Accessory List

Glossary
F-27382-1 © Copyright 2006 TAC. All Rights Reserved. 387
Offset: The amount of difference between control point and
setpoint in a proportional control system.
Packaged Equipment: Off-the-shelf HVAC equipment.
Parallel Fan: For air terminal units, a system in which fans
are located outside the primary airstream to allow intermittent
fan operation.
Paralleling: A control arrangement in which several
controlled devices are operated in unison as the signal from
the controller changes. If these controlled devices are
actuators they will operate over the same range.
Parameter: Any specific characteristic of a device. When
considered together, all the parameters of a device describe
its operational and physical characteristics.
P/E (Pneumatic-Electric Switch): An air pressure operated
switch in which the contacts are made or broken in order to
operate electrical devices in a pneumatic control system.
Peak Load: The maximum electrical or thermal load reached
during a period of time.
Percent Authority: The adjustment of a receiver-controller
which determines the effect of the reset signal of the
secondary transmitter as a percentage of the signal of the
primary transmitter.
Pneumatic: Controls powered by low-pressure compressed
air.
Positive Positioner: Used where accurate positioning of the
controlled device is required. Example: Pneumatic positive
positioners provide up to full main air to the actuator for any
change in position required by the controller. Positive
positioners may also be referred to as pilot positioners.
Positive Positioning: The characteristic of a controlled
device in which it has the maximum force available at any
point of the stroke.
Pressure Drop: The difference in pressure between two
points in a flow system, usually caused by frictional resistance
to fluid flow in a conduit, filter, or other flow system.
Pressure Independent VAV: A control technique in which
the flow of air (usually through a VAV terminal unit) is
maintained essentially at the setpoint of a flow controller
regardless of variations (reasonably controlled) in supply duct
static pressure.
Proportional Control: A mode of control in which the
controlled device may assume any position from fully closed to
fully open, depending on the load at any given point in time.
PSIG (Gauge Pressure): The amount of pressure above
atmospheric pressure.
Radiation: Heat energy transmitted from one body to
another without the need of intervening matter. Moves in
waves.
Range: (1) The minimum to maximum setpoint capability of
a controller, (2) the minimum to maximum sensing capability of
a transmitter, or (3) the start point to finish point of an actuator.
Example: Controller - 55 to 85°F
Transmitter - 40 to 240°F
Actuator - 5 to 10 psi
Recovered Energy: Energy utilized which would otherwise
be wasted from an energy utilization system.
Reheat: The process by which heat is added to
preconditioned (cooled, humidified, etc.) primary air or
recirculated room air.
Relative Humidity: The ratio of the amount of moisture that
is present in the air to the amount that can be in the air at that
temperature.
Relief Damper: A damper usually associated with an air
handling unit. Usually modulates open as the outdoor air
damper opens and the return air damper closes. Also called
an exhaust damper.
Reset: Making use of a second (open loop) sensor whose
function is to change the effective/desired setpoint of a
controller automatically according to changes in the open loop
conditions. Not to be confused with Automatic Reset.
Restrictor: A device which has a minute opening (0.005” or
0.0075”) which changes the velocity pressure of the air line to
static pressure.
Return Air: Air returning to the heater or conditioner from
the heated or conditioned space.
Return Stroke (Pneumatic Actuators): Refers to the
retraction of the actuator shaft as a result of the control
pressure being less than opposing force of the spring.
Reverse Acting (R.A.): A decrease in the sensed media
causes an increase in controller output (and vice-versa).
Reverse Reset: On multiple (typically two) input
applications, when a decrease at the second (open loop)
sensor causes the controller setpoint to be increased.
Rooftop Unit: Packaged heating/cooling or heating/cooling/
ventilating unit designed to be mounted on the roof of a
building. May be a small, single-zone unit; a large, complex
unite supplying air to many VAV terminals; or anything in
between.
RTD (Resistance Temperature Device): An electronic
device which sensor temperature. As the temperature sensed
changes the resistance changes. Example: Balco.
Run Time: For HVAC equipment, the total hours of actual
running time since installation, the last maintenance, or a
specified date.
Safety Shutdown: A process or device that prevents
equipment from operating manually, such as during
maintenance work.
Sensible Heat: The heat which changes the temperature of
the air without a change in moisture content. Changes in dry
bulb thermometer readings are indicative of changes in
sensible heat.
Span: The difference between the start and finish point of
range.
Example: Transmitter range 50° to 100° = span of 50°
Voltage Range 6 to 9 Volts = span of 3 volts
Spring Range of 3 to 8 psi = span of 5 psi