18
15.3 LOW AMBIENT CONTROL (LAC)
This component senses compressor head pressure and shuts the heat pump fan off
when the head pressure drops to approximately 220 PSIG [1516.8 kPa]. This
allows the unit to build a sufficient head pressure at lower ambient in order to main-
tain system balance and obtain improved capacity. Low ambient control should be
used on all equipment operated below 70°F [21.1°C] ambient.
15.4 HIGH AND LOW PRESSURE CONTROLS (HPC & LPC)
These controls keep the compressor from operating in pressure ranges which can
cause damage to the compressor. Both controls are in the low voltage control cir-
cuit.
High pressure control (HPC) is an auto-reset which opens near 610 PSIG and
closes at 420 PSIG.
The low pressure control (LPC) is an automatic reset which opens near 50 PSIG
and closes near 95 PSIG.
16.0 SERVICE
16.1 OPERATION
Single phase units are operated PSC (no starting components). It is important that
such systems be off for a minimum of 5 minutes before restarting to allow equaliza-
tion of pressure. The thermostat should not be moved to cycle unit without waiting 5
minutes. To do so may cause the compressor to go off on an automatic overload
device or blow a fuse. Poor electrical service can also cause nuisance tripping on
overloads, trip a breaker, or cause light dimming. This generally can be corrected
by adding start components. Check with factory for recommended start compo-
nents, if required. For PSC type operation, refrigerant metering must be done with
fixed orifice, cap tubes or bleed type expansion valves because of low starting
torque. If non-bleed expansion valve coils (supplied by factory) are used, start com-
ponents are required.
16.2 SINGLE-POLE COMPRESSOR CONTACTOR (CC)
Single-pole contactors are used on all standard single phase units up through 5
tons. Caution must be exercised when servicing as only one leg of the power sup-
ply is broken with the contactor.
FIGURE 3
CONTROL WIRING FOR GAS OR ELECTRIC HEAT
FOR TYPICAL ELECTRIC HEAT
FOR TYPICAL GAS OR OIL HEAT
