A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 210 terms in this directory beginning with the letter B.
Babo’s law
addition of a nonvolatile solid to a liquid in which it is soluble lowers the vapor pressure of the solvent in proportion to the amount of substance dissolved.

back pressure
the static pressure existing at the outlet of an operating pressure relief device due to pressure in the discharge line.

back pressure valve
(also known as evaporator pressure regulator), an automatic valve located between the evaporator outlet and the compressor inlet that responds to its own inlet pressure that prevents the evaporator pressure from falling below a selected value.

back siphonage
reverse flow in a water system caused by negative pressure in an incoming pipe when the point of use is at atmospheric pressure. Note: back siphonage generally is more evident in an open water system.

back-siphonage preventer
see [[backflow preventer]].

backdraft damper
device which, when mounted in a duct or opening, permits the flow of air in one direction only. Can be gravity (counter weighted) or power operated.

reverse flow in a water system from the normal or intended direction. Note: backflow generally is more evident in an open water system.

backflow preventer
device designed to prevent reverse flow in a water system. Note: term normally used where back-pressure-type backflow is implied.

backflow prevention
program, ordinance, or code designed to prevent backflow into a potable water system and to discover, eliminate, and prevent all uncontrolled cross connections, existing or potential.

background irradiance
irradiance at the entrance aperture of the infrared sensing system that is not radiated directly from the object being investigated.

background noise
total noise from all sources other than a particular sound that is of interest. Compare to [[ambient noise]].

backward-curved impeller
centrifugal rotor in which the convex sides of blades face in the direction of rotation.

backward-inclined impeller
component of a centrifugal fan or pump consisting of simple flat blades backwardly inclined to match the velocity pattern of the fluid passing through the impeller wheel for high-efficiency operation. Impeller blade width and length affect flow rate and pressure or lift performance. See [[centrifugal fan]]; [[centrifugal pump]].

surface, usually in the form of a plate or wall, used for separating spaces or deflecting fluids.

balance of system
Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

balance point
the outdoor temperature at which a building's heat loss to the environment is equal to internal heat gains from people, lights, and equipment.

balanced draft
two fans connected to a combustion unit, one to supply the combustion air and the other to induce draft.

balanced flow
fluid flows where the design supply flow volumetric rate equals the design return/exhaust volumetric rate. Also called balanced ventilation or balanced system.

balanced heat recovery
occurs when internal heat gain equals recovered heat, and no external heat is introduced to the conditioned space. Maintaining balance may require raising the temperature of recovered heat.

balanced relief valve
a pressure relief valve that incorporates means of minimizing the effect of back pressure on the operational characteristics of the valve (opening pressure, closing pressure, and relieving capacity).

a commonly used term for the person or firm that performs testing, adjusting, and balancing of HVAC systems.

the methodical proportioning of air and hydronic flows through the system mains, branches, and terminal devices using acceptable procedures to achieve the specified airflow or hydronic flow within testing, design, and installation limitations.

balancing area
A metered segment of the power system, maintained by a balancing area authority, that ensures the total of all electrical generation equals the total of all system loads.

balancing damper
adjustable blade device to control airflow.

balancing station
assembly to measure and control fluid flow; composed of a measuring device, a volume control device, and recommended lengths of straight ductwork or pipe leading into and out of the measuring device.

balancing valve
metering-type valve with locking positions to control hydronic flow.

ball valve
valve consisting of a rotatable ball with a hole through its center. Typically manufactured in either full port (opening) or conventional (reduced) port (opening) construction.

a device used in conjunction with an electric discharge lamp to cause the lamp to start and operate under the proper circuit conditions of voltage, current, wave form, electrode heat, etc.

ballast factor
ratio of commercial electric ballast lamp lumens to reference ballast lamp lumens, used to correct the lamp lumen output from rated to actual.

ballast losses
electric power (watts) consumed internally by the ballast components.

band gap
In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

band gap energy (Eg)
The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

an instrument for measuring atmospheric pressure.

barometric condenser
apparatus in which steam is condensed at reduced pressure by direct contact with water.

barometric damper
(1) counterweighted damper set so that variations in chimney barometric pressure will cause the damper to open or close gradually to maintain a constant draft directly upstream of the damper. (2) mechanically balanced damper that rotates from changes in pressure within breeching to bleed air into the breeching to maintain a steady draft.

