Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
INDIVIDUAL ROOM CONTROL APPLICATIONS
410
CONTROL
Contr ol of unit hea ter s may be modula ting b ut is usuall y two
position. Lo w-limit contr ol (sensing the w ater or condensa te
temper atur e) is usuall y pr ovided for night or summer shutdo wn
if the hea ting system is of f, and pr events the f ans fr om blowing
cold air if the hea ting system f ails.
Two-Position Contr ol
Figur e 15 shows two-position contr ol of a steam or hot w ater
unit. When space temper atur e falls belo w the ther mosta t
setpoint, the ther mosta t star ts the f an. The fan forces air acr oss
the coil to w ar m the space . When space temper atur e r ises to
the setpoint, the ther mosta t contacts open and the f an tur ns off.
Low-limit contr ols ar e typicall y installed to pr event the f an fr om
operating until there is heat in the coil.
Modulating Contr ol
Modulating control (Fig. 17) throttles the heating medium in
pr opor tion to c hang es in space temper atur e. The fan oper ates
contin uously to pr event air sta gnation. On a dr op in temper atur e,
the ther mosta t sends a signal to r eposition the v alve and the f an
forces air acr oss the coil to r aise space temper atur e. When the
valve is completel y closed, the coil cools do wn and the lo w-
limit contr ol shuts of f the f an. In some cases, the f an r uns
contin uousl y or under time contr ol without a lo w limit switch.
Fig. 15. Unit Heater Two-Position
Control–Steam or Hot Water Heat.
Unit heaters with hot water or steam valves can also be
operated from two-position thermostats. The thermostat can
operate the valve and the fan or just the valve and let the low
limit cycle the fan.
Figur e 16 shows tha t contr ol of an electr ic-hea t unit hea ter is
similar to steam or hot w ater tw o-position contr ol except tha t
the ther mosta t cycles the fan and the r elay or contactor ener gizes
the electr ic hea ting elements w hen the f an is r unning .
Fig. 16. Unit Heater Two-Position Control–Electric Heat.
C3026
UNIT HEATER
LOW LIMIT
ROOM
THERMOSTAT
COIL
FAN
STEAM OR
HOT WATER
SUPPLY
HOT WATER OR
CONDENSATE
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
C3027
UNIT HEATER
RESISTANCE
HEATING ELEMENTS
FAN
RELAY
THERMOSTAT
TO POWER
SUPPLY
Fig. 17. Unit Heater with Modulating Control.
Pneuma tic contr ol of the valve and electr ic switching of the
fan motor b y the lo w-limit contr ol ar e the most economical
means of accomplishing modula ting contr ol. The low-limit
control is usually a strap-on type mounted on the return pipe of
either w ater or steam systems. In a steam system,
the contr oller
sensing element should be mounted between the unit and the
steam trap.
DOWN-BLOW UNIT HEATER
The down-blow unit hea ter (F ig. 18) circulates war m air tha t
nor mally str atifies near the ceiling . Contr ol for this a pplica tion
requir es two ther mosta ts: one in the r oom (occupied ar ea) and
the other near the ceiling . The r oom ther mosta t contr ols the
suppl y valve. The fan c ycles with the v alve. As the ceiling
temper atur e rises above ceiling ther mosta t setpoint, the ceiling
ther mosta t overrides the f an contr ol and star ts the f an. The fan
r uns until the ceiling ther mosta t is sa tisfied. This a pplica tion
recycles war m air and r educes the amount of hea ting medium
circulated through the coil.
C3025
UNIT HEATER
LOW LIMIT
THERMOSTAT
COIL
FAN
VALVE
STEAM OR
HOT WATER
SUPPLY
HOT WATER OR
CONDENSATE
RETURN
STEAM TRAP
(IF STEAM SUPPLY)