Glossary
1409
excelsior. A tight lattice of fibers made from fine curled
wood shavings. Used in a greenhouse as part of an evapora-
tive cooling system. (27)
excess air. Any secondary air that exceeds the amount of air
necessary for complete combustion in a furnace. (35)
exfiltration. The natural leakage of inside air out of a build-
ing through doors, windows, and other construction joints.
(22, 30)
exosphere. The outermost layer of earth’s atmosphere that
extends outward from the edge of the thermosphere to a dis-
tance of 6,200 miles, as it gradually blends with space. (21)
expansion steam trap. A steam trap that has an inter-
nal material that expands and contracts with temperature
change to trigger the steam’s trap valve. (33)
expansion tank. In a hydronic system, a tank that helps
maintain a stable system pressure by storing water as it
expands and contracts due to temperature changes. Also
called compression tank. In the secondary loop refrigeration
system of a commercial system, a cylinder containing a pres-
sure responsive bladder that is used to account for changes
in secondary loop pressure. (33, 45)
expendable refrigerant. A refrigerant that typically has a
very low boiling temperature and is used only once in a sys-
tem for purposes such as freezing food. (6)
expendable refrigeration system. A refrigeration system
in which the refrigerant is discarded or released into the
atmosphere after it has evaporated. Also called an open-cycle
refrigeration system or chemical refrigeration system. (6, 43)
external drive compressor. A compressor that is driven by a
motor that is separate from the compressor unit. Also called
an open-drive compressor. (15)
external static pressure (ESP). The pressure difference
between a blower’s inlet and outlet. (23)
Fahrenheit scale. A temperature scale used in US Customary
system. At sea level, the boiling point of water is 212°F and
the freezing point of water is 32°F. (1)
fan convector. A type of terminal unit in a hydronic system
that uses a fan to blow air across the surface of the heat-
ing element, which increases the rate of heat transfer and
improves convection. Also called fan coil. (33)
fan heater. A type of convection heater that uses electric
heating elements to produce warm air, which is distributed
to a conditioned space by a fan. (37)
farad (F). A unit used to measure a capacitor’s capacitance.
A farad is equal to an electrical charge of one coulomb with
the potential to discharge one volt of electrical pressure. Most
capacitors used in HVACR systems are rated in microfarads. (9)
fast-acting fuse. A fuse that blows immediately after the
maximum rating of the fuse is exceeded. (13)
feedback. The information detected by a sensor in a condi-
tioned area that is sent to the controller to determine what
action needs to be taken. (13)
field pole. An electromagnet in an ac motor’s stator whose
polarity changes as the flow of current alternates in the field
windings. (12)
field winding. In an electric motor, the wires wrapped
around the field poles of the stator that create a magnetic
field when current flows through them. (12)
field-erected air-conditioning system. A system in which
all components are purchased individually and assembled
at the spot where the system is to be used. (26)
file. A hand tool with cutting ridges, or teeth, used to clean
and shape metal surfaces. (3)
fill. In a cooling tower’s wet decks, specially designed sheets
or structures with different ridges, corrugations, or designs
used to increase air-water contact with the lowest possible
air pressure drop. (45)
filter-drier. A device used to remove moisture, dirt, metal,
and other debris from refrigerant. (2)
fin comb. A tool used to straighten the bent fins of condens-
ers and heat pumps. Also called a fin straightener. (53)
fin evaporator. An evaporator that features metal fins
of various styles and types connected to the evaporator
tubing. (46)
fixed filter humidifier. A humidifying unit with a pad that
consists of numerous layers of a tight metallic mesh wetted
by water metered through a solenoid valve. (28)
flame failure response time (FFRT). The amount of time for
an oil furnace’s primary control unit to sense that there is no
flame when the thermostat is calling for heat. (36)
flame rectification. The process of using a pilot flame or
gas burner flame to change a small electric current from
alternating current to direct current. Ignition control mod-
ules rely on flame rectification to verify the presence of a
flame in a furnace. (35)
flame retention oil burner. An oil burner with a combustion
head that directs combustion air in a manner that retains its
flame, making it more compact and more efficient. (36)
flame rollout. When the flame in a furnace spills backward
out of the burner. (35)
flammability. A substance’s capacity to ignite. Refrigerants
have a flammability rating of 1, 2, 2L, or 3 under ASHRAE
Standard 34. (6)
flammability limits. The range of fuel concentrations in
a mixture of fuel and air within which the fuel will burn
when an ignition source is present. (35)
flare. An enlargement at the end of a piece of tubing, usu-
ally made at about a 45° angle, by which the tubing is con-
nected to a fixed fitting or to another piece of tubing using
threaded couplings. (4)
flare nut wrench. A wrench with a small opening in the grip-
ping head, similar to an open end wrench, but capable of
providing maximum surface area contact with flare nuts. (3)
flash gas. The instantaneous evaporation of liquid refrig-
erant. In an evaporator, flash gas occurs from the pressure
drop between the high side and low side of the system. This
pressure drop allows flash gas to absorb heat, which helps
to cool the rest of the liquid refrigerant. In a liquid line,
the evaporation of high-pressure liquid refrigerant, which