barometric effect
variations in barometric pressure caused by altitude or weather changes.

barometric pressure
the pressure of the atmosphere relative to zero absolute pressure (a perfect vacuum).

barrier energy
The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier.

see [[building automation system]].

base load
The average amount of electric power that a utility must supply in any period.

base load generating plants
Typically coal or nuclear generating units that are committed and dispatched at constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost of energy when run at very high capacity factors.

base temperature
temperature from which temperature departure is calculated (e.g., degree-day).

base use
primary HVAC system losses incurred and auxiliary system energy consumed in maintaining a central HVAC energy source available for consumption by all residents.

baseboard radiator
steam, hydronic, or electric heating device located at or near the floor.

baseline building design
a computer representation of a hypothetical design based on the proposed building project. This representation is used as the basis for calculating the baseline building performance for rating above standard design.

baseline building performance
the annual energy cost for a building design intended for use as a baseline for rating above standard design.

baseline data
the measurements and facts describing facility operations and design during the baseline period. This will include energy use or demand and parameters of facility operation that govern energy use or demand.

baseline model
the set of arithmetic factors, equations, or data used to describe the relationship between energy use or demand and other baseline data. A model may also be a simulation process involving a specified simulation engine and set of input data.

baseline period
the period of time selected as representative of facility operations before retrofit.

basis of design (BOD)
a document that records the concepts, calculations, decisions, and product selections used to meet the Owner’s Project Requirements and to satisfy applicable regulatory requirements, standards, and guidelines. The document includes both narrative descriptions and lists of individual items that support the design process.

Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

battery available capacity
The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage.

battery capacity
The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

battery cell
The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all the above.

battery cycle life
The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

battery energy capacity
The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

battery energy storage
Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

battery life
The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

Baudelot cooler
liquid cooler in which the liquid to be cooled passes by gravity over a refrigerated surface. (Also known as falling film surface cooler)

Board for the Coordination of Model Codes (a Board of CABO)

Building Design and Construction

beam irradiance
irradiance received from the sun without significant change in direction from the sun’s apparent position.

Beer’s law
absorption of light by a solution changes exponentially with the concentration, all else remaining the same.

(1) flexible, corrugated chamber that converts pressure variation into mechanical movement. (2) flexible, fluid-containing vessel that will expand or contract as a result of a change in the pressure of the contained fluid. Can be used to transmit force and/or motion in a pneumatic or hydraulic system or as a sensor of temperature or pressure when the bellows is sealed.

bellows seal
mechanical seal of flexible, corrugated-metal bellows with one end attached to a ring pressed against the shoulder of the shaft, the other end to a disc pressed against the housing.

bellows valve
packless valve in which a bellows forms the seal between the adjustment spindle and the valve body.

Building Energy Performance Standards

tendency of an estimate to deviate in one direction from a true value (a systematic error). See [[error]]. Compare to [[precision]].

bias error
the difference between the true value to be measured and the indicated value from the measuring system that persists and is usually caused by the particular instrument or technique of measurement.

Building Industries Consulting Services International

bifurcated fan
tubeaxial (ducted) fan whose motor is mounted outside a bifurcated (divided) duct with only the blades of the fan located in the airstream.

billing data
information collected from invoices sent to an owner from the energy supplier (e.g., an electric or gas bill).

billing demand
the demand used to calculate the demand cost. In the United States, this is very often the monthly peak demand of the customer or the peak demand over a several month timeframe (a demand ratchet). Billing demand in many countries may be the contract demand. Other variations are possible.

bimetallic element
actuating element consisting of two strips of metal with different coefficients of thermal expansion attached so that internal strains caused by temperature changes bend the compound strip.

bimetallic thermometer
a thermometer that indicates temperature from the flexing of two strips of materials of different coefficients of expansion bonded together.

a class interval or grouping, typically for outdoor air temperature.

bin method
energy calculation method, usually used in prediction, in which the annual (or monthly) energy use of a building is calculated as the sum of the energy used for all of the outdoor temperature bins. The bin method allows heat pump (or other heater or cooler) performance, which is different for each bin, to be accounted for.

characteristic or property involving a selection, choice, or condition in which there are two possibilities (as binary numbers in a computer, on/off or open/closed switch position).

binary vapor cycle
thermodynamic cycle relative to the varying condensing and evaporating temperatures of a nonazeotropic mixture of two fluids.

chemical substance capable of killing living organism, usually in a selective way.

biological deposits
water-formed deposits of biological organisms or the products of their life processes. Biological deposits include barnacles, algae, or slimes.

