Product data

Controls - 38HDS
Unit operation
NOTE: AUTO fan mode is used as the unit operation example for ALL fan coil units in this section. Contact your
local Carrier 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 38HDS 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 indoor unit refrigerant circuit; including opening a liquid line solenoid
valve in the outdoor unit. As the set point at each fan coil is satisfied, its individual 24-v signal to the 38HDS unit
stops, and the respective solenoid valve for each fan coil unit closes. The indoor fan cycles off. The 40QNB 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).
38HDS 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 cooling requests to control up to 2 compressors. The 38HDS 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 38HDS-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 transducers (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 temperature thermistor to determine if the head pressure
should be used to control the fan output.
Operation — The 38HDS 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 38HDS 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 the controller initiates compressor operation. The microprocessor control in the
38HDS 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 beginning 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 38HDS048 units (circuit A and circuit B). Each circuit
operates independently, 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 appropriate compressor starts.
2. The solenoid valve connected to the fan coil sending the demand signal is energized (at the 38HDS units).
3. The outdoor fan starts, and its operation is controlled by the microprocessor as described in 38HDS
Microprocessor Control of Outdoor Fan section below. When a second fan coil unit signals a demand for
cooling, its associated solenoid valve is energized immediately, allowing refrigerant to flow to both fan coil units
simultaneously.
Decreased Demand for Cooling — When a fan coil unit’s demand for cooling ends, the 24-v signal to the 38HDS
unit stops, and the appropriate solenoid valve closes. If the other fan coil unit on this circuit still has a demand, the
appropriate compressor 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 solenoid 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 continue under control of (MORE)