressure drop, which in turn increases refrigerant flow rate,
decreases overall defrost time, and enhances defrost efficiency.
Step 1—Problems Affecting TXV
LOW SUCTION PRESSURE
1. Restriction in TXV
2. Low refrigerant charge
3. Low indoor load
4. Low evaporator airflow
HIGH SUCTION PRESSURE
1. Overcharging
2. Sensing bulb not secure to vapor tube
3. High indoor load
4. Large evaporator face area
When installing or removing TXV, wrap TXV with a wet cloth.
When reattaching TXV, make sure sensing bulb is in good thermal
contact with suction tube.

PISTON BODY CLEANING OR REPLACEMENT

Do not vent refrigerant to atmosphere. Recover during system
repair or final unit disposal.
Damage may occur to the scroll compressor if operated at a
negative suction pressure during a system pumpdown.
1. Pump down outdoor unit. Close service valves at outdoor unit.
2. Recover remaining refrigerant from tubing and coil through
gage port on vapor-tube service valve.
3. Disconnect refrigerant (liquid) tube from piston body. (See
Fig. 30.)
4. Avoid damaging seal ring or machined surfaces on piston,
bore, and retainer.
5. Using small wire with a hook on end of it, remove piston from
body.
TEFLON SEAL
PISTON
RETAINER
BRASS
HEX NUT
Fig. 30—Refrigerant Flow-Control Device
(For FA, FB, and FF)
When cleaning the piston orifice, be careful not to scratch or
enlarge the opening, as this will affect operation.
6. Install new or cleaned piston into body.
36
PISTON
BRASS
HEX BODY
A93530