GA HVAC key terms

An industry-standard method for calculating how much heating and cooling a home needs by accounting for its size, insulation, windows, air leakage, and local weather data. Contractors use the results to size new equipment correctly.

The companion procedure to Manual J that matches calculated room-by-room loads to specific equipment models. It checks that the chosen unit's capacity and airflow fall within the manufacturer's published range for the application.

The portion of total cooling capacity that removes heat without changing humidity, expressed as a decimal. A higher ratio means the system is doing less dehumidification relative to its total cooling.

The outdoor and indoor temperature and humidity values an engineer assumes when sizing equipment. They represent a severe but not record weather day and are usually taken from published climate tables for the local area.

A combination of indoor coil, outdoor unit, and air handler that a manufacturer has tested together and listed with a certified capacity and efficiency rating. Mixing unmatched components can void ratings and reduce performance.

The number of degrees a vapor refrigerant is above its saturation temperature at a given pressure. Measured at the evaporator outlet, it confirms the expansion valve is feeding the coil properly.

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

A valve that meters refrigerant into the evaporator by sensing suction-line temperature and pressure, then adjusting flow to hold a set superheat. It responds to load changes better than a fixed orifice.

The temperature difference between the bubble point and dew point of a zeotropic blend as it changes phase. Technicians must read pressure-temperature charts carefully because the temperature slips during evaporation and condensation.

Annual fuel utilization efficiency, a percentage that estimates how much of a fossil fuel's energy a furnace or boiler turns into usable heat over a typical heating season. Higher numbers mean less waste up the flue.

The air a gas appliance needs for complete burning plus dilution of flue gases. If it is inadequate or blocked, the appliance can produce carbon monoxide and spill flue products into the building.

The outdoor temperature at which a heat pump's capacity exactly equals the building's heat loss. Below it, supplemental heat must run to keep the space at the thermostat setpoint.

A timed or demand-initiated sequence that temporarily reverses a heat pump to melt frost off the outdoor coil. The system switches to cooling mode, warms the coil, and usually energizes backup heat to avoid a cold draft.

A flame-sensing method in which the control board sends an AC signal through the burner flame and detects a small DC current. A weak or missing microamp reading shuts the gas valve for safety.

The resistance to airflow measured across the blower, from return inlet to supply outlet, in inches of water column. It tells whether the duct system is restrictive and whether the blower is moving its rated airflow.

A procedure that pressurizes the duct system with a calibrated fan and measures how much air escapes at a specified pressure. Results show whether sealing work is needed for efficiency and comfort.

Minimum Efficiency Reporting Value, a number from 1 to 16 describing how well an air filter captures particles of different sizes. Higher ratings trap more particles but add static pressure to the blower.

A widely adopted standard that sets minimum whole-building and local exhaust ventilation rates for homes. It is used to size bath fans and fresh-air intakes so indoor pollutants are diluted.

A short drop leg of pipe installed ahead of a gas appliance's controls to catch dirt, scale, and moisture before they reach the valve. It must be installed with a capped bottom and the correct orientation.

Corrugated stainless steel tubing, a flexible gas piping material often used in homes. It must be bonded to the electrical grounding system and protected from physical damage per code and manufacturer instructions.

Flowing dry nitrogen through refrigerant lines while brazing so the inside stays oxygen-free. This prevents carbon and scale from forming, which would later clog the expansion valve and contaminate the system.

The current a motor draws at the instant it starts with the rotor held still, shown on the nameplate as LRA. Technicians compare measured inrush to this value to judge starting components and winding condition.

Electronically commutated motor, a brushless DC motor with built-in electronics that can hold programmed airflow and use far less electricity than a conventional PSC motor. It often needs a dedicated control signal to run.

The temperature range between the switch-on and switch-off points that keeps equipment from short cycling. A wider deadband means fewer cycles but larger temperature swings in the conditioned space.

A small step-down transformer that converts line voltage to the low voltage used by thermostats, gas valves, and control boards, typically 24 volts. A shorted control circuit or failed transformer leaves the system unresponsive.

The documented process of verifying that installed equipment and controls perform as designed, including checklists, functional testing, and recording final settings before the owner accepts the system.

Pulling a vacuum on refrigerant lines to remove air and moisture before charging. A standing micron gauge reading confirms the system is tight and dry enough to receive a proper charge.

Adding refrigerant by weight using a scale and the manufacturer's specified amount instead of relying on pressure gauges. It is required for systems with no superheat or subcooling target, such as units with fixed restrictions.

An adjustable plate or blade inside ductwork that is set to shift airflow between branches. Once adjusted during balancing, it is locked in place so the design air quantities stay stable.

A method of measuring airflow in a duct by taking velocity pressure readings at several points across the cross-section with a pitot tube and averaging them to find average velocity and volume.

The difference between return air temperature and supply air temperature across the equipment. Comparing the measured split to the manufