Glossary
air changes per hour. The number of times in one hour that the entire volume of air in a space is replaced with 100-percent fresh, outside air.
air handler. A metal HVAC cabinet inside the house connected to the supply and return air ducts. It has a fan, blower, air filter, and heating/cooling coils to transfer heat to and from the refrigerant.
air tightness. The extent to which a building element prevents infiltration and exfiltration.
alternating current. The electrical current in your house is not a smooth, uniform flow but three phases of sinusoidal waves 120° apart.
attic ventilator. A thermostat-controlled exhaust fan installed in the roof used to purge hot air from the attic, reducing the thermal load on the ceiling of the uppermost floor and, by extension, the load on the air conditioning system overall.
audit. The analysis of a building’s energy use profile, to include energy-saving possibilities.
backdraft damper. A special damper that allows air in a duct or opening to flow in only one direction.
ballast. A magnetic or electronic device that regulates the current to fluorescent lamps and provides enough voltage to start the lamp.
barycentric damper. An HVAC damper placed in the outside wall of a negative pressure zone, controlling an exhaust fan to maintain the pressure negative.
British Thermal Unit (BTU). The amount of thermal energy needed to raise the temperature of 1 pound of water from 59 to 60° F.
BTU. British Thermal Unit.
building envelope. The external shell of a building that encloses the conditioned spaces, e.g., the floors, wall, and roof.
certification program. A group of professional organizations that develops energy efficiency standards for buildings. The program usually includes a standardized inspection process to verify compliance with energy efficiency standards.
CFL. Compact Fluorescent Lamp.
chimney effect. The principle in passive ventilation methods that air passing over the top of a tall duct column draws in air at the bottom, similar to a fireplace chimney.
clean electricity. Electrical current with no harmonic distortions, i.e., perfect alternating current.
clerestory window. A high window, usually facing south, that allows sunlight to penetrate deep into the northern side of a building.
Coefficient Of Performance. A measure of the efficiency of heat pump systems, where a COP of 1.0 is straight-electric heat. COP is equal to the heat output of the system divided by the energy input.
Color Rendering Index. A percentage value (relative to incandescent lamps which have a CRI of 100) that indicates how natural or normal objects appear when illuminated by a light source. The higher the CRI, the better objects seem, and the more realistic people appear.
combustion air. All gas appliances need a steady, dependable supply of outside air for the burning of fossil fuels.
compact fluorescent lamp. A small bulb suitable for desk and table lamps with an electronic ballast to operate a fluorescent-based light source.
condensing unit. The outside part of a heat pump where a fan blows air across finned heat-transfer surfaces to move heat to and from the refrigerant.
conditioned. Heated or cooled areas of a building inside the thermal (building) envelope or shell, e.g., “conditioned spaces.”
conduction. The transfer of heat from molecule-to-molecule via physical contact.
convection. Heat that is transferred by a moving fluid such as air or water.
cooling tower. A heat-transfer device that lowers the temperature of water pumped through it by evaporative cooling. This process is accelerated by a forced airflow that accelerates the heat-transfer rate.
cooling load. The power necessary to keep a building cool in a warm climate
CO2. Carbon dioxide.
COP. Coefficient of Performance.
CRI. Color Rendering Index.
damper. An air-volume control device that adjusts the volume of air in an HVAC duct.
daylighting. Using natural, outside light for illumination inside a building.
degree day. A total of twenty-four hours during which the outside temperature varies by 1° F from a standardized benchmark (e.g., 65° F inside temperature).
dehumidification. The process of reducing the relative humidity of air in an HVAC system, usually by condensation at the cooling coils.
dew point. The temperature to which air must be cooled to become saturated with water vapor, after which airborne water condenses into droplets.
diffuser. A lighting device placed over a window that transforms direct sunlight into diffuse light spread out over a large area. Similarly, for point-sources in lighting applications.
dirty power. Alternating current that has been adversely modified by electronic devices in the circuit so that other devices on that same circuit operate less efficiently.
energy audit. The measurement and tabulation of total energy usage in a building during a calendar year. A computer simulation of the building is matched to the utility bills so that modifications to the building can be quantified as a dollaramount savings.
energy balance. The idea that energy must be input to a building at the same rate that it’s lost to the environment.
energy recovery ventilator. A device that transfers latent and sensible thermal energy from the exit airflow to the incoming airflow.
evaporative cooling. To lower the temperature by the evaporation of a liquid which removes latent heat from a surface where the evaporation takes place.
envelope. The external surfaces (floor, roof, walls) of a building that surround the conditioned space.
exfiltration. Air flowing out of a building shell.
footcandles. A standard measurement of the light level established by the Illuminating Engineering Society equal to one lumen per square foot.
frost line. The maximum depth to which the ground freezes in the winter.
