Installation and Maintenance Manual

Unit Information
12 VAV-SVX08E-EN
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.
(SeeFigure 5, p. 12 thru Figure 8, p. 12.)
Typical Fan-Powered Units
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, heater is off while 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 5. Parallel fan-powered terminal unit (top) &
series fan-powered terminal units (bottom)
Figure 6. Low height series: LSCF (top) & low height
series: LSWF (bottom)
Figure 7. Low height series: LSEF (top) & low height
parallel: LPCF (bottom)
Figure 8. Low height parallel: LPWF (top) & low height
parallel: LPEF (bottom)