Product Overview
20
W7100A-E
To provide DA reset from outdoor air, connect a
C7031G1016 Sensor to the W7100 reset terminals 6 and 7
(Fig. 5).
Reset From Space Temperature
A VAV system can be made more economical by reset-
ting the discharge air temperature from the average build-
ing temperature. This type of reset is most effective in
buildings where there is not a wide diversity of cooling
loads between zones. Typical examples are office build-
ings, department stores, discount stores, single-story build-
ings, and installations where minimum-volume terminal
boxes are used.
VAV systems that discharge constant temperature air
tend to overcool a building during cold weather for three
reasons. One reason is that the air distribution system may
be oversized. A second reason is that buildings with mini-
mum volume boxes may not be able to pinch off enough air
to match the decreasing load on the building. A third reason
for overcooling is that during cold weather the VAV sys-
tem economizer may be exhausting BTUs from the build-
ing, placing a greater demand on the heating system.
To help prevent the building from overcooling, the
discharge temperature can be reset from average building
temperature. Resetting the DA temperature upward has the
effect of reducing the cooling capacity of the central air
handling unit. The W7100 contains a unique, patented,
reset algorithm. The graph of this algorithm is shown in
Fig. 10.
Space sensors are strategically located in the building to
measure average space temperature. It is recommended
that at least four sensors be installed and wired to provide a
signal proportional to the average sensor temperature. The
setpoint of these sensors should be at the maximum desired
cooling temperature of the building. If it is desired to
conform to the Emergency Building Temperature Restric-
tion Act, adjust the setpoint of the reset sensor to 78° F
[26° C] or higher, the W7100 control point to 60° F [16° C]
and the reset adjustments to 20° F [11.1° C].
In operation, as the average temperature drops below the
setpoint of the reset sensors, the setpoint of the mechanical
cooling is reset upward by an adjustable amount. The
setpoint of the economizer is not reset upward until the
average building temperature drops 6° F [3.3° C] below the
setpoint of the reset sensors. As long as free outside air is
available, the W7100 will try to subcool the building some-
what to force the fan system to run at a lower horsepower.
When the average temperature in the building drops
more than 6° F [3.3° C] below the setpoint of the reset
sensors, the economizer setpoint is reset upward to mini-
mize the load on the heating system in the building.
In reset applications where high humidity may be a
concern, an outdoor temperature control (T675) can be
connected to W7100 reset input (terminals 6 and 7) to lock
out reset whenever outdoor temperature is above 55° F to
60° F [13° C to 16° C]. Another alternative is to use an
indoor humidity sensor. With either alternative, reset sen-
sors can be bypassed by connecting a jumper across the
W7100 reset terminals. DA temperature will then be main-
tained at the W7100 setpoint without reset.
Outdoor Reset
As an alternative to space temperature reset, outdoor
reset may be employed. The advantages of outdoor reset
are that it is simple to apply and field wiring is not needed.
The disadvantage of outdoor reset is that outdoor tempera-
ture change has little effect on the cooling load on many
VAV systems; for example, the interior mall of a large
shopping center. In this application, the major cooling load
consists of solar heat, people and lighting.
In applications where outdoor temperature has a major
effect on the cooling load in an application, outdoor reset
has many of the same energy saving advantages of space
reset. Fig. 11 shows the schedule for outdoor reset. Since
the outdoor reset does not start setting down the mechanical
cooling setpoint until the outdoor temperature reaches 70° F
[21° C], the reset should be set so that the maximum
discharge temperature is 65° F [18° C]. An example would
be a 60° F [16° C] W7100 setpoint, and 5° F [2.8° C] of
reset, or a 55° F [13° C] W7100 setpoint and a 10° F
[5.6° F] reset. If high humidity is a concern, the reset input
can be bypassed any time the outdoor temperature is higher
than 55° F or 60° F [13° C or 16° C] by jumpering reset
terminals 6 and 7. The W7100 will then establish control at
the W7100 setpoint.
Outdoor reset should be employed only in those applica-
tions where outdoor conditions represent a significant fac-
tor of 50 percent or more on the cooling load of the
building.
VARIABLE AIR VOLUME MODULATING
SYSTEMS
Modulating Heat/Cool Systems
The W7100 has a modulating output for use in either
modulated heating or modulated cooling systems such as
boiler or chiller applications. The output commonly modu-
lates the supply water valve to the cooling or heating coil as
shown in Fig. 12.
The W7100E has a modulated heat output, a staged heat
output, four stages of cooling, and is normally used in
indirect fired gas heat/cool systems. A typical system con-
figuration is shown in Fig. 13.
When used for heating/cooling applications, use the
contacts on a switching relay to short terminals 9 and 10 for
heating, and to enable or disable the heating and cooling
actuators. The modulating motors must be spring return.
SPECIAL APPLICATIONS
Additional Stages of Heat
Control of up to six additional stages of on-off heating or
cooling can be provided by adding a W7101A Satellite
Sequencer to the W7100 System. The W7101A can pro-
vide either 2-heat/4-cool or 4-heat/2-cool stages, selected
by a jumper at terminals 6 and 7 on the W7101A.
The fixed resistor value across the W7100 terminals 7
and 8 (Table 2) must agree with the combined number of
switched heating/cooling stages controlled by the W7100
and W7101A. Do not include any modulating stages con-
trolled by the W7100.
OPERATION