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

TWO-PIPE HEATING/COOLING

The flow of medium through a fan coil unit can be controlled
in two ways. One method uses a two-way valve to control the
flow of steam or hot or chilled water. The second method, shown
in Figure 31, uses a three-way valve to control hot or chilled
water flow.
3-WAY
MIXING
VALVE
HOT
WATER
RETURN
HOT
WATER
SUPPLY
FAN
SWITCH
POWER
Fig. 31. Two-Pipe Heating Fan Coil Unit
Using Three-Way Mixing Valve.
The three-way valve provides constant flow in the supply
and return lines and minimizes pressure fluctuations at the valve.
A modulating thermostat controls the three-way valve to
regulate water flow through the coil. In a heating application,
when the thermostat senses space temperature below the
thermostat setpoint, the valve is positioned to allow full flow
through the coil. As space temperature rises, the thermostat
control signal modulates the valve to decrease the amount of
flow through the coil and increase the bypass flow around the
coil. At the upper end of the thermostat throttling range, the
valve is in the coil bypass position, eliminating flow through
the coil. The fan runs continuously or is stopped as space
temperature rises above setpoint. Cooling-only systems operate
in a similar way.

TWO-PIPE HEATING/COOLING, SINGLE COIL

Figure 32 shows a two-pipe fan coil application that uses a
single coil for heating and cooling with seasonal changeover.
Control for this application requires a heating/cooling room
thermostat that reverses its action from a remote heating/cooling
changeover signal. One method of automatic changeover is to
install a pipe-mounted sensor that switches thermostat action
when it senses a change in supply medium between hot and
chilled water. Because it cannot offer simultaneous heating and
cooling capability, this system can cause problems during
intermediate seasons when hot water is in the system and
cooling is needed, or vice versa.
DISCHARGE
AIR
THERMOSTAT
HEATING
COIL
FAN
FILTER
RETURN
AIR
C3033
INDIVIDUAL ROOM CONTROL APPLICATIONS
3-WAY
MIXING
VALVE
HOT
RETURN
WATER
OR
CHILLED
SUPPLY
WATER
PIPE-MOUNTED
SENSOR
TIME
CLOCK
Fig. 32. Two-Pipe Heating/Cooling Fan Coil Unit.
The room thermostat opens the coil valve on a fall in space
temperature during the heating cycle. In the cooling cycle,
chilled water in the pipes causes the pipe-mounted sensor to
reverse the thermostat action and the thermostat opens the valve
as space temperature rises. The fan can be operated from a local
switch or a central time clock.
The surface area of the coil is large to accommodate cooling
requirements. Therefore, when a single coil is used for both
heating and cooling, the hot water supply temperature should
be lower. Typical hot water supply temperatures range between
90 and 140F.
77
46
FAN
ON
FILTER
RA
Fig. 33. Digitally Controlled Two-Pipe
Heating/Cooling Fan Coil Unit.
423 ENGINEERING MANULA OF AUTOMATIC CONTROL
DISCHARGE
AIR
THERMOSTAT
HEATING OR
COOLING COIL
FAN
POWER
FILTER
RETURN
AIR
C3032
PERCENT
OPEN
WALL MODULE
TEMPERATURE
75
COOLING
75
SETPOINT
CONTROL
2.5
PROGRAM
HEATING
DEADBAND
OUTDOOR AIR
55
M10533