Thermostatic Expansion Valve (Txv); Txv Operation; Replacing Txv - Carrier WeatherMaster 50HCQ04-12 Service And Maintenance Instructions

NO CHARGE
Use standard evacuating techniques for Puron (R-410A) refrig-
erant. After evacuating system, weigh in the specified amount
of refrigerant.

THERMOSTATIC EXPANSION VALVE (TXV)

All 50HCQs have a factory-installed nonadjustable thermostat-
ic expansion valve (TXV). The TXV will be a bi-flow, bleed
port expansion valve with an external equalizer. TXVs are spe-
cifically designed to operate with Puron
Use only factory-authorized TXVs. See Fig. 15 for a typical
piping schematic.

TXV Operation

The TXV is a metering device that is used in air conditioning
and heat pump systems to adjust to changing load conditions
by maintaining a preset superheat temperature at the outlet of
the evaporator coil.
The volume of refrigerant metered through the valve seat is de-
pendent upon the following (see Fig. 17 and 18):
1. Superheat temperature is sensed by cap tube sensing bulb
on suction tube at outlet of evaporator coil. This tempera-
ture is converted into pressure by refrigerant in the bulb
pushing downward on the diaphragm which opens the
valve using the push rods. As long as this bulb and cap
tube contain any liquid refrigerant, this temperature is con-
verted into suction pressure pushing downward on the dia-
phragm, which tends to open the TXV valve through the
push rods.
2. The suction pressure at the outlet of the evaporator coil is
transferred through the external equalizer tube to the
underside of the diaphragm.
3. The needle valve on the pin carrier is spring loaded, exert-
ing pressure on the underside of the diaphragm. Therefore,
the bulb pressure equals evaporator pressure (at outlet of
coil) plus spring pressure. If the load increases, the tem-
perature increases at the bulb, which increases the pressure
on the top side of the diaphragm, pushing the carrier away
from the seat, opening the valve and increasing the flow of
refrigerant. The increased refrigerant flow causes
increased leaving evaporator pressure which is transferred
through the equalizer tube to the underside of the dia-
phragm. This causes pin carrier spring pressure to close
the TXV valve. The refrigerant flow is effectively stabi-
lized to the load demand with a negligible change in
superheat.
or R-22 refrigerant.
®
12

Replacing TXV

CAUTION
PERSONAL INJURY HAZARD
Failure to follow this caution can result in injury to person-
nel and damage to components.
Always wear approved safety glasses, work gloves, and
other recommended Personal Protective Equipment (PPE)
when working with refrigerants.
1. Recover refrigerant.
2. Remove TXV support clamp using a
3. Remove TXV using a wrench and an additional wrench on
connections to prevent damage to tubing.
4. Remove equalizer tube from suction line of coil. Use file
or tubing cutter to cut brazed equalizer line approximately
2 inches above suction tube.
5. Remove bulb from vapor tube inside cabinet.
6. Install the new TXV using a wrench and an additional
wrench on connections to prevent damage to tubing while
attaching TXV to distributor.
7. Attach the equalizer tube to the suction line. If the coil has
a mechanical connection, then use a wrench and an addi-
tional wrench on connections to prevent damage. If the
coil has a brazed connection, use a file or a tubing cutter to
remove the mechanical flare nut from the equalizer line.
Then use a new coupling to braze the equalizer line to the
stub (previous equalizer line) in suction line.
8. Attach TXV bulb in the same location (in the sensing bulb
indent) where the original was when it was removed, using
the supplied bulb clamps. See Fig. 19.
9. Route equalizer tube through suction connection opening
(large hole) in fitting panel and install fitting panel in
place.
10. Sweat the inlet of TXV marked "IN" to the liquid line.
Avoid excessive heat which could damage the TXV valve.
Use quenching cloth when applying heat anywhere on
TXV.
5 / -in. nut driver.
l6