ID HVAC key terms

Rate of heat removal equal to 12,000 British thermal units per hour, roughly the cooling needed to melt one ton of ice in 24 hours. Equipment capacity in North America is commonly rated in tons.

Heat that changes the temperature of a substance without changing its phase. In air conditioning, it is the portion of the load tied to dry-bulb temperature difference between room air and supply air.

Heat absorbed or released when a substance changes phase, such as water evaporating or vapor condensing, with no temperature change. In comfort cooling it is mostly the energy used to condense moisture removed from air.

The fraction of total coil load that is sensible heat, calculated as sensible load divided by total load. A low ratio means a large share of capacity goes to moisture removal rather than temperature drop.

The temperature of air measured by an ordinary thermometer, unaffected by moisture content. It is the value most people mean when they say room temperature and is plotted on the horizontal axis of a psychrometric chart.

The lowest temperature air can reach by evaporating water into it, measured with a wetted sensor in moving air. It indicates the evaporative cooling potential and is used for evaporative coolers and condenser performance.

The temperature to which air must be cooled, at constant moisture content, for water vapor to begin condensing. A surface colder than the dew point will sweat; cooling coils must run below it to remove moisture.

The amount of water vapor in air expressed as a percentage of the maximum the air could hold at that temperature. It changes as air warms or cools even when the actual moisture content stays the same.

The mass of water vapor carried in a given mass of dry air, usually stated in grains per pound or kilograms per kilogram. Unlike relative humidity, it does not change when air is simply heated or cooled.

A graph of moist air properties that lets you find dry-bulb, wet-bulb, dew point, humidity ratio, and enthalpy from any two known values. It is the standard tool for analyzing coil and air-mixing processes.

The total heat content of moist air per unit mass of dry air, combining sensible heat and the latent heat of its moisture. Coil capacity can be found from the enthalpy difference between entering and leaving air.

The effective surface temperature of a cooling coil, found where the straight line between entering and leaving air conditions crosses the saturation curve. It shows how cold the coil must be to reach a target supply condition.

The fraction of air passing through a coil that does not contact the coil surface and leaves at essentially entering conditions. Lower values mean better coil performance and a supply temperature closer to the coil surface.

A repeating process in which refrigerant absorbs heat at low pressure and temperature, is compressed, rejects heat at high pressure, then expands back to low pressure. It moves heat from a cooled space to outdoor air.

The pump of a refrigeration system that raises refrigerant vapor from low to high pressure, adding energy so the vapor can reject heat to outdoor air. Common types include reciprocating, scroll, screw, and centrifugal.

The indoor coil where low-pressure liquid refrigerant boils and absorbs heat from air passing over the fins. Its surface runs colder than the air dew point, so condensate forms and drains away.

The coil that rejects heat from hot refrigerant vapor to outdoor air or water, causing the refrigerant to condense into a liquid. Air-cooled and water-cooled designs are the two common arrangements.

A metering device that drops refrigerant pressure from condensing to evaporating level and regulates flow to match load. Thermostatic and electronic types control flow by maintaining a set superheat at the evaporator outlet.

The temperature of refrigerant vapor above its saturation temperature at the same pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant to protect the compressor.

The amount by which liquid refrigerant is cooled below its condensing temperature at the same pressure. Measured at the condenser outlet, it shows how much liquid is stacked in the condenser and helps confirm charge.

A finned heat exchanger that removes heat and moisture from air passing across it. Its capacity depends on surface area, refrigerant or chilled-water temperature, and airflow, and it must drain condensate reliably.

The volume of air moved per unit time, usually cubic feet per minute or liters per second. Too little airflow causes coil icing and high temperature rise; too much reduces dehumidification and temperature drop.

The outward push of air against duct walls, measured in inches of water column. External static pressure indicates the resistance a blower works against and is used to check duct sizing and filter loading.

The layout and sizing of air distribution passages to deliver required airflow with acceptable pressure loss, noise, and space use. Equal-friction and static-regain methods are common approaches to sizing trunk and branch ducts.

An outlet that spreads supply air into a room with controlled velocity and direction, promoting mixing before it reaches occupants. Ceiling, slot, and floor types are chosen to suit throw, drop, and noise limits.

An intake opening that draws room air back to the air handler, sized to keep face velocity and noise low. Poor return location can cause short-circuiting, where supply air returns before mixing with room air.

The number of times the full volume of a space is replaced with supply or outdoor air in one hour. It is used for ventilation checks, dilution of contaminants, and required exhaust in specialized rooms.

The quantity of outdoor air supplied to a space per unit time or per person, set to control odors, carbon dioxide, and other contaminants. Codes give minimum rates that vary by occupancy type.

The louvered opening and damper through which fresh air enters a building's air system. Its location must avoid exhaust fumes, loading docks, and snow, and the damper must close tightly when not needed.

A control strategy that uses cool outdoor air instead of mechanical cooling when outdoor conditions are favorable. Air-side economizers use damper modulation; water-side types use a fluid cooler or tower.

A movable blade assembly inside ductwork or an opening that varies or stops airflow. Manual dampers balance branches; motorized dampers respond to controls for economizer, smoke, or zone functions.

A system that holds supply temperature roughly constant while varying fan and box airflow to match zone loads. It saves fan energy but needs careful minimum airflow settings to maintain ventilation and avoid stratification.

A graph showing how a fan's airflow and pressure output relate, along with power and efficiency at each point. The operating point is where the fan curve intersects the system resistance curve.

A fuel-fired or electric appliance that heats air for distribution through ducts. Gas units are rated by input and output capacity, and steady-state efficiency reflects how much heat reaches the air.

In a furnace, the metal passages that separate hot combustion gases from the air being heated. Cracks or holes allow carbon monoxide into supply air, so inspection and combustion testing are essential.

The air supplied to a burner to support complete burning of fuel. Inadequate supply causes incomplete combustion, soot, and carbon monoxide, and codes require a permanent source for confined spaces.

A colorless, odorless gas produced by incomplete combustion that binds to hemoglobin and can be fatal at low concentrations. It is the main safety reason for venting checks and combustion analysis on fuel-burning equipment.

The pressure difference that moves combustion gases up a flue or chimney and out of the building. Natural-draft appliances rely on buoyancy, while induced-draft units use a fan to maintain safe venting.

Annual fuel utilization efficiency, the percentage of fuel energy delivered as heat over a heating season, accounting for cycling and standby losses. Standard furnaces are around 80 percent; condensing models exceed 90 percent.

The calculated heat loss a building experiences on a design winter day, including transmission through the envelope and infiltration. It sets equipment capacity and is paired with the cooling load for sizing.

The calculated rate of heat gain a building must remove on a design summer day, including solar, conduction, occupants, and equipment. It determines capacity and airflow, and differs from the connected equipment capacity.

A measure of how far the average daily temperature falls below or rises above a base, summed over a season. Heating and cooling degree days estimate seasonal energy use and compare climates.

A fluid chosen to evaporate and condense at useful pressures and temperatures inside a refrigeration system. Modern selections favor low global-warming-potential blends, which require careful charging and handling.

The removal of water vapor from air, usually by cooling it below its dew point on a coil or by desiccant absorption. Proper dehumidification requires adequate run time and airflow low enough for the coil to condense moisture.

A widely used residential method for calculating heating and cooling loads from envelope, orientation, and occupancy data. Its output sizes equipment, while separate methods size ducts and select equipment.