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VAV HEATING AND COOLING WITH SYSTEM-
POWERED CHANGEOVER — This application uses a CV
control package along with a wall-mounted thermostat pack-
age (cooling/heating) and a system-powered changeover con-
trol package. The cooling thermostat is direct acting (DA)
while the heating thermostat is reverse acting (RA).
Operation is very similar to system-powered warm-up. The
changeover switch assembly is field installed in the unit ple-
num and is piped in-line between the volume controller and
the thermostat. It consists of 2 thermally operated pneumatic
valves, one for each thermostat. The tube from the volume
controller is connected by a ‘T’ into the valve assembly,
making control air available to each valve. The valve for the
DA cooling thermostat is normally open as long as there is
cool supply air in the plenum (Fig. 62). The other valve is
normally open when warm air is being supplied, matching
the RA heating thermostat (Fig. 63). Each valve is con-
nected to the corresponding thermostat by a separate tube.
In the cooling mode (Fig. 64), the cooling valve is open,
activating the cooling thermostat, while the heating thermo-
stat is isolated by the closed heating valve. On changeover
from cooling to heating, the cooling valve closes and the
heating valve opens, shifting control to the heating thermo-
stat (Fig. 65).
Fig. 61 — Location of CV Unit for Warm-Up Application
60