Specifications

CONTROLS
UNIT OPERATION
NOTE: AUTO fan mode is used as the unit operation example
for ALL fan coil units in this section. Contact your local dealer for
operation information in other fan modes.
Fan coil units — Each fan coil unit has a self-contained control
system that determines the set point for fan coil operation, fan
mode operation, and heating mode operation (if provided). The
fan coil units are equipped with either a wired or wireless remote
controller set. Set points and fan modes may be determined
separately for each fan coil unit.
Each fan coil unit may call for cooling operation independently.
On a call for cooling operation by a single fan coil unit, a 24-v
signal is sent to the 538S outdoor unit and energizes a control
relay. The indoor blower starts according to the normal fan coil
unit sequence of operation. The control relay (in the outdoor
unit) initiates operation of a cooling cycle for the respective in-
door unit refrigerant circuit; including opening a liquid line sole-
noid valve in the outdoor unit.
As the set point at each fan coil is satisfied, its individual 24-v
signal to the 538S unit stops, and the respective solenoid valve
for each fan coil unit closes. The indoor fan cycles off. The 619E
fan coil units then begin room sampling operation after a
3-minute off period (during sampling, the fan motor runs at low
speed for 1 minute to sample the demand in the space; then
cycles off until the next sampling period if no demand is found).
538S outdoor condensing unit — The outdoor unit is
equipped with a control that monitors the indoor fan coil cooling
request. The control turns on solenoid valves for the appropriate
indoor fan coil unit system. The control also combines the cool-
ing requests to control up to 2 compressors.
The 538S control provides a 2-minute compressor time-delay
circuit which disables the compressor for 2 minutes at unit start-
up. The time delay device timers are initialized when the unit is
powered up (for the first 2 minutes).
The 538S unit control includes an integral head pressure control
function. This function maintains a minimum head pressure by
cycling the outdoor-fan motor in response to inputs from the
thermistor (for outdoor ambient temperature) and the transduc-
ers (for system discharge pressures in each compressor
circuit).
The head pressure for the compressor(s) is monitored to control
the outdoor fan output. The control reads an outdoor tempera-
ture thermistor to determine if the head pressure should be used
to control the fan output.
Operation — The 538S unit receives a 24-v control signal from
each fan coil unit as each fan coil unit initiates a demand for
cooling. The 24-v signal energizes a control relay in the 538S
unit (one relay per fan coil unit). The refrigerant flow to each fan
coil unit is controlled through a solenoid valve (one valve per fan
coil unit). The solenoid valve(s) will not open until compressor
operation is initiated by the controller.
The microprocessor control in the 538S unit includes a
2-minute, anti-short cycle, time-delay function. This function
provides a minimum off delay between compressor run stages
(2 minutes each from the end of the last on period to the begin-
ning of the next on period). If more than 2 minutes have passed
since the end of the last on period, the compressor is ready to
restart with initiation of demand from any fan coil unit on its
circuit.
Increased Demand for Cooling
NOTE: There are 2 separate compressor circuits in the
538S048 units (circuit A and circuit B). Each circuit operates in-
dependently, and will operate as follows (as will the single-
circuit 024 units) upon receiving the first 24-v cooling demand
signal from a fan coil unit:
1. After the 2-minute time-delay function is satisfied, the appro-
priate compressor starts.
2. The solenoid valve connected to the fan coil sending the de-
mand signal is energized (at the 538S units).
3. The outdoor fan starts, and its operation is controlled by the
microprocessor as described in 538S Microprocessor Con-
trol of Outdoor Fan section below.
When a second fan coil unit signals a demand for cooling, its as-
sociated solenoid valve is energized immediately, allowing re-
frigerant to flow to both fan coil units simultaneously.
Decreased Demand for Cooling — When a fan coil unit’s de-
mand for cooling ends, the 24-v signal to the 538S unit stops,
and the appropriate solenoid valve closes. If the other fan coil
unit on this circuit still has a demand, the appropriate compres-
sor continues to run as long as necessary for the second fan coil
unit.
When the second fan coil unit’s demand for cooling ends, its so-
lenoid valve closes, and the appropriate compressor stops. The
compressor will not start for at least 2 minutes after the end of
this cycle due to the time-delay function.
On size 024 units, when the compressor stops, the outdoor fan
also stops. On size 048 units, outdoor fan operation may con-
tinue under control of the 538S microprocessor if the other re-
frigerant circuit is still operating. The outdoor fan only stops
when both compressors are off. Refer to 538S Microprocessor
Control of Outdoor Fan section below for more details.
538S microprocessor control of outdoor fan — The micro-
processor control has a built-in head pressure control system
that cycles the outdoor-fan motor to maintain a selected dis-
charge pressure. The microprocessor senses outdoor ambient
temperature using a thermistor and refrigerant pressure using a
pressure transducer mounted on the compressor circuit dis-
charge line. The 538S024 units have one transducer, and the
048 units have two.
The factory set points for the head pressure control operation
are 55 F for outdoor ambient temperature and 250 psig for dis-
charge pressure.
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