Global Warming Potential. The total contribution to global warming by the emission of one unit of a gas (typically 1 kilogram) measured relative to carbon dioxide, which has a GWP of 1.0.
gray water. Outflow from sinks, baths, and showers is collected and treated for subsequent use in the garden or to flush toilets.
Ground Source Heat Pump. A heat-transfer-system that transfers thermal energy to and from a ground-loop field.
GSHP. Ground Source Heat Pump.
GWP. Global Warming Potential.
harmonics. Electronic devices like computers use power from a circuit in a nonlinear fashion, switching on and off many times each second. This corrupts the power in the rest of the circuit so that it’s no longer uniform alternating current.
heat equation. The flow of heat through a barrier is directly proportional to (1) the area, (2) the temperature difference, and (3) the amount of insulation.
heat of vaporization. The amount of thermal energy (540 calories) required to vaporize (change into steam) 1 gram of water at 100° C.
heat transfer. The three ways thermal energy moves are via conduction, convection, and radiation.
heating load. The heat energy needed to keep a building warm in a cold climate.
HVAC. Heating, Ventilating, and Air Conditioning.
Indoor Air Quality. The quality of the air volume inside a building with respect to both comfort and health.
infiltration. Air flowing out of a building shell.
infra-red scan. A thermal scan to determine where heat is escaping from the building envelope.
intelligent membrane. A type of vapor barrier that is vapor-open when there is a higher humidity outside, allowing the building assembly to breathe and dry out.
latent heat. The amount of heat required to cause a phase change (gas-to-liquid or liquid-to-gas) of water without a change in the actual temperature.
LED. Light Emitting Diode.
Light Emitting Diode. An energy-efficient light source whose function is opposite to that of a photovoltaic solar cell, in that it converts electricity into light (instead of sunlight into electricity).
light quality. Illumination without glare and with low overall brightness contrast.
light shelf. A horizontal, reflective shelf in the middle of a large window façade that reflects some of the incoming sunlight deep into the space.
light tubes. A circular duct with a highly reflective inner surface that transports light from a rooftop fixture down into the interior of a building.
low-flow shower. An aerating shower-head attachment that uses less water than the usual “power” head.
lumens. A standard measure of the amount of light emitted from a light source.
makeup air. Instead of, for example, a clothes dryer, drawing all of the air from the air-conditioned zone where it’s situated, part of the air is drawn from outside.
negative pressure zone. An area inside a building with a lower HVAC pressure than the surrounding areas, for example, to keep airborne pathogens from escaping a COVID-19 ward in a hospital.
net metering. A utility company’s guarantee to purchase excess energy from residential customers (created by solar panels or wind turbines), either as a credit or cash.
passive solar. Solar energy that’s used indirectly to heat massive objects, which store the thermal energy and re-radiates that heat later for space heating.
passive solar design. Using architectural elements, geometry, and building orientation to optimize heating and daylighting without artificial energy input.
payback. The cost of a project divided by the energy saved per year, yielding a result in years, the length of time for the energy savings from the project to pay off the initial cost of the project.
peak demand. The maximum energy used by a utility customer during any given 15-minute period of the day.
peak load. HVAC systems are sized to meet the maximum heating (cooling) load on the hottest (coldest) day of the year.
peak load shifting. The practice of shifting the electricity use away from peak hours (4-6 P.M.) into a time of day when the rates are lower.
phantom loads. Electrical devices that continue to draw power even when they’re turned off.
radiation. The transfer of thermal energy without the necessity of a medium (i.e., air).
radiant barrier. A layer of metal foil placed in the roof or attic to block the passage of high-energy UV radiation from the sun; or as a facing on fiberglass batts to reflect heat back into the house.
radiant floor. Flooring in which hot water is circulated to heat a space, operating by the same principle as a radiator.
reflectance. The ability of a material to reflect radiant heat energy.
refrigerant. A substance which exhibits a refrigerating effect while expanding or vaporizing, used to transfer thermal energy to and from a “heat sink.”
resistance losses. Electrical flow through conductors is not perfect: some energy is lost, and that lost energy is manifested as heat.
return air. Air recirculated back to the central air handler.
rooftop sprinkler. A system that periodically emits a fine spray of water on the roof. The water evaporates, extracting the heat of vaporization from the roof assembly, lowering the summertime loads on the air conditioning.