See building integrated photovoltaics.

(1) body that absorbs all the radiant energy falling on it. (2) body that has the maximum, theoretical, radiant-energy emittance at a given temperature. See also [[absorber]].

blackbody equivalent temperature
apparent temperature of an object as determined from the measurement of its radiance and the assumption that it is an ideal blackbody with emissivity of 1.0.

blade pitch
the angle between the chord of the blade and the plane of rotation (axial fan) or centerline of the rotor hub. The pitch may be constant for the length of the blade, or it may be larger at the blade root than at the tip.

blade twist
the property of a propeller blade describing the variation of the pitch from the blade root to the tip.

blanket thermal insulation
relatively flat and flexible insulation in coherent form, furnished in units of substantial area.

blast coil
heat transfer surface, most frequently of an extended surface arrangement, over which air is blown to be heated or cooled, depending on the temperature of the fluid within the coil.

blast freezer
chamber in which cold air is circulated rapidly around products to be frozen so that freezing occurs rapidly enough to avoid formation of large ice crystals which may damage the product.

blast gate
heavy duty duct closure device.

blast gate damper
in an air-handling system, a sliding damper.

blast heater
set of heat transfer coils or sections used to heat air which is drawn or forced through them by a fan. A unit heater.

bleed valve
valve that has a fixed orifice incapable of being closed by an action of the valve, permitting a flow through or in parallel with the main valve port.

pipe attached to a unit, such as a condenser, to bleed off liquid refrigerant parallel to main flow.

a refrigerant consisting of a mixture of two or more different chemical compounds, often used individually as refrigerants for other applications.

refrigerants consisting of mixtures of two or more different chemical compounds, often used individually as refrigerants for other applications.

blind flange
flange used to seal the end of a pipe.

block thermal insulation
rigid insulation preformed into rectangular units.

blocking diode
A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

(1) discharge of water from a steam boiler or open recirculating system that contains high total dissolved solids. The addition of makeup water will reduce the concentration of dissolved solids to minimize their precipitation. (2) in pressure relief-devices, the difference between actuation pressure of a pressure relief valve and reseating pressure, expressed as a percentage of set pressure or in pressure units.

a ducted centrifugal fan used in a heating, ventilating, and air-conditioning system. See also [[centrifugal fan]]; [[fan]].

blower door
an assembly consisting of a fan/blower and a calibrated flow measuring station [orifice plate, flow nozzle(s), flow ring, etc.] used for pressurizing or depressurizing a building envelope.

blowoff valve
safety valve that maintains a predetermined pressure in a vessel by discharging excess gas to the atmosphere.

blowthrough unit
air-handling unit with a section or sections downstream of the supply air fan.

blue energy
sometimes called ocean energy, blue energy is a term for the method of generating electricity through the convergence of both fresh and salt water. This energy can be extracted through a variety of means, including tidal power, current power, wave power, thermal energy conversion, and osmosis. Power may also be collected by harnessing the wind power associated with the body of water, usually the ocean.

see [[building management system]].

Building Officials and Code Administrators

boil-off gas
liquefied gas lost by vaporization during storage in a tank.

a closed, pressure vessel that uses fuel or electricity for heating water or other fluids to supply steam or hot water for heating, humidification, or other applications.

boiler burner unit
boiler designed especially for gas and oil and sold integrally with the burner.

boiler capacity
design maximum rate of heat output.

boiler crown sheet
part of a boiler forming the top of the furnace in a firebox boiler or the equivalent surface in other types.

boiler feed pump
pump that returns condensed steam, makeup water, or both directly to the boiler. Normally installed with a large receiver.

boiler feedwater
water supplied to a boiler by pumping.

boiler feedwater heater
apparatus for raising the temperature of the boiler feedwater, usually with exhaust steam.

boiler foaming
carryover of slugs of water into the piping from overloading of the boiler. Compare to [[boiler priming]].

