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VAV-SVN01E-EN 17
Unit
Information
Fan-Powered and Fan-Powered
Low-Height Units
VariTrane fan-powered and low-height
fan-powered units can be either parallel
or series, with or without re-heat. (See
Figure 3.)
The fan on a series unit runs
continuously whenever the main air
handler unit is in operation. There are
various options for starting the fan. The
fan can be started three ways: 1)
remotely, 2) by a duct pressure switch,
or 3) by a combination of both. The
particular fan control method may vary
from unit to unit, depending upon job
needs.
Typically, the heater is off while the air
valve modulates primary air and
responds to zone temperature. If zone
temperature decreases to the point
where a decrease in primary air will not
maintain the desired temperature, the
re-heat will be activated to increase the
temperature of the discharge air.
On a parallel unit, the VariTrane air valve
delivers primary cooling air to the unit
outlet. When the space temperature
decreases beyond air valve control, the
fan is turned on as the first stage of
heat. The fan delivers plenum air from
above the occupied space to the unit
outlet, which is mixed with primary air
and delivered to the occupied space.
Note: Either the fan, the air valve, or both
can deliver airflow into the occupied
space. In order to prevent primary airflow
from exiting through the fan when the
fan is not running on a parallel unit, a
back draft damper is provided. When the
fan is not running, the efficiency of this
system is the same as a standard single-
duct VAV unit.
Typically, the control systems applied to
parallel units cause the air valve to
close to zero or a minimum flow before
the fan is activated. After the fan is
activated, the optional heat will be
activated upon further reduction in
zone temperature. Therefore, minimal
primary air is mixed with the heated air.
VariTrane fan-powered unit fan sizes
02SQ–05SQ and 08SQ–10SQ were
performance tested at .12 in. w.g. and
sizes 06SQ and 07SQ were tested at .15
in. w.g. Units are not designed to
operate unducted and below these
tested static pressures.
Note: Fan-powered units are available
with rectangular discharge connection
only. The optional heater is mounted on
the discharge of the unit. Hot water coils
are connected to either the plenum inlet
or on the discharge on parallel units, and
to the discharge of series units.
Figure 3 – Typical Fan-Powered Units
VSCF
VSEF
VSWF
VPCF
VPEF
VPWF