Accessory List

Glossary
386
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Humidity Controller: A device which senses and controls
the moisture content of air.
Humidistat: An instrument which measures humidity and
controls a device(s) for maintaining a desired humidity.
Hunting: The action of a controller which causes the
controlled device to continuously travel from one end of its
stroke to the other. Normally associated with proportional
control. Hunting is an undesirable condition.
HVAC: Heating, ventilating, and air conditioning.
Hydraulic Actuator: A controlled device that operates
dampers or valves by producing a linear motion as a result of
the fluid pressure developed from a continuously running
motor pumping oil through a transducer. As the control signal
increases the fluid pressure increases and as the control
signal decreases the fluid pressure decreases allowing the
spring to retract the output shaft.
Hydronics: The science dealing with the control of and use
of water as a heat transfer medium in air conditioning systems.
Hygroscopic: Water absorbing.
Immersion Sensor: A device with an extended element,
which can be inserted into a well in order to sense the
temperature in liquid lines and tanks.
Infiltration: The uncontrolled inward air leakage through
cracks and joints in any building element and around windows
and doors of a building, caused by the pressure effects of wind
and/or the effect of difference in the indoor and outdoor air
density.
Inlet Vane: An attachment to a centrifugal fan that restricts
the flow of air into the fan housing. Also used on centrifugal
chillers to restrict refrigerant flow.
Integral Control: A control action designed to eliminate/
reduce offset in proportional control.
Interface: The point at which a connection is made between
two devices so that they can work together. Software
interfaces allow the user to interact with a computer. Hardware
interfaces are cards, plugs, and other devices that connect
hardware with the computer.
in. W.C. (Inches Water Column): A unit of pressure
measurement used to measure and control low differential
pressures. These pressures include duct static pressure
relative to space static pressure, space statue pressure
relative to that of other spaces or outside atmospheric
pressure, and the velocity pressure of air flowing in ducts.
Latent Heat: The amount of heat necessary to change a
given quantity of water at 212°F (100°C) from liquid to vapor at
constant barometric pressure.
Load Shedding: The turning off of electrical loads to limit
peak electrical demand.
Low Limit: A control/application to prevent a sensed
variable from falling below a dangerous or undesirable
condition.
Low Temperature Thermostat: A duct thermostat with a
capillary-type, vapor-filled sensing element installed across a
duct. When any given section of the element (usually one foot)
falls below setpoint, the thermostat is actuated, usually to stop
the supply fan of an air handling unit and close the outdoor air
and relief dampers. Available with manual or automatic reset.
Main or Supply Pressure (Pneumatic Controls): The force
per unit area (psi) of the compressed air supplied to a
controller. It is usually constant at 15 or 20 psig, but may have
some other value in special cases.
Make-Up Air: Outdoor air brought into a building for
ventilation and/or pressurization.
Make-Up Water: Water supplied to replenish that lost by
leaks, evaporation, etc.
Minimum Position: A control sequence in which the
controlled device is prevented from moving to the fully closed
position even though the signal from the controller is at a value
that would cause the controlled device to be fully closed.
However, at a total loss of power or signal from the minimum
position, the controlled device will typically go to a fail safe
position.
I.E. Minimum Position of the outside air damper, for purposes
of ventilation may require that a minimum of X% of outside air
be introduced to the building when occupied. However, if there
is a loss of power or a low limit that could freeze the coil, the
outside dampers will close fully.
Mixing Box: A box containing dampers in the hot and cold
air stream, mixing the two and delivering the air to a space at a
specified temperature.
Mixing Valve: The three-way valve which has two inlets,
one outlet, and can direct full flow from either inlet or
proportion the flow from the two inlets.
National Electrical Manufacturers Association (NEMA):
Defines a product, process, or procedure with reference to one
or more of the following: nomenclature, composition,
construction, dimensions, tolerances, safety, operating
characteristics, performance, quality, rating, testing and the
service for which the product is designed.
Night Setback (Heating): An application by which the
setpoint is shifted to a lower value during unoccupied hours
during the heating season.
Night Setup (Cooling): An application by which the setpoint
is shifted to a higher value during unoccupied hours during
cooling season.
Normally Closed (N.C.): Applies to the condition of a
controlled device which closes when all operating force
(control pressure or electric energy) is removed. i.e., power
failure.
Normally Open (N.O.): Applies to the condition of a
controlled device which is open when all operating force is
removed.
Occupied Mode: A control mode used to heat or cool a
building when it is occupied.