ACCA Residential HVAC Design key terms

The industry-standard method for calculating how much heat a home gains in summer and loses in winter, based on construction details, insulation, windows, and local weather data. Its result sizes the equipment.

The procedure for selecting specific equipment that meets the load calculated by Manual J, confirming the chosen model's capacity and sensible-to-total ratio match the home's needs at design conditions.

The method for designing a duct system that delivers the required airflow through each room, sizing trunks, branches, and fittings so that external static pressure stays within the blower's capability.

The calculated heating or cooling capacity a home needs on the coldest and hottest days expected for its location, expressed in Btu per hour and used to size equipment.

The specific outdoor and indoor temperature and humidity values chosen as the worst-case basis for load calculations, typically taken from published weather tables for the local area.

The fraction of total cooling capacity that removes sensible heat (temperature) rather than latent heat (moisture). A mismatch with the home's ratio causes poor humidity control or short cycling.

Heat energy absorbed or released when moisture changes phase, such as water evaporating or condensing. In cooling, latent capacity is what dehumidifies the air.

The total pressure resistance of a duct run expressed in feet, combining straight duct with the added resistance of fittings, transitions, and grilles converted to an equivalent straight-run distance.

The pressure drop per unit length of duct, usually inches of water per 100 feet, used in duct design to size each section so total system resistance stays acceptable.

The total resistance the air handler's blower must overcome from the duct system, grilles, and components outside the unit itself, measured in inches of water column.

The volume of air moved through the system per unit time, usually stated in cubic feet per minute (cfm). Proper airflow is required for capacity, efficiency, and safe equipment operation.

The outdoor temperature at which a heat pump's heating capacity exactly equals the home's heat loss, so auxiliary heat begins to be needed below that temperature.

Supplemental heating, often electric resistance strips, that operates when a heat pump cannot meet the load alone, such as during very cold weather or defrost cycles.

A refrigeration system that can reverse its refrigerant flow to provide both heating and cooling by moving heat between indoor and outdoor air rather than generating it by combustion.

A periodic operating mode in which a heat pump temporarily reverses or stops heating to melt frost from the outdoor coil, during which it may activate auxiliary heat to keep the space warm.

The documented process of verifying that a newly installed system performs as designed, including measuring airflow, refrigerant charge, temperature split, and static pressure before handover.

An installation performed to recognized industry standards, covering proper sizing, sealed and tested ducts, correct charge, verified airflow, and documented performance measurements.

A field measurement of pressures within the air distribution system, used to confirm the blower can deliver design airflow and to find restrictions or excessive duct resistance.

A procedure that pressurizes or depressurizes the duct system and measures how much air escapes, expressed as a percentage of system airflow, to verify sealing quality.

The difference between return-air and supply-air temperatures across an operating system, used as a quick check that airflow and refrigerant charge are in a normal range.

The number of degrees a refrigerant vapor is above its saturation temperature at a given pressure. It is the primary charging indicator for fixed-orifice and piston-metering systems.

The number of degrees a liquid refrigerant is below its saturation temperature at a given pressure. It is the primary charging indicator for systems using a thermostatic expansion valve.

A thermostatic expansion valve that meters refrigerant into the evaporator while holding a target superheat, adjusting flow as load changes. Systems with a TXV are typically charged by subcooling.

The quantity of refrigerant contained in a system. Correct charge is essential for capacity, efficiency, and compressor life and is verified by superheat or subcooling measurement.

The process of removing refrigerant from a system into a separate container for reuse, recycling, or disposal, required by law before opening a circuit or scrapping equipment.

Cleaning recovered refrigerant on site to remove oil, moisture, and contaminants so it can be returned to the same or another system, distinct from reclaiming to specification.

Evacuation of a refrigerant circuit to a deep low pressure to remove air, moisture, and non-condensables before charging, verified with a micron gauge and standing vacuum test.

An instrument that measures very low absolute pressure during evacuation, used to confirm the system has been pulled down to the required vacuum level and holds it.

A federally required credential for technicians who service, maintain, or dispose of equipment containing regulated refrigerants, obtained by passing an EPA-approved certification exam.

An ASHRAE classification (such as A1, A2L, or B1) describing a refrigerant's toxicity and flammability, which determines handling, storage, and installation requirements.

An official authorization from the local building authority to perform specified work, required before installation or major alteration and tied to inspection milestones.

A review by the authority having jurisdiction, or a qualified third party, confirming that installed work matches approved plans and applicable codes before it is covered or placed in service.

The Authority Having Jurisdiction: the organization or official responsible for enforcing code requirements and approving permits and inspections for a given project.

Meeting all applicable building, mechanical, energy, and refrigerant regulations for a project, including adopted editions of model codes and any local amendments.

A written, signed modification to the original contract that documents added or removed work and its effect on price and schedule, protecting both contractor and customer.

A legal claim against a property for unpaid work or materials, which a contractor or supplier may file if the customer fails to pay, potentially affecting the owner's title.

A computer program that performs Manual J procedures using entered building data, producing room-by-room loads and reports required for permitting and equipment selection.