Control; Ashrae Control Cycles; Standby/Warm-Up Stage - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
In the "draw-through" unit ventilator (Fig. 21) the fan draws
filtered outdoor and return air across the coil and blows the
conditioned air into the space.
WALL
HEATING
COIL
OUTDOOR MIXING
AIR
DAMPER
Fig. 21. Draw-Through Unit Ventilator.
Some unit ventilators use separate heating and cooling coils
or a combination hot-water/chilled-water coil. Figure 22 shows
a typical air conditioning unit ventilator with two separate coils.
The heating medium may be hot water, steam, or electric
resistance elements, and the cooling medium may be chilled
water or DX refrigerant. If heating and cooling sources are both
water, they are sometimes combined in a single coil by providing
separate sections of the coil for each function. With DX cooling,
the condensing unit may be an integral part of the unit ventilator
or may be remotely located.
WALL
HEATING
COIL
DRAIN PAN
OUTDOOR
AIR
Fig. 22. Unit Ventilator with Separate
Heating and Cooling Coils.
ENGINEERING MANULA OF AUTOMATIC CONTROL
DISCHARGE
AIR
FAN
FACE AND
BYPASS
DAMPER
FILTER
RETURN
AIR
C3036
DISCHARGE
AIR
FAN
COOLING
COIL
FILTER
MIXING
DAMPERS
RETURN
AIR
C3035
Face and bypass dampers are frequently found on unit
ventilators wherein the room controller modulates the dampers,
and the coil valve closes after the face damper is closed.

CONTROL

Unit ventilator control regulates the amount of outdoor air
introduced into a space and the amount of heating or cooling
medium required to heat or cool the room. Day/night systems
can lower the setpoint and cycle the fan during unoccupied
hours to maintain minimum temperatures and save energy.

ASHRAE Control Cycles

ASHRAE classifies the control of unit ventilators as follows:
— Standby/Warm-up Stage (can be used with any of the
following cycles)
— Cycle I–Fixed Maximum Percentage of Outdoor Air
— Cycle II–Fixed Minimum Percentage of Outdoor Air
— Cycle III–Variable Outdoor Air
These classes, defined around the 1950's, simplified the
specifications for unit ventilators, unit ventilator controls and
sequences, and the factory mounting of control components.
These cycle definitions are for heating only, although ASHRAE
recognized that cooling coils may also be required. Cycles I,
II, and III differ in the sequence of damper action in response
to a rise in space temperature and in the amount of outdoor air
admitted at various temperatures. ASHRAE control cycles may
be implemented by pneumatic, electric, electronic, or digital
control.

STANDBY/WARM-UP STAGE

During cold room periods, all three cycles position the valves
and dampers the same. Figure 23 shows the standby/warm-up
stage in which the unit ventilator fan is shut down (manually or
by time clock). The outdoor air damper is closed, and the return
air damper is full open. The heating coil valve is open. The coil
acts as a convector as air circulates by convection across the
coil. The room thermostat modulates the valve, closing the valve
as space temperature rises above setpoint. The fan does not
operate in the standby stage. During the warm-up stage, the
fan energizes and the unit recirculates space air for a rapid rise
in space temperature. The thermostat signal operates the valve
to control the heat.
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