air separator. A component designed to remove air from
a hydronic system. A wire mesh element creates a swirling
motion in the circulating water causing the formation of tiny
bubbles that merge and rise to the top of the chamber where
they are vented. (33)
air tube. The passage through which air from an oil burner fan
is blown into the combustion chamber. Also called blast tube. (36)
air vent. A float-operated valve that allows air to escape
from a hydronic system while preventing water from leak-
ing out and outside air from coming in. (33)
Air-Conditioning, Heating, and Refrigeration Institute (AHRI).
A trade association representing manufacturers of air condi-
tioning, heating and commercial refrigeration equipment. (54)
air-cooled condenser. A condenser that uses the air
movement of either natural convection or forced drafts to
de superheat, condense, and subcool a system’s high-pressure
refrigerant. (45)
air-cooling evaporator. An evaporator that is designed to
directly cool the air in a conditioned space. (46)
airflow friction chart. A graph of duct diameters, airflow
rates, pressure drops from friction, and air velocities. (23)
air-source heat pump. A heat pump that uses the outside
air as a heat source or a heat sink for producing the desired
temperature in a conditioned space. (34)
air-to-air heat pump. Type of air-source heat pump that
transfers heat between outside air and air inside a conditioned
space using forced air as the heat distribution method. (34)
air-to-water heat pump. Type of air-source heat pump that
transfers heat between outside air and air inside a condi-
tioned space using a hydronic system as the heat distribu-
tion method. (34)
alternating current (ac). Electric current in which the direc-
tion of electron flow reverses or switches at regular inter-
vals. In 60 Hz current, electrons reverse direction 120 times
per second. (9)
ambient temperature. Temperature of the air surrounding
an object. (1)
American Society of Heating, Refrigerating, and Air-
Conditioning Engineers (ASHRAE). An international orga-
nization advancing the arts and sciences of HVACR through
research, standards writing, continuing education, publica-
tions, and examinations. (54)
American Wire Gage (AWG). Standardized system of des-
ignating wire size based on a wire’s diameter. For sizes
18 AWG to 1 AWG, the wire diameter increases as the AWG
number decreases. For larger aught (/0) sizes, wire size
increases as the numbers increase. (10)
ammeter. An instrument that measures the rate of current
flow through a conductor in amperes. (14)
ampere (A). Unit used to measure the flow of electric current.
One ampere equals the flow of one coulomb per second. (9)
anhydrous ammonia. Ammonia that is free of water. (31)
annealing. Cooling a metal slowly from a high temperature
to make the metal soft. (4)
annual fuel utilization efficiency (AFUE) rating. Measurement
of furnace efficiency that compares a furnace’s yearly or sea-
sonal energy output to the energy input for the same time
period. The higher the AFUE rating, the more efficient and cost
effective a furnace is. (35)
anode. In electronics, a positively charged segment of a
semiconductor consisting of a P-type material, such as on a
diode. (11)
anticipator. A device that assists the controller by causing
the sensor to detect the cut-in or cut-out temperature slightly
before the conditioned area actually reaches that tempera-
ture. Often this is a resistor used in a thermostat. (13)
anti-short cycle control. A type of solid-state relay that will
stop a compressor from cycling on before a preset time has
passed after shutdown. (51)
apparent power. A circuit’s calculated power value (P = I × E),
which does not take into account the effects of inductive
reactance or capacitive reactance. The value is calculated in
volt-amperes (VA). (10)
aquastat. A hydronic system control that measures the tem-
perature of the boiler water and turns the burner or heat-
ing element on and off as needed to maintain the preset
temperature. (33)
asbestos. A naturally occurring mineral that forms fiber
bundles. It was once used in building construction for its
strength and fire resistance but is no longer used because it
is now a known cancer-causing agent. (22)
ash content. The amount of noncombustible contaminants
in fuel oil. (36)
aspect ratio. The ratio of a duct’s wide side to its narrow
side. (23)
atmosphere. The air surrounding the earth. This term also
refers to atmospheric pressure at sea level, which is 14.7 psia
(0 psig). (1, 21)
atmospheric balancing. Bringing up the pressure of an
evacuated part of an HVACR system that is to be opened
to atmosphere for service. This is commonly done by
bypassing a small amount of vapor refrigerant through
the gauge manifold till pressure reads 0 psig (14.7 psia
or 101.3 kPa). Also called pressure equalizing or balancing
pressures. (53)
atmospheric dust spot efficiency. A method used to mea-
sure the ability of a filter to remove atmospheric dust by
evaluating the flow rates on both sides of the filter and the
quantity of the material it captures. (22)
atmospheric gas burner. A type of gas burner that uses the
siphoning action of gas flow through its orifice to induce air-
flow through the burner without the need for a blower. (35)
atom. The smallest part of an element. (9)
atomization. The breaking up of fuel oil into tiny droplets
so that the fuel oil will vaporize rapidly and ignite in the
combustion chamber. (36)
atomizing humidifier. A humidifying unit that sprays small
water droplets into the air by mechanically flinging water
against a diffuser or by forcing it through a nozzle. (28)
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