IA HVAC key terms

The temperature of air measured by an ordinary thermometer, unaffected by moisture content or radiant effects. It is the value reported in weather data and used as one axis of the psychrometric chart.

The lowest temperature air can reach by evaporating water into it, read from a thermometer with a wetted wick in moving air. It indicates how much evaporative cooling is available and approximates the supply-air limit of an evaporative cooler.

The temperature at which air becomes saturated and water vapor begins to condense. Any surface colder than the dew point will collect moisture, which drives decisions about duct insulation, chilled-water piping, and coil drain pans.

The amount of water vapor in air expressed as a percentage of the maximum it could hold at that dry-bulb temperature. It changes when air is heated or cooled even if the actual moisture content stays constant.

The mass of water vapor contained in a given mass of dry air, typically expressed in grains per pound or kilograms per kilogram. Because it does not change with sensible heating or cooling, it is the preferred measure for tracking moisture in a system.

A graphical plot of moist-air properties such as dry-bulb, wet-bulb, dew point, humidity ratio, and enthalpy. Plotting entering and leaving air conditions on it shows whether a coil is cooling, dehumidifying, or humidifying.

The total heat content of moist air per unit mass, combining sensible heat and the latent heat of the water vapor it carries. Differences in enthalpy across a coil represent total cooling capacity.

Heat that changes the temperature of a substance without changing its phase. In air conditioning it is the portion of the load that raises or lowers dry-bulb temperature, separate from moisture-related latent heat.

Heat absorbed or released during a phase change, such as water evaporating or vapor condensing, with no change in temperature. In buildings it comes mainly from people, cooking, showers, and infiltration of humid outdoor air.

The fraction of total cooling load that is sensible, calculated as sensible heat divided by total heat. It determines the coil's required apparatus dew point and whether the system can hold design humidity.

The coil surface temperature at which the straight line between entering and leaving air conditions on the psychrometric chart intersects the saturation curve. It represents the effective average coil surface temperature during dehumidification.

The outdoor and indoor temperature and humidity values selected for sizing equipment, drawn from published climate data at a stated frequency of exceedance. Equipment sized to them may not meet load during rarer extremes.

A room-by-room estimate of the heat that must be removed or added to maintain indoor design conditions, accounting for conduction, solar gain, internal gains, and infiltration. It precedes equipment selection and duct sizing.

The total heating or cooling load of a whole building treated as one zone, used to size central equipment. It is typically smaller than the sum of individual zone peaks because zones peak at different times.

Unintentional leakage of outdoor air into a building through cracks, joints, and penetrations, driven by wind and pressure differences. It adds both sensible and latent load and is distinct from deliberate outdoor-air ventilation.

Outdoor air intentionally supplied to a space to dilute pollutants and meet occupancy requirements. It is a controlled, code-driven load, unlike uncontrolled infiltration, and must be conditioned before delivery.

The number of times the volume of air in a space is replaced in one hour, calculated as airflow in cubic feet per minute times 60 divided by room volume. It is used for ventilation checks and for sizing exhaust in special spaces.

Dividing a building into separately controlled areas that share or use independent equipment so each can hold its own setpoint. Zones are defined by similar load patterns, orientation, and occupancy rather than by floor area alone.

Deliberate oversizing added to a calculated load to cover uncertainty in assumptions or future changes. Excessive safety factors cause short cycling, poor dehumidification, and inefficient part-load operation.

A refrigeration machine that can move heat either direction between indoors and outdoors, providing both heating and cooling. In heating mode its capacity and efficiency fall as outdoor temperature drops.

The outdoor temperature at which a heat pump's heating capacity exactly matches the building's heat loss. Below it, supplementary or backup heat must make up the difference.

Supplementary heating, often electric resistance, staged on to supplement a heat pump when its capacity is insufficient or during defrost. It has a much lower efficiency than the compressor and should run only when needed.

A timed or demand-initiated reversal of a heat pump that melts frost from the outdoor coil. During defrost the system temporarily stops heating the space and may energize auxiliary heat.

Air supplied to a fuel-burning appliance for complete combustion. It must be provided from indoors or outdoors per code, and inadequate supply causes incomplete combustion, soot, and carbon monoxide production.

The pressure difference that moves combustion gases up a flue and out of a building. Natural-draft appliances rely on buoyancy and require proper vent height, while induced-draft appliances use a fan.

The repeating sequence of compression, condensation, expansion, and evaporation that moves heat from a cooler space to a warmer one. Each component changes refrigerant pressure and state to make heat flow opposite its natural direction.

