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

TWO-PIPE HEATING/COOLING

The flow of medium thr ough a f an coil unit can be contr olled
in tw o ways. One method uses a tw o-way valve to contr ol the
flow of steam or hot or c hilled water. The second method , shown
in F igur e 31, uses a thr ee-way valve to contr ol hot or c hilled
water f low.
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 thr ee-way valve pr ovides constant f low in the suppl y
and r etur n lines and minimiz es pr essur e fluctua tions a t the v alve.
A modula ting ther mosta t contr ols the thr ee-way valve to
regulate water f low thr ough the coil. In a hea ting a pplica tion,
when the ther mosta t senses space temper atur e below the
ther mosta t setpoint, the valve is positioned to allo w full flow
thr ough the coil. As space temper atur e rises, the ther mosta t
contr ol signal modula tes the v alve to decr ease the amount of
flow thr ough the coil and incr ease the b ypass f low ar ound the
coil. At the upper end of the ther mosta t thr ottling r ang e, the
valve is in the coil b ypass position, elimina ting flow thr ough
the coil. The fan r uns contin uousl y or is stopped as space
temper atur e rises above setpoint. Cooling-onl y systems oper ate
in a similar w ay.

TWO-PIPE HEATING/COOLING, SINGLE COIL

Figur e 32 shows a two-pipe f an coil a pplica tion tha t uses a
single coil for hea ting and cooling with seasonal c hang eover.
Control for this application requires a heating/cooling room
ther mosta t tha t r everses its action fr om a r emote hea ting/cooling
chang eover signal. One method of automa tic chang eover 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 of fer sim ultaneous hea ting and
cooling ca pa bility, this system can cause pr oblems dur ing
inter media te seasons w hen hot w ater is in the system and
cooling is needed , or vice v er sa.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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 r oom ther mosta t opens the coil v alve on a fall in space
temper atur e dur ing the hea ting c ycle. In the cooling c ycle,
chilled w ater in the pipes causes the pipe-mounted sensor to
reverse the ther mosta t action and the ther mosta t opens the v alve
as space temper atur e rises. The fan can be oper ated fr om a local
switch or a central time clock.
The surf ace ar ea of the coil is lar ge to accommoda te cooling
requir ements. Ther efore, when a single coil is used f or both
heating and cooling , the hot w ater suppl y temper atur e should
be lower. Typical hot w ater suppl y temper atur es range between
32 and 60 C.
77
8
FAN
ON
FILTER
RA
Fig. 33. Digitally Controlled Two-Pipe
Heating/Cooling Fan Coil Unit.
419
DISCHARGE
AIR
THERMOSTAT
HEATING OR
COOLING COIL
FAN
POWER
FILTER
RETURN
AIR
C3032
PERCENT
OPEN
WALL MODULE
TEMPERATURE
24
COOLING
24
SETPOINT
CONTROL
-1.5
PROGRAM
HEATING
DEADBAND
OUTDOOR AIR
13
M15317