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Application Considerations

Operating and Maintenance Costs—DDC controls can be networked together via wired or wireless

networks to provide system-control strategies for energy savings. Multiple controllers can be

easily monitored and adjusted from a remote location. DDC controls also have system and

individual diagnostic capability.

Pneumatic Control Systems

Pneumatic control systems use compressed air through simple mechanical control devices, such

as diaphragms, springs, and levers to change an output in response to a change in a monitored

variable. With VAV terminal units, the output is typically a primary airflow and the monitored

variable is zone temperature.

Benefits

Performance—Pneumatic controls are a proven technology that is effective and has a long life

cycle.

Installed Cost—When a source of compressed air exists at the facility, pneumatics generally have

a lower installed cost than other types of controls when only a basic functionality is required.

Operating and Maintenance Costs—Pneumatics are still the most familiar control technology to

many building designers and maintenance people.

Large Installed Base—Pneumatic systems are very common in existing buildings. This eliminates

the need to purchase the most expensive piece of equipment in a pneumatic control system—the

control air compressor. Extensions to existing pneumatic systems are generally very simple and

extremely cost-effective.

Disadvantages

Performance—Pneumatic controls provide proportional-only control for VAV terminal unit

systems. This control scheme is less accurate than the more advanced control schemes. Improper

calibration of pneumatic controls leads to poor energy utilization.

Versatility—A central pneumatic control system, where each of the control zones can be monitored

and adjusted from a remote location, is extremely costly to configure and to modify.

Operating and Maintenance Costs—Pneumatics easily drift and require constant upkeep and

scheduled maintenance. Diagnostic capability for pneumatics is not available. A main compressor

which is not maintained and becomes contaminated with oil or water can pump those

contaminants into the compressed-air-distribution system. This may require costly cleaning of the

system and a possible replacement of system components.

DDC Controls Basic Information

DDC controls have become the industry standard for VAV terminal unit control systems. DDC

systems use electronic field devices such as a flow transducer, a primary air modulating damper,

and an electronic thermostat. These field devices report software instructions of how the outputs

are positioned in relation to the inputs to a controller. The VariTrane™ system uses a primary air

valve and flow transducer for both DDC systems and analog electronic systems. However, the DDC

zone sensor is different from the analog electronic thermostat.

DDC controls provide much flexibility and considerable diagnostic capability. DDC controllers can

be connected together to form a network of controllers. Once the controllers are networked, they

can be monitored for proper operation from a remote location. Commands and overrides can be

sent for groups of controllers at one time to make system-wide changes. Commands and overrides

can be sent to individual units to allow problem diagnosis, temporary shutdown, startup schedules

or other specialized changes. When integrated into a building management system, the operation

of the VAV terminal unit system can be modified to do such things, as coincide with occupancy

schedules and reduce energy charges.

DDC control of VAV terminal units is a key element in providing intelligent and responsive building

management. Precision control, flexible comfort, and after hours access are all available with the

VariTrane™ DDC control system for VAV terminal units.

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