The number of degrees a vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding enough refrigerant to protect the compressor from liquid.

The number of degrees a liquid is below its saturation temperature at a given pressure. Measured at the condenser outlet, it indicates how much liquid refrigerant is stacked in the condenser and confirms adequate charge.

A metering device that drops refrigerant pressure from condensing to evaporating level and regulates flow to maintain a set superheat. Thermostatic and electronic types respond to evaporator outlet conditions.

The ratio of useful heating or cooling delivered to the energy input, expressed in consistent units. Higher values mean greater efficiency; electric resistance heat has a COP of about 1.0 by definition.

Seasonal Energy Efficiency Ratio, the total cooling output over a typical season divided by total electrical energy input, in Btu per watt-hour. Higher ratings indicate more efficient equipment, and regional standards set minimums.

Heating Seasonal Performance Factor, the seasonal heating output of a heat pump divided by its seasonal electrical input in Btu per watt-hour. It accounts for defrost and cycling losses that steady-state ratings ignore.

The outward force exerted by air on duct walls, measured in inches of water column. A fan must overcome the total static pressure of the system, and high readings signal restricted filters, coils, or ducts.

The pressure associated with the speed of moving air, equal to total pressure minus static pressure. It is used with a pitot tube to measure duct velocity and to calculate airflow in a duct.

The resistance to airflow outside the air handler, including ducts, filters, coils, and terminal devices. Manufacturers publish a fan curve relating airflow to external static pressure so the installer can verify the delivered airflow.

A graph showing how a fan's airflow changes as system resistance varies, along with power and efficiency. The operating point is where the fan curve intersects the system resistance curve.

A variable air volume terminal that modulates airflow to a zone in response to a thermostat while the central fan holds a duct static pressure setpoint. It may include a reheat coil for heating or minimum airflow.

Adjusting dampers, valves, and fan speeds so each space receives its design airflow or water flow within tolerance. It is verified with measured readings and documented in a balancing report at commissioning.

A control device that senses space temperature and cycles or modulates heating and cooling to hold a setpoint. Deadband, anticipation, and sensor location all affect comfort and equipment cycling.

The target value a control loop tries to maintain, such as a supply-air temperature or duct static pressure. Setpoints are compared with sensor readings to generate the control signal.

A control strategy combining proportional, integral, and derivative actions to reduce error, eliminate offset, and damp response. It is common in modulating valves, dampers, and variable-speed drives.

A control sequence that uses cool outdoor air for free cooling when its enthalpy or temperature is suitable, reducing compressor or chiller operation. Dampers modulate outdoor and return air while relief air is exhausted.

A safety procedure requiring the energy source of equipment to be isolated, locked, and tagged before servicing. It prevents accidental startup or release of stored energy and is a legal requirement for maintenance work.

An instrument that measures voltage, current, and resistance. Technicians use it to verify power is off before work and to diagnose electrical faults in motors, contactors, and control circuits.

An electrically operated switch that connects and disconnects power to a compressor, fan motor, or heater in response to a control signal. Pitted or welded contacts are a common cause of equipment that will not start or stop.

A colorless, odorless gas produced by incomplete combustion. It is a serious health hazard from faulty heating equipment, so combustion analysis and CO detectors are required safety measures.

The documented process of verifying that systems are installed, function, and perform according to design intent and the owner's requirements. It includes functional testing, balancing, and training before handover.

A scripted test that operates equipment and controls through normal and failure modes to confirm they respond as designed. Results are recorded and any deficiencies are logged for correction.

Rapid, repeated starting and stopping of equipment, usually from oversizing, low load, or a control fault. It wastes energy, reduces dehumidification, and wears out compressors and motors.

Scheduled service such as filter changes, belt inspection, coil cleaning, and lubrication performed to prevent failures. It preserves efficiency and capacity and is documented in a maintenance log.

A capture device over cooking or process equipment that removes grease, moisture, and contaminants at the source. Makeup air must be supplied to replace exhausted air and maintain building pressure.

The ventilation standard that sets minimum outdoor-air rates and indoor air quality procedures for commercial buildings. Designers use its rates and calculations to determine required ventilation airflow.

A widely used residential load calculation method that estimates room-by-room heating and cooling loads from construction details and climate data. It is the accepted basis for sizing residential equipment.

A fluid that absorbs and releases heat as it changes phase in a refrigeration system. Its selection and handling are governed by safety and environmental regulations, including leak detection and recovery requirements.

Cooling achieved by evaporating water into air, lowering its dry-bulb temperature while raising its humidity. Its effectiveness depends on the difference between dry-bulb and wet-bulb temperatures.