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VAV Heating and Cooling With Changeover — (Function
5.) A VAV control arrangement for cooling/heating includes
a wall-mounted cooling/heating thermostat and provision to
change the thermostat from cooling to heating configuration
and back again. Figure 46 shows the control arrangement
with Moduline control for heating and cooling. The wall
thermostat is shown in Fig. 47. The system-powered heating
and cooling changeover uses an assembly consisting of 2
temperature-operated pneumatic switches located inside the
unit plenum and installed in-line between a cooling/heating
thermostat and the volume controller. These switches se-
quence the thermostat between the cooling and heating modes,
depending upon the temperature of the primary air supply.
VAV Heating — (Function 9.) Variable air volume heating
controls are the same as for VAV cooling except that only a
wall-mounted heating thermostat is offered (Fig. 48). The
heating thermostat is reverse acting (RA); thus the output
pressure decreases as the space temperature increases.
SYSTEM-POWERED CONTROLS WITH ELECTRIC
INTERFACE
VAV Cooling With Electric Warm-Up — (Function 6.) Elec-
tric warm-up uses an electric changeover valve to perform
the same function as system-powered warm-up; that is, to
provide heated air to reestablish comfortable temperatures
when occupancy resumes after an extended unoccupied pe-
riod. Since the cooling thermostats are satisfied at the re-
duced temperature, the units will be shut off and the system
will not be able to deliver warm air. It is necessary, there-
fore, to provide a means of temporarily overriding the cool-
ing thermostat.
The changeover control is an electrically operated, re-
motely controlled 3-way solenoid valve. The normally closed
port of the valve is capped. The valve usually is wired to
activate automatically with the supplemental heating switch;
however, it can also be manually activated. This option may
be used with either a unit-mounted or wall-mounted ther-
mostat. See Fig. 49.
VAVHeating and Cooling With Electric Changeover — (Func-
tion 7.) A VAV control arrangement for cooling/heating in-
cludes a wall-mounted cooling/heating thermostat (Fig. 47)
and provision to change the thermostat from a cooling to
heating configuration and back again. System-powered cooling/
heating with electric changeover uses a 3-way electric so-
lenoid valve to switch control between the cooling and heat-
ing functions of a cooling/heating wall-mounted thermostat.
It is installed in-line between the volume controller of a CV
cooling control package and the thermostat. The valve may
be activated separately or interlocked with an electric heat-
ing system. See Fig. 50.
70
50
90
Fig. 46 — System-Powered Changeover
Switch Arrangement with Heating/Cooling
Wall Thermostat
50 70 90
Fig. 47 — Cooling/Heating Wall Thermostat
50 70 90
Fig. 48 — Wall-Mounted Heating Thermostat
50
70
90
Fig. 49 — VAV Control Arrangement
for Electric Warm-Up
50
70
90
Fig. 50 — VAV Control Arrangement
for Electric Changeover
48