Trane HUV Installation Operation & Maintenance page 34

Control Cycles-General
There are a variety of control sys-
tems available today in unit venti-
lators. The exact method of
controlling the amount of outside
air and heating element capacity
can vary. However, all systems
provide a sequence of operation
designed to provide rapid class-
room warm-up and increasing
amount of ventilation air to offset
classroom overheating.
Classroom air conditioners are
normally controlled according to
ASHRAE Cycle I, II or III and varia-
tions of these control cycles. The
Trane classroom unit ventilators
only utilize cycles I and II. See Fig-
ure 31 for cycle charts.
ASHRAE Cycle I
(Spring Return)
ASHRAE Cycle I admits 100% out-
door air at all times except during
a warm-up cycle. During warm-up,
the outside air damper is closed.
As room temperature approaches
the thermostat setting, the outside
air damper opens fully, and the
unit handles 100% outside air. Unit
capacity is then controlled by
modulating the heating element
capacity.
ASHRAE Cycle I can be used in
those areas where a large quantity
of outdoor air is required to offset
the air being exhausted relieving
the room of unpleasant odors and
particles.
3 4
Controls
ASHRAE Cycles
ASHRAE Cycle II
(spring return)
ASHRAE Cycle II is the most wide-
ly used of the three types of series.
Similar to ASHRAE Cycle I, the
outside air damper is closed dur-
ing warm-up. But with Cycle II, the
unit handles recirculated air
through the return-air system. As
temperature approaches the com-
fort setting, the outside air damp-
er opens to admit a
predetermined minimum amount
of outside air. This minimum has
been established by local code re-
quirements and good engineering
practices per ARI 840 to provide
adequate ventilation. Unit capaci-
ty is controlled by varying the
heating element output. If room
temperature rises above the ther-
mostat setting, the heating ele-
ment is turned off and an
increasing amount of outside air is
admitted until only outside air is
being delivered.
This cycle may incorporate a min-
imum discharge air temperature
sensor that overrides the other
controls to maintain an acceptable
discharge temperature. When the
outside air temperature is very
cold, the minimum air tempera-
ture control modulates the
amount of outside air being deliv-
ered. This keeps the mixture tem-
perature delivered to the room at
55 F or 60 F.
ASHRAE Cycle II is a very econom-
ical control sequence since only
the minimum amount of outside
air is heated and free natural cool-
ing is available to offset the heat
gains in occupied classrooms.
Figure 31: Cycle charts
UV-PRC001-EN