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VAV HEATING — It is suggested that the following section
be reviewed before reading this section: VAV Cooling,
page 56.
Combining a diffuser-mounted constant volume control pack-
age with a wall-mounted thermostat package (heating only)
allows the use of Moduline units in VAV heating
applications.
The operation of the heating only thermostat is opposite
to that of the cooling only thermostat in that the heating only
thermostat is reverse acting (RA) instead of direct acting (DA).
The thermostat’s bleed valve is open when the room tem-
perature is above set point. It bleeds air from the low
pressure chamber of the volume controller, lowering its pres-
sure, and allowing the relatively stronger pressure of the air
in the controller’s high pressure chamber to hold closed the
bleed from the controller’s bellows pressure chamber. The
bellows pressure becomes equal to the high pressure in the
unit plenum, closing the unit damper and shutting off the
unit discharge. See Fig. 66.
As the room temperature falls, the thermostat bleed closes
proportionally. This raises the pressure of the air in the low
pressure chamber of the controller, forcing the bellows pres-
sure chamber bleed open and lowering the bellows pressure
proportionally. The damper opens and the unit delivers air to
meet the existing demand in the conditioned space.
See Fig. 67.
Fig. 66 — Variable Volume Controls — Thermostat Open, Controller Shut Off, Unit Shut Off
63