Control; Two-Position Control; Down-Blow Unit Heater; Modulating Control - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS

CONTROL

Control of unit heaters may be modulating but is usually two
position. Low-limit control (sensing the water or condensate
temperature) is usually provided for night or summer shutdown
if the heating system is off, and prevents the fans from blowing
cold air if the heating system fails.

Two-Position Control

Figure 15 shows two-position control of a steam or hot water
unit. When space temperature falls below the thermostat
setpoint, the thermostat starts the fan. The fan forces air across
the coil to warm the space. When space temperature rises to
the setpoint, the thermostat contacts open and the fan turns off.
Low-limit controls are typically installed to prevent the fan from
operating until there is heat in the coil.
STEAM OR
HOT WATER
SUPPLY
FAN
COIL
UNIT HEATER
LOW LIMIT
HOT WATER OR
CONDENSATE
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
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.
Figure 16 shows that control of an electric-heat unit heater is
similar to steam or hot water two-position control except that
the thermostat cycles the fan and the relay or contactor energizes
the electric heating elements when the fan is running.
RELAY
FAN
RESISTANCE
HEATING ELEMENTS
UNIT HEATER
Fig. 16. Unit Heater Two-Position Control–Electric Heat.
ENGINEERING MANULA OF AUTOMATIC CONTROL
ROOM
THERMOSTAT
C3026
THERMOSTAT
TO POWER
SUPPLY
C3027
414

Modulating Control

Modulating control (Fig. 17) throttles the heating medium in
proportion to changes in space temperature. The fan operates
continuously to prevent air stagnation. On a drop in temperature,
the thermostat sends a signal to reposition the valve and the fan
forces air across the coil to raise space temperature. When the
valve is completely closed, the coil cools down and the low-
limit control shuts off the fan. In some cases, the fan runs
continuously or under time control without a low limit switch.
STEAM OR
HOT WATER
SUPPLY
FAN
COIL
UNIT HEATER
LOW LIMIT
HOT WATER OR
CONDENSATE
RETURN
STEAM TRAP
(IF STEAM SUPPLY)
Fig. 17. Unit Heater with Modulating Control.
Pneumatic control of the valve and electric switching of the
fan motor by the low-limit control are the most economical
means of accomplishing modulating control. The low-limit
control is usually a strap-on type mounted on the return pipe of
either water or steam systems. In a steam system, the controller
sensing element should be mounted between the unit and the
steam trap.

DOWN-BLOW UNIT HEATER

The down-blow unit heater (Fig. 18) circulates warm air that
normally stratifies near the ceiling. Control for this application
requires two thermostats: one in the room (occupied area) and
the other near the ceiling. The room thermostat controls the
supply valve. The fan cycles with the valve. As the ceiling
temperature rises above ceiling thermostat setpoint, the ceiling
thermostat overrides the fan control and starts the fan. The fan
runs until the ceiling thermostat is satisfied. This application
recycles warm air and reduces the amount of heating medium
circulated through the coil.
VALVE
THERMOSTAT
C3025