Marvair Scholar III™ VAISA I&S Manual 03/2019 Rev. 8
30
If the W2 indicator is on and the ODS indicator is on then, in addition to the CC, IBM
and OFM indicators, the HTR indicator will be on indicating the call for second stage
heat.
If the SCKT LED is on indicating that the S Circuit function is selected, the CC and
OFM indicators will be off and the HTR indicator will be on. Refer to table below.
G Y W2
ODS
SCKT
IBM
CC
OFM
HTR
ON
OFF OFF OFF OFF ON
OFF OFF OFF
ON
ON OFF OFF OFF ON
ON ON OFF
ON
ON ON OFF OFF ON
ON ON OFF
ON
ON ON ON
OFF ON
ON ON ON
ON
ON ON ON
ON ON
OFF OFF ON
Defrost Mode (heat pump only)
When the system has been operating in the heat pump mode for a period of time (set
by the Defrost Timer), the system will examine the Defrost Control thermostat Input
(DFC). If this input is on (the thermostat is closed), indicated by the LED being on, the
system will go into Defrost mode. At this point the Outdoor Fan Motor relay (OFM) is
de-energized and the reversing valve is energized. In this mode heat is being applied
to the outdoor coil to remove any possible buildup of ice on the coil. The Defrost
Control Switch (DFC) comes on at roughly 28°F and goes off at approximately 56°F.
During the Defrost Cycle, the unit will continuously examine the DFC input and when
it switches off OR the system has been in defrost for 10 minutes, the system will
revert back to normal heating mode. By having a maximum time for the Defrost Cycle
to operate, the system will not go into Defrost and remain in Defrost mode if a Defrost
Switch malfunctions. If the Electric Heat During Defrost (EHDD) function has been
selected, the Heat Contactor (HTR) will come on to supply supplemental heat during
the Defrost Cycle.
Low Pressure Lockout
The Low Pressure Switch is designed to guard against the operation of the system
in the event of a loss of refrigerant. If the Low Pressure Switch stays on for more
than eight minutes, the system will turn off and a Low Pressure Lockout fault will be
indicated by a slow flashing on the “A” LED. The interval for the flash is once per
second.
In cold weather the pressure in the refrigerant system is low prior to operation. When
the Scholar III™ unit starts in the heat pump mode during cold weather, low pressure
could cause the system to lock out. To guard against nuisance lockouts, the Scholar
III unit will not shut off if the Low Pressure Switch (LPS) comes on during the first
eight minutes of operation and the switch has not cycled more than three times in
an hour. In other words, the compressor will start and operate for eight minutes even
with the LPS switch off three times before causing a lockout on low pressure.
High Pressure Switch
The system has a High Pressure Switch (HPS) that indicates a high system pressure.
When this occurs, the system will run for five seconds then set a High Pressure
Lockout. This lockout condition shuts the system off and flashes the “A” indicator
quickly at a rate of twice per second.