R-value. A measure of the insulating quality of a material or assembly.
sensible heat. The quantity of thermal energy needed to raise the temperature of a liquid without a change in state.
shrink house wrap. A plastic vapor barrier installed as part of the wall assembly for the entire building envelope to inhibit the migration of moisture into the enclosed, conditioned space.
solar gain. Energy from the sun captured via windows and glass.
standby losses. Thermal energy lost from a water heater tank through the shell because the water temperature inside is higher than the outside temperature.
strip heat. Thermal energy from a pure electrical source, i.e., a bare wire glowing red by virtue of resistance to electrical current flowing through the wire.
task light. A small light, such as a desk lamp, used to deliver a higher level of illumination than is available via area fixtures.
thermal bridge. A gap in the insulation with a lower R-value (higher conductivity to heat) to allow heat to flow across the building element (e.g., wall) more readily. A two-by-four wood stud is a thermal bridge in a wall assembly otherwise filled with fiberglass batt insulation( in the cavities between studs).
thermal bypass. A type of thermal bridge resulting from the convective air movement within insulation.
thermal interlocking. In houses with a narrow crawl space, the temperature of the bottom of the house closely tracks the ground temperature.
thermal mass. The ability of a material to absorb and store thermal energy, to later be released when the inside ambient temperature drops and the direction of heat flow reverses.
translucent insulation. Semi-transparent or opaque insulation that passes up to 50 percent of light.
stack effect. Wind moving against a structure simulates a positive pressure on one side and a negative pressure on the opposite side, with increased infiltration and exfiltration, respectively. This effect is more pronounced with higher floors.
standby losses. Thermal energy lost from a water heater and associated hot-water piping when water isn’t actually flowing. Also called standing losses.
storm door. A second, glass outer door to an existing doorway, to add insulation and reduce infiltration, usually in the wintertime.
storm window. A layer of glass framed to the outside of a window, increasing its insulation value and also reducing infiltration, both of which reduce HVAC loads.
sun room. A ground-floor room extension with a high glazing area but a normal roof. Also called a conservatory.
supplemental heating. Alternate-energy technologies (solar, wind) require a second, more dependable backup source (electric, gas) to guarantee service at all times, upon demand.
swamp cooler. An air-conditioning device that uses the evaporative cooling of water to lower the temperature of the air.
tankless water heater. A water-heating method that doesn’t have a storage tank but heats water on demand.
task lighting. Lighting in an area where the actual work is being done (e.g., a desk lamp).
tempered air. Air that’s partially cooled or heated, though not to the extent of air supplied to a conditioned zone.
thermal envelope. The architectural elements that enclose conditioned space and within which constant environmental conditions (i.e., temperature and humidity) are maintained.
thermal mass. Heavier materials that store heat and release it as the ambient temperature varies. Appropriate use of thermal mass can enhance the performance of a building HVAC system.
transmittance. The percent of visible light transmitted through a material.
two-speed motor. A motor that can run at two speeds, allowing the motor to be matched more closely to the load, thereby saving energy.
ultraviolet radiation. Light with a wavelength shorter than visible light.
unconditioned space. An area inside a building that is not heated or cooled.
utilization factor. A measure of the efficiency of a heat pump. Same as the coefficient of performance.
vapor barrier. A physical barrier (e.g., a vapor closed material impermeable to the passage of water vapor) installed as part of the outside wall assembly of all buildings to mitigate the flow of moisture into the conditioned space.
vapor open. A material that is breathable, allowing movement of moisture.
vapor pressure. The pressure exerted by water vapor. When the vapor pressure is higher on one side of a wall, moisture migrates through that wall until the moisture content of the air is the same on both sides of the wall (or other barrier).
vaporize. To change from a liquid to a gas.
variable speed motor. A motor that can operate at a continuum of speeds, allowing the motor to be matched exactly to the load, saving a lot of energy.
ventilation. In this book, this refers to mechanical ventilation by a fan.
wall assembly. The composite components of (typically an external) wall, including the framing, insulation, exterior cladding, vapor barrier, and the interior finish.
water heater blanket. An insulating wrap placed around a water heater to insulate the tank and reduce standby losses, saving energy.
Water Source Heat Pump. A thermal “heat pump” device (the same as for a GSHP) which can be used to transfer heat between two water sources, for example, to and from a well or lake instead of the ground loop in a GSHP.
WSHP. Water Source Heat Pump.
work surface. The horizontal plane at which the light level is measured, e.g., the surface of a table, desk, or countertop.
whole house fan. A system to cool a house by exhausting a large volume of warm indoor air, replacing it with cool outside air.
zone. A room or rooms separated by an air barrier (e.g., wall or partition) from the rest of the building.