Four-Pipe Heating/Cooling, Split Coil; Four-Pipe Heating/Cooling, Single Coil - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
Digital Control of the two pipe heating/cooling fan coil unit
should above could be as Figure 33. The fans could be batch
scheduled via one or more optimum start programs. Wall
modules could have a cooling setpoint knob (with software
bounds) and a heating deadband. The entering water
temperature (from a plant sensor) and the outside air temperature
are shown for operator information. The heating/cooling plant
could be controlled similarly to that described in the paragraph
Hot Water Plant Considerations.
The four-pipe heating/cooling fan coil unit control would be
identical except the valves would be sequenced through the
deadband.

FOUR-PIPE HEATING/COOLING, SPLIT COIL

Year-round heating and cooling is possible at each unit with a
four-pipe heating/cooling, split coil application (Fig. 34). A fan
coil unit in one zone can cool while a unit in another zone heats.
The control system comprises a room thermostat connected to
two valves, one controlling hot water flow and the other controlling
chilled water flow. Valves can be two-way or three-way.
3-WAY
MIXING
VALVE
RETURN
CHILLED
WATER
SUPPLY
3-WAY MIXING
VALVE
RETURN
HOT
WATER
SUPPLY
TIME
CLOCK
Fig. 34. Four-Pipe Heating/Cooling Fan
Coil Unit with a Split Coil.
ENGINEERING MANULA OF AUTOMATIC CONTROL
THERMOSTAT
DISCHARGE
AIR
COOLING
COIL
HEATING
COIL
FAN
POWER
FILTER
RETURN
AIR
C3030-1
When space temperature is below the thermostat setpoint,
the hot water supply valve modulates open and hot water flows
through the heating coil. As space temperature increases, the
hot water valve modulates closed. As space temperature rises
above setpoint, the thermostat signal starts to open the chilled
water valve. The room thermostat throttling range and valve
actuator movement should be selected to provide a "deadband"
between heating and cooling so that both valves are closed when
space temperature is satisfied. The fan can be operated by a
local fan switch or a central time clock.

FOUR-PIPE HEATING/COOLING, SINGLE COIL

Figure 35 shows the single-coil method for four-pipe heating/
cooling. A thermostat controls two three-way valves to regulate
the flow of chilled or hot water into a single coil.
SUPPLY
CHILLED
WATER
RETURN
RETURN
HOT
WATER
SUPPLY
Fig. 35. Four-Pipe Heating/Cooling Fan Coil Unit.
This system requires special three-way sequencing valves that
shut off all flow at the middle of the thermostat throttling range.
On a fall in space temperature, with the chilled water supply and
return ports close, the hot water return port opens 100 percent,
and the supply valve hot water port modulates the hot water flow
to maintain space temperature. As space temperature rises, the
hot water supply and return ports close and flow through the coil
is stopped. When space temperature rises above the thermostat
setpoint, the chilled water return port opens to 100 percent flow
and the supply port modulates chilled water flow through the coil
to maintain the space temperature. Both the chilled water and hot
water supply ports are closed in a deadband between heating and
cooling operations. The control is similar to having sequenced
modulating two-way valves on the HW and CW supplies and
sequenced two-position two-way valves on the returns. The fan
can be operated by a local fan switch or a central time clock.
424
THERMOSTAT
3-WAY
DISCHARGE
MIXING
AIR
VALVE
HEATING/
3-WAY
COOLING
DIVERTING
COIL
VALVE
FAN
FAN
SWITCH
TIME
CLOCK
POWER
FILTER
RETURN
AIR
C3031