Thermostatic Expansion Valve (Txv); Txv Operation - Bryant Preferred 551J Service And Maintenance Instructions

PROBLEM
RDV Valve Operation
(NOTE: Normally Closed
When De-energized)
Low Latent Capacity in
Subcooling or Hot Gas
Reheat Modes
Low Sensible Capacity in
Normal Cool or Subcooling
Reheat Modes
Low Suction Pressure and
High Superheat During
Normal Cool Mode
Low Suction Pressure and
High Discharge Pressure
RDV Valve Cycling On/Off
Circuit B Will Not Operate
With Circuit A Off
LEGEND
CRC — Cooling/Reheat Control
CLV — Cooling Liquid Valve
RLV — Reheat Liquid Valve
RH — Relative Humidity
RDV — Reheat Discharge Valve

THERMOSTATIC EXPANSION VALVE (TXV)

All 551J units have a factory installed nonadjustable thermostatic
expansion valve (TXV). The TXV will be a bi-flow, bleed port ex-
pansion valve with an external equalizer. TXVs are specifically
designed to operate with Puron or R-22 refrigerant, use only facto-
ry authorized TXVs. See Fig. 18.

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 main-
taining a preset superheat temperature at the outlet of the evapora-
tor coil.
The volume of refrigerant metered through the valve seat is de-
pendent upon the following:
1. Superheat temperature is sensed by the cap tube sensing
bulb on suction the tube at the outlet of evaporator coil.
This temperature is converted into pressure by refrigerant
in the bulb pushing downward on the diaphragm which
opens the valve using 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 the evaporator pressure (at the
outlet of the coil) plus the spring pressure. If the evapora-
tor load increases, the temperature increases at the bulb,
which increases the pressure on the topside of the dia-
phragm, pushing the carrier away from the seat, opening
the valve and increasing the flow of refrigerant. The
increased refrigerant flow causes increased leaving evapo-
rator pressure which is transferred through the equalizer
Table 4 — Perfect Humidity Troubleshooting (cont)
CAUSE
No 24V signal to input terminals
Solenoid coil burnout
Stuck valve
CLV valve open or leaking
RDV valve open or leaking
General cooling mode problem
RDV valve open or leaking
General cooling mode problem
Both RLV and CLV valves closed
Hot Gas Reheat mode low suction pressure
limit
Normal operation. Motormaster outdoor fan
control requires operation of circuit A.
Check using Cool→Reheat1 Valve Test (Service Test → HMZR →
RHV.A or RHV.B) using ComfortLink Scrolling Marquee.
Check MBB relay output.
Check wiring.
Check transformer and circuit breaker or fuses.
Check continuous over-voltage is less than 10%.
Check under-voltage is less than 15%.
Check for missing coil assembly parts.
Check for damaged valve enclosing tube.
Replace valve. Replace filter drier.
See CLV Valve Operation.
See RDV Valve Operation.
See Cooling Service Troubleshooting.
See RDV Valve Operation.
See Cooling Service Troubleshooting.
See RLV and CLV Valve Operation.
Normal Operation During Mixed Circuit Subcooling and Hot Gas
Reheat Modes at Lower Outdoor Temperatures.
None
tube to the underside of the diaphragm. This causes the pin
carrier spring pressure to close the TXV valve. The refrig-
erant flow is effectively stabilized to the load demand with
a negligible change in superheat.
REPLACING TXV
PERSONAL INJURY HAZARD
Failure to follow this caution can result in injury to personnel
and damage to components.
Always wear approved safety glasses, work gloves and other
recommended Personal Protective Equipment (PPE) when
working with refrigerants.
1. Disconnect all AC power to the unit. Use approved lock-
out/tag-out procedures.
2. Using the gage set approved for use with Puron (R-410A)
refrigerant, recover all refrigerant from the system.
3. Remove the TXV support clamp.
4. Disconnect the liquid line at the TXV inlet.
5. Remove the liquid line connection at the TXV inlet.
6. Remove the equalizer tube from the suction line of the
coil. Use a tubing cutter to cut the brazed equalizer line
approximately 2-in. (50 mm) above the suction tube.
7. Remove the bulb from the vapor tube above the evaporator
coil header outlet.
15
REMEDY
CAUTION