Accessory List

Glossary
F-27382-1 © Copyright 2006 TAC. All Rights Reserved. 385
Degree Day, Heating: A unit, based upon temperature and
time, used in estimating fuel consumption and specifying
nominal heating load of a building in winter. For any one day,
when the mean temperature is less than 65°F (18°C), there
exist as many degree days as there are Fahrenheit (Celsius)
degrees difference in temperature between the mean
temperature for the day and 65°F (18°C).
Dew Point: The point at which a given mixture of air and
water vapor is saturated.
Differential Pressure Control: A system in which two
pressure sensors transmit their respective signals to a
controller; the controller, in turn, produces an output to the
controlled device that will vary in accordance with the
difference of the two sensed pressures.
Differential Pressure Switch: A switch activated by air
pressure, often used in supply ducts to detect flow or pressure
drop across filters.
Digital: An On/Off or two-position signal.
DIP Switch (Dual In-line Package Switch): A ganged array
of switches on a circuit board. Each switch can be set to one of
two positions.
Direct Reset: On multiple (typically two) input applications,
when a decrease at the second (open loop) sensor causes the
controller setpoint to decrease.
Discharge Damper: A damper located directly in the
discharge of a fan or duct.
Diverting Valve: A three-way valve which has one inlet, two
outlets, and can direct full flow to either outlet or proportion the
flow between the two outlets.
Dry-Bulb Temperature: The air temperature as measured
by a conventional thermometer.
Dry Contact: A contact closure that does not impose an
electronic signal from an outside source. A direct short of
normally open contacts.
Duct Fan: An axial flow fan mounted in, or intended for
mounting in, a section of duct.
Duty Cycling: An energy management function that reduces
consumption by periodically turning off electrical equipment for
short intervals during normal operating hours.
Economizer Band: The range of temperatures within which
an air handler is in economizer mode.
Economizer Deadband: The range of temperatures
between the high end of the economizer band and the first
stage of mechanical cooling.
Economizer Mode: A control mode in which outside return
and relief dampers are controlled by air temperature to provide
the most economical heating and cooling.
Electronic Controls: Using very low voltages (20V or less)
and currents for sensing and transmitting.
Energy Management: A number of techniques for reducing
a building’s energy consumption, while maximizing operating
efficiency, all without drastic degradation of comfort.
Engineering Units: The units that a medium is measured in,
represented by an abbreviation. Examples include degrees
Fahrenheit (DEGF), kilowatts (KW), and feet per minute
(FPM).
Enthalpy: For most HVAC applications, a measure of total
heat (sensible plus latent) of air, measured above an arbitrary
datum. The specific enthalpy of dry air is assigned a value of
zero at 0 [degrees] F and U.S. standard atmospheric pressure
(29.92 in. mercury), and is measured in Btu per pound of dry
air.
E/P (Electric-Pneumatic Switch): An electrically operated
air flow switch with normally closed and normally opened
inputs which lead to a common output. Also known as solenoid
air valve.
Evaporative Cooling: The adiabatic exchange of heat
between air and a water spray or wetted surface. The wet bulb
temperature of the air remains constant, but the dry bulb is
decreased. Example: a swamp cooler.
Exhaust 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 a
relief damper.
Finish Point (TAC System 8000): The voltage necessary to
drive an actuator to complete its stroke. Example: An actuator
with a 6-9 Vdc range, “9” is the finish point.
Finish Point (Pneumatic): The pressure necessary to
completely compress the spring of an actuator and cause the
actuator to complete its stroke. Example: An actuator with a 5-
10 psi spring range, “10” is the finish point.
Floating Control/Action: While definitions vary, floating
control is essentially two position control in which the
controlled device (i.e., MF-XXXX Actuators) can stop at any
point in its stroke at loss of control signal. The controlled
device will hold this position until the controller senses another
signal to reposition the controlled device.
FPM (Feet Per Minute): A unit of measure to quantify the
velocity of air flow.
Freezestat: Refer to Low Temperature Thermostat.
GPM (Gallons Per Minute): A unit of measure to quantify
water flow.
Gear Train Actuator: A controlled device that operates
dampers or valves by producing a rotary motion as a result of
an induction motor driving the output shaft through a series of
gears. The motor is driven in either direction and can be
stopped at any position so as to obtain proportional control.
The electronic actuator drive is necessary to interface the DC
signal of the controller and the induction motor.
Heat Pump: A refrigeration machine which is arranged to
either heat or cool a building by using heat from the condenser
section or by using cooling from the evaporator section.
High Limit: A controller generally located in the return air,
that will turn off the fan of air handling units when the
temperature increases above setpoint.