Carrier 50ET - Manual (Page 5)

HEATING A COOLING
50ET,QT
Single-Package Units
To Start Unit — Check that main power is on and,
if temperature is below 65 F, that compressor crankcase
heater has been energized for at least 24 hours.
1. Set selector switch at <; -O—^ if thermostat is
mounted on chassis, or at OFF if thermostat accessory
is remote mounted.
2. Set fan switch as desired (FAN) (AUTO.).
3. Set thermostat lever at the desired temperature.
4. Set selector .switch at HEAT or COOL. Check sy.stem
refrigerant charge. See Refrigerant Charging.
SERVICE
Service Port Connections — High and low side
pressure connections are accessible from the indoor
portion of the unit for charging. (See Fig. 3.)
Low-Pressure Switch (50QT only) (Safety
Control)
is located on liquid line downstream of Accu-
Rater™ control during cooling mode (or upstream of
AccuRater control during heating mode). Switch opens
at 5 psig and shuts down compressor to protect it from
overheating if refrigerant charge is too low.
High-Pressure Relief Valve (Safety Control) is
located in compressor. Relief valve opens at a pressure
differential of approximately 450 ± 50 psi between
suction (low side) and discharge (high side) to allow
pressure equalization.
Internal Current and Temperature Sensitive
Overload (Safety Control)
resets automatically
when compressor motor temperature drops to a safe level
(overloads may require up to one hour to reset). When an
internal overload is suspected of being open, check by
using an ohmmeter or continuity tester. If necessary,
refer to Carrier Standard Service Techniques Manual,
Chapter 2, Electrical, for complete instructions.
Defrost Control consisting of a control board and
defrost thermostat, interrupts normal system heating
operation to remove frost and iee formation on outdoor
coil. Frost impairs unit performance. Defrost control
simultaneously stops outdoor fan, energizes reversing
valve solenoid to switch system into cooling cycle (out­
door unit as condenser, indoor unit as evaporator), and
activates accessory electric heater. Unit can defrost every
90 minutes, but will do so only if outdoor temperatures
are in the frosting temperature zone.
For heat pump to defrost, 2 eonditions are necessary;
1 Defrost circuit board contacts must be closed.
2. Coil temperature must be cold enough to cause defrost
thermostat contacts to close.
Contacts clo.se at 27 ±5 F. Every 90 minutes of elap.sed
running time, the defrost circuit board contacts close for
10 seconds. If the defrost thermostat contacts are closed,
the unit defrosts The defrost circuit board limits defrost­
ing period to 10 minutes. Normally the frost is removed
and the defrost thermostat contacts will open to terminate
defrosting before 10 minutes have elapsed. Defrost
thermostat contacts open at 80 ±5 F. When defrosting is
terminated, the outdoor fan motor is energized and
reversing valve solenoid is de-energized, returning unit
to heating cycle.
HEAT PUMP CIRCUITS shown in Fig. 5 are refrig­
erant flow diagrams for heating and cooling cycles.
Refrigerant Charging
A CAUTION
To prevent personal injury, wear safety glasses and
gloves when handling refrigerant.
Do not overcharge system. An overcharge can
cause compressor damage.
Unit refrigerant system is factory charged. When
recharging is necessary, weigh in total charge indicated
in Table 3. (Charge must be weighed in during heating
season.) Remove any refrigerant remaining in system
before recharging. If system has lost complete charge.
3 Unit Piping
Fig. 5 — 50QT Refrigerant Flow Diagrams
Manulacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obiigations.
Bookll h |4 |4 PC131 Catalog No 535-078 PrintedinUSA Form 50ET,QT-3S1 Pg5 5-85 Replaces: 50QT-10SI
FROcTlT^r^ Pgj replacement items use Carrier Specified Parts