12
ComPac 9/04-12
Time Delay/Anti-Short Cycle Timer
The time delay (delay on make) prevents the compressor from restarting immediately after
interruption of power. The delay interval, which is adjustable from .2 to 10 minutes (factory
set at 3 minutes), protects the compressor by allowing internal refrigerant pressures to
equalize. For dual units, set one timer at 3 minutes and the other at 5 minutes.
The time delay does not effect the electric heat circuit.
Low Ambient Control
The low ambient control permits cooling when outdoor ambient temperatures are low. The
control uses a reverse-acting high pressure switch to cycle the condenser fan motor according
to liquid refrigerant pressure conditions. Switch closure and fan operation occurs when the
pressure reaches 250 PSIG. The switch opens again when the refrigerant pressure falls to 190
PSIG. Therefore, the outdoor fan always starts after the compressor, and it will cycle
frequently during normal operation at low outdoor conditions.
High Pressure Switch
The high pressure switch is mounted on the compressor discharge line. It is electrically
connected to a lockout relay which shuts down the system if the refrigerant pressure rises to
400 PSIG. This protects the unit if airflow through the condenser is blocked or if the outdoor
fan motor fails.
Although the contacts of the high pressure switch close when the refrigerant pressure falls to
approximately 300 PSIG, the system must be manually reset once the lockout relay is
activated. A manual reset is necessary to prevent harmful short-cycling. To reset switch, turn
primary power off, then back on or turn thermostat system switch off, then back on.
Low Pressure Switch
The low pressure switch is mounted on the compressor suction line. It is designed to open
if the refrigerant pressure drops to 35 PSIG; it resets when the pressure rises to 60 PSIG.
The switch protects the unit if airflow through the indoor blower is impeded, if the blower
motor fails, or if there is a loss of refrigerant.
Lock-Out Relay
The lockout relay prevents the unit from cycling on the pressure switch by providing positive
shutdown if the high pressure limit is exceeded. An extra set of dry contacts (normally open)
on the relay (Figure 8) may be used for signaling a lockout condition. Once triggered, the relay
must be manually reset.
Timed Low Pressure By-Pass
The timed safety bypass provides a current path around the low pressure switch following
each start for a period of one to three minutes. This bypass is essential when operating
mechanical cooling when ambient temperatures are less than 20°F (-6°C).
Hard Start Kit
Used on single phase equipment to give the compressor higher starting torque under low
voltage conditions. Generally not recommended on units with scroll compressors.
Extreme Duty Package
The Extreme Duty Package allows selected Marvair
®
ComPac
®
I & ComPac
®
II air
conditioners to operate in extremely cold and hot ambient conditions. The Extreme Duty Kit
1.6
B
M
ECHANICAL
C
ONTROLS
(AVP20-72)
1.7 O
PTIONAL
C
ONTROLS
&
P
ACKAGES
