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144 VAV-PRC011M-EN

Application Considerations

Key features of the system include:

• An advanced unit controller

• Flexible system design

• User-friendly interaction

Pneumatic Controls Basic Information

Pneumatic controls modulate air pressure of a controller to maintain setpoint. For VAV systems,

there are two primary types of pneumatic controllers—the room thermostat and the pneumatic

volume regulator (PVR).

Room Thermostats

The most visible controller to the customer is the room thermostat. Pneumatic room thermostats

can be classified by two characteristics: the tubing connection(s) to the thermostat and the action

of the thermostat output in response to a change in the input.

Room thermostats are available in models that require a one-pipe or a two-pipe configuration. The

name is derived from the number of tubes that must run to the thermostat location. The difference

is really in the construction of the thermostats. The two-pipe thermostats have a constant pressure

supply connected via an air tube to the thermostat supply air port. The supply air travels through

the thermostat’s relays, levers, diaphragm, and bleed port to produce an output. The output line

is connected to the output port of the thermostat and extends to the controlled device. The one-

pipe thermostat has, as its name suggests, only one air line connection. The thermostat works by

opening and closing an air bleed valve. This will either decrease or increase the pressure on the

controlled device, which is connected to the same line that runs to the thermostat.

Room thermostats also can be classified by their reaction to a change in temperature. Room

thermostats classified this way are denoted as either direct-acting or reverse-acting. Direct-acting

thermostats will increase their output pressure as the temperature the thermostat measures

increases.

On the contrary, reverse-acting thermostats will decrease their output pressure as the temperature

the thermostat measures increases.

Pneumatic Volume Regulators

These controllers accept the room thermostat signal and modulate the VAV terminal unit primary

air damper. The primary air damper is controlled for an airflow setpoint that is determined by the

room thermostat. The thermostat increases the PVR’s airflow setting when the temperature in the

space is warm. On the other hand, the thermostat decreases the PVR’s airflow setting when the

temperature in the space is cold.

Figure 13. Direct-acting thermostat response (L) and direct-acting thermostat response (R)

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