BPI AC/Heat Pump key terms

The amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit. It is the basic unit used to rate heating and cooling capacity in North America.

A cooling capacity equal to 12,000 Btu per hour, roughly the heat needed to melt one ton of ice in a day. Equipment capacity and load estimates are often expressed in tons.

Heat that changes the temperature of a substance without changing its state. A thermometer measures it directly, and it accounts for most of the cooling load in a typical home.

Heat absorbed or released when a substance changes state, such as water evaporating or refrigerant boiling, with no temperature change. In comfort cooling it is mainly the energy removed while dehumidifying air.

A condition in which a vapor is at its dew point and any further cooling causes condensation, or a liquid is at its boiling point. Refrigerant tables use it as the reference for pressure-temperature relationships.

The number of degrees a vapor is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms that only vapor, not liquid, is returning to the compressor.

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

The repeating process in which refrigerant absorbs heat at low pressure and rejects it at high pressure. It moves through compression, condensation, expansion, and evaporation, transferring heat from indoors to outdoors.

The pump that raises refrigerant pressure and temperature so heat absorbed indoors can be rejected outdoors. Common types include reciprocating, scroll, and rotary, and many are now inverter-driven variable-speed units.

The indoor heat exchanger where low-pressure liquid refrigerant boils and absorbs heat from passing air. Its surface temperature is below the air dew point, so condensate forms and drains away.

The heat exchanger where high-pressure refrigerant vapor condenses and releases heat to outdoor air or, in winter, absorbs heat from it. Keeping it clean directly affects capacity and efficiency.

A component that drops refrigerant pressure between the condenser and evaporator while regulating flow. Common types are the fixed orifice, thermostatic expansion valve, and electronic expansion valve.

A modulating metering device that adjusts refrigerant flow to hold a target superheat at the evaporator outlet. It responds to bulb temperature and evaporator pressure, improving part-load efficiency.

A four-way valve that swaps the roles of the indoor and outdoor coils so a heat pump can heat or cool. It is switched by a solenoid pilot and is a common no-heat diagnostic suspect.

A system that moves heat rather than generating it, providing both heating and cooling by reversing refrigerant flow. Its heating efficiency is expressed as COP or HSPF rather than steady-state efficiency.

The ratio of heat delivered to energy consumed at a specific operating condition. A COP of 3 means three units of heat are moved per unit of electricity, which is why heat pumps outperform resistance heat.

The current seasonal efficiency rating for central air conditioners, measured under updated test conditions with higher external static pressure. Minimum allowed ratings depend on region and equipment size.

A seasonal efficiency rating for heat pumps in heating mode, dividing total seasonal heat output by total electrical input. HSPF2 is the updated version tested under the newer procedure.

Cooling capacity in Btu per hour divided by power input in watts at a single rated condition. Unlike seasonal ratings, it reflects performance at a specific outdoor temperature and is useful for hot-climate comparisons.

An industry method for estimating how much heat a building gains and loses under design conditions. It accounts for envelope, windows, infiltration, and internal gains, and it is the basis for correct sizing.

The process of matching equipment capacity to the calculated load using manufacturer performance data. It verifies the unit can meet both sensible and latent loads at design conditions, not just nominal tonnage.

A method for sizing ducts and selecting fittings so the air distribution system delivers required airflow within available static pressure. It uses equivalent length and friction rate to size each run.

The outdoor and indoor temperature and humidity values a system is sized to handle, such as a summer outdoor design temperature. They define the worst-case load the equipment must meet.

The outdoor temperature at which a heat pump's heating capacity exactly equals the building's heat loss. Below it, supplemental heat must make up the difference to maintain indoor temperature.

Supplemental heating, usually electric resistance strips, that operates when a heat pump cannot meet load alone. It is staged by outdoor temperature or time and is a major driver of winter energy cost.

A mode in which the compressor is locked out and only supplemental heat runs, used when the heat pump has failed. It is expensive to operate and intended only until repairs are made.

Guidance for selecting supply and return air outlets and locating them for proper throw, spread, and mixing. Correct selection prevents drafts and stratification while supporting the required room airflow.

Air drawn from conditioned spaces back to the air handler for reconditioning. Undersized or restricted returns raise static pressure, reduce airflow, and can cause coil icing or high superheat.

The resistance to airflow measured across the air handler, expressed in inches of water column. It combines duct, filter, and coil resistance and must be compared with the blower's rated performance.

The volume of air moved per minute, usually targeted at about 400 cubic feet per minute per ton of cooling. Too little airflow causes coil freezing; too much reduces dehumidification and sensible capacity.

The documented process of verifying that a newly installed system is charged, adjusted, and performing as designed. It includes airflow verification, refrigerant charge confirmation, and control checks before handover.

Determining refrigerant charge by measuring the exact weight added, typically against the nameplate charge plus an allowance for line-set length. It is the most reliable method for new installations.

A timed or demand-initiated sequence in heating mode that melts frost from the outdoor coil by temporarily reversing the cycle. It is triggered by coil temperature and pressure sensors.

Equipment that varies compressor speed electronically to match load instead of cycling on and off. It improves part-load efficiency and humidity control but requires specific diagnostic procedures.

The difference between return-air and supply-air temperatures across the indoor coil, often expected near 18 to 22 degrees Fahrenheit in cooling. It is a quick indicator of airflow and charge problems.

The process of finding charge loss using electronic detectors, pressure testing with dry nitrogen, or fluorescent dye. It precedes any repair and recharge, since adding refrigerant without repair only delays failure.

The ordinary air temperature measured by a standard thermometer, unaffected by moisture content. It is paired with wet-bulb or humidity readings to assess comfort and coil performance.

The percentage of moisture the air holds compared with the maximum it could hold at that temperature. Comfort and mold risk depend on indoor levels, typically targeted between 30 and 50 percent.

A written record of readings and adjustments taken when a system is placed in service, including voltages, pressures, temperatures, and airflow. It documents performance and supports future warranty claims.

A safety procedure that isolates electrical or other hazardous energy and applies a personal lock and tag before work begins. It prevents accidental energizing of equipment while a technician is servicing it.

Safety gear such as gloves, eye protection, and insulated tools worn for specific tasks. For refrigerant and electrical work it also includes frostbite protection and appropriate electrical-rated gloves.

Removing refrigerant from a system into a recovery cylinder rather than venting it to the atmosphere. It is legally required in most jurisdictions and protects the environment and the technician.