boiler generator
part of an absorption machine in which the refrigerant vapor is driven off by heat.

boiler heating surface
surfaces of the boiler that are exposed to the products of combustion on one side and water on the other, expressed in area units (of the side receiving the heat).

boiler horsepower
equivalent evaporation of 34.5 lb of water per hour from and at 212°F (100°C). This is equal to heat output of 970.3 Btu/lb/h × 34.5 lb = 33,475 Btu/h, approximately 9809.5 W.

boiler priming
carryover of boiler water with the steam due to insufficient steam space, faulty boiler design, or operating conditions. Compare to [[foaming]].

boiler rating
design maximum rating of a steam or water boiler expressed as the total heat transferred by the heating surfaces in Btu/h (kW). Sometimes expressed in horsepower or pounds (torque, kilograms) of steam per hour. Compare to [[boiler capacity]].

boiler water leg
space that contains water between the outer shell and furnace shell.

boiler waterline
level at which water is maintained within a steam boiler.

boiling ebullition process
vaporization of a liquid with formation of bubbles.

boiling point
temperature at which the vapor pressure of a liquid equals the absolute external pressure at the liquid vapor interface.

sensitive thermal device for measuring radiant energy.

Building Owners' and Managers' Association

(also known as electrical ground), connection to ground potential of a metal part on an appliance or component which may become energized by an electrical fault or may develop a static charge.

accessory to increase output of, for example, a compressor, pump, or header.

sheet metal transformation piece used to make connection between round and rectangular ductwork.

inside diameter of a hollow cylinder or of a hole.

boron (B)
The chemical element commonly used as the dopant in photovoltaic device or cell material.

A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.

boundary conditions
Physical conditions (values of physical quantities, conditions of energy interchange, etc.) imposed upon a system at the start or end of a process (temporal boundary conditions) and/or at the boundaries separating the system from its surroundings (local boundary conditions).

boundary layer
region of retarded fluid flow near the surface of a body moving through the fluid or past which the fluid moves. See also [[flow]].

boundary layer flow
flow of that portion of a viscous fluid in the area of a body in contact with the fluid and in motion relative to the fluid.

Bourdon gage
mechanical pressure measuring instrument that senses pressure with a curved oval tube that tends to straighten when the pressure increases and recurves when the pressure decreases.

Boyle’s law
product of the volume of a gas times its pressure is a constant at fixed temperature. Also known as Mariotte’s law.

brackish water
impure water with a lesser content of salt than seawater, but higher than that of potable water.

brake horsepower
brake power expressed in horsepower.

brake power
actual power delivered by a shaft (from the use of a brake to measure power).

(1) in ducts, piping, or conduit, another section of the same size or smaller, at an angle with the main. (2) section of pipe or duct from a main to a terminal device.

branch circuit
the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s). The final wiring run to the load.

branch line
air supply line connecting a controller and controlled device.

to join metals by fusion of nonferrous alloys that have melting points above 800°F (427°C) but lower than those of the metals being joined. May be accomplished by means of a torch (torch brazing), in a furnace (furnace brazing), or by dipping in a molten flux bath (dip or flux brazing).

joined by fusion using a spelter of brass. Considered equivalent to hard soldering.

brazed tube
tube made from sheet or strip with a longitudinal brazed joint.

break-even temperature
outdoor temperature at which total heat losses from conditioned spaces equal internally generated heat gains.

a condition that occurs in a gas phase filter when, as contaminated air passes through a filter, the outlet contaminant concentration reaches a predetermined percentage of the challenge.

breakthrough time
for gas phase filters, the sudden decline in efficiency, defined as the operating time (at constant operating conditions) before a certain penetration is achieved.

breather plug
removable plug, cap, or other means of venting a space containing insulating material through vaportight sheathing.

breathing zone
the region within an occupied space between planes 3 and 72 in. (75 and 1800 mm) above the floor and more than 2 ft (600 mm) from the walls or fixed air-conditioning equipment.

the connection to any fuel-fired equipment, device, or appliance and the horizontal section of the exhaust system used for conducting the products of combustion from a fuel-fired equipment, device, or appliance to the vertical section, which is identified as the vent, chimney, or flue.

Brewster’s law
index of refraction for a material is equal to the tangent of the polarizing angle for the material.

Building Research Institute

brine expansion tank
vented reservoir in a closed, circulating brine system for the accommodation of volume expansion of brine due to temperature change.

brine return tank
reservoir in an open, circulating brine system for storing brine at the pump suction and for inspecting the condition and flow of brine.

brine tank
(1) in a brine circulating system, a storage or balance tank for brine. (2) in an ice plant, a main freezing tank in which cans are immersed while ice is being produced. (3) in domestic and commercial fields, a container surrounding the evaporator and filled with brine for storing refrigerant or equalizing temperature at various points of the evaporator (especially an ice cream cabinet).

brine-spray refrigerating system
cooling of air by spraying brine into the airstream. Note: process discontinued in meat refrigerating plants because of salt corrosion of meat-carrying rails.

briquette ice
ice produced as small, regularly shaped and sized pieces.

British thermal unit (Btu)
The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

British Standards Institute

Btu meter
metering system capable of measuring the energy added to or extracted from an electric or fluid stream. Also called thermal energy meter, heat meter, or thermal meter. Compare to [[watt meter]].

bubble-point temperature
a liquid-vapor equilibrium point for a volatile pure liquid or for a multicomponent mixture of miscible, volatile, pure component liquids, in the absence of noncondensables, where the temperature of the mixture at a defined pressure is the minimum temperature required for a vapor bubble to form in the liquid.

budget building design
a computer representation of a hypothetical design based on the actual proposed building design. This representation is used as the basis for calculating the energy cost budget.

(1) digital circuit element used to increase the number of outputs a circuit can drive or to convert input or output levels for signal level compatibility. Also, any isolating amplifier stage. (2) insulating circuit used to avoid reaction of a drive circuit on any driven circuit. (3) salts or other compounds that reduce the changes in the pH of a solution upon the addition of an acid or alkali. (4) storage device used to compensate for a difference in rate of flow, data, time, or occurrence of events when transmitting data from one device to another.

buffer tank
tank fitted into a circuit to dampen fluctuations in flow.

buffer zone
an unconditioned part of a building containing some or all of the distribution system.

a structure wholly or partially enclosed within exterior walls, or within exterior and party walls, and a roof, affording shelter to persons, animals, or property.

building automation and control network (BACnet)
data communications protocol for building automation and control networks

building automation system (BAS)
an energy management system, usually with additional capabilities, relating to the overall operation of the building in which it is installed, such as equipment monitoring, protection of equipment against power failure, and building security. Compare to [[building management system]].

building envelop leakage
overall leakage of the building characterized by the airflow rate at a given pressure difference across the envelope of the building. Compare to [[air infiltration]]).

building envelope
(1) outer elements of a building, including walls, windows, doors, roofs, and floors, including those in contact with earth. (2) the exterior plus the semi-exterior portions of a building. For the purposes of determining building envelope requirements, the classifications are defined as follows. Building envelope, exterior: the elements of a building that separate conditioned spaces from the exterior. Building envelope, semi-exterior: the elements of a building that separate conditioned space from unconditioned space or that enclose semiheated spaces through which thermal energy may be transferred to or from the exterior, to or from unconditioned spaces, or to or from conditioned spaces.

building envelope opaque areas
all exposed areas of a building envelope which enclose space, except for openings for windows, skylights, doors, and building service systems (i.e., all areas of a building envelope that permit passage of radiant energy in or out of the building space).

building envelope void
any localized area of the building envelope that has a thermal resistance significantly different from the area surrounding it. The void may be due to partial or complete absence of thermal insulation.

building exit
any doorway, set of doors, or other form of portal that is ordinarily used only for emergency egress or convenience exit.

building height
vertical distance from level grade or average grade to the highest finish roof surface, in the case of flat roofs, or to a point at the average height of the highest roof having a pitch.

building information
The object (or set of objects) represents not only the geometry required to represent the component or assembly graphically (visually) but also has the ability to have much more information about that object associated with it or related to other intelligent objects associated with it. Rule-based relationships between intelligent objects that enable related properties to be updated when one property changes.

building information model (BIM)
a data specification for representing building information for the purposes of interoperable data exchange. Also, a data model of a specific building or its systems, components, or other information elements based on that specification.

Building Information Modeling (BIM)
the human activity of using BIM software and other related software, hardware, and technologies to create and use in a building information model. See [[building information model]].

building integrated photovoltaics
A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading eyebrows over windows; or other building envelope systems.

building management system
an energy management system relating to the overall operation of the building in which it is installed. It often has additional capabilities, such as equipment monitoring, protection of equipment against power failure, and building security. It may also be a direct digital control (DDC) system where the mode of control uses digital outputs to control processes or elements directly.

building material
any element of the building envelope, other than air films and insulation, through which heat flows and that is included in the component U-factor calculations.

building official
the officer or other designated representative authorized to act on behalf of the authority having jurisdiction.

building power line carrier
method of transmitting control signals using existing electrical wiring. Signals are introduced by superimposing on, or altering, existing waveforms. Signals are sensed by detecting these modifications of the wave forms with a remotely controlled receiver.

building project
building or group of buildings proposed or under construction, including on-site energy conversion or electric generating facilities, which utilize a single submittal for construction permit or are within the boundary of a contiguous area under one ownership.

building services
utilities (such as electricity, gas, steam, telephone, and water) supplied and distributed within a building.

building space
the volume of a building that exchanges air with outdoor (ambient) air. The building volume is the space that is deliberately conditioned for human comfort (ft³ [m³]).

building thermal envelope
elements of a building that enclose spaces and which control or regulate heat and mass transfer (air, water vapor, and entrained moisture) between the interior spaces and the building exterior. Note: the interior surfaces of insulated floors, walls, windows, and ceilings generally comprise the thermal envelope. Compare to [[thermal envelope]]. See [[air infiltration]].

building use type
the classification of a building based on its intended use as determined by the use and occupancy classification for the building that has been established by the authority having jurisdiction (AHJ).

portion of a thermal sensing system that is placed in the controlled (or measured) variable.

bulk modulus of elasticity
ratio of the compressive or tensile force applied to a material per unit surface area to the change in volume of the material per unit volume. Also known as bulk modulus, compression modulus, or hydrostatic modulus.

buoyancy inertial parameter
a parameter derived from the dimensionless Richardson number relating fluid inertial forces to buoyancy forces in a storage device.

part of a fuel burning device (as a stove or furnace) where flame is produced.

burner ignition
(1) initiation of combustion. (2) process or means of igniting a fuel/air mixture.

burner low fire setting
setting that determines the fuel firing rate at which burner ignition occurs where low/high/low/off or modulating combustion controls are used. The firing rate corresponds to approximately one third or less of the full burner delivery rate for rotary burners. It corresponds to approximately one fifth or less of the full burner delivery rate of air for mechanical atomizing burners.

burner register
series of air-directing vanes (usually adjustable) that are used to direct and/or control the combustion airflow through the burner.

burner throat
sleeve, usually formed of refractory, located at the burner exit, within which combustion starts.

burner window box
device to introduce secondary combustion air into the furnace in a 360° pattern around the flame and, with an ignition cone, to cause accelerated vaporization of the oil.

butt joint
joint between two members lying approximately in the same plane.

butt weld
weld formed by placing two edges or ends one against the other and welding them. Compare to [[lap weld]].

butt weld pipe
pipe welded along a butted longitudinal seam and not scarfed or lapped.

butterfly damper
(1) damper consisting of a plate turning on a diametral axis inside a duct. (2) pair of flaps hinged to a common diametrical spindle and permitting flow in one direction only. (3) two flaps in "V" arrangement.

butterfly valve
valve in which the regulating mechanism is a circular or elliptical disc that is rotatable about an axis.

a pipe or duct, usually controlled by valve or damper, for conveying a fluid around an element of a system.

bypass air
any portion of the mixed air that circumvents conditioning.

bypass diode
A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.

bypass factor
the percentage of the air that does not come into contact with the coil; the remaining air is assumed to exit the coil at the average coil temperature. See also [[apparatus dew point]].

bypass leakage
unwanted passing of untreated air into the treated air between the components within casings such as filters or coils within a section.

bypass terminal
air terminal unit using a method of volume modulation whereby airflow is varied by distributing that volume required to meet the load, the balance of primary air being diverted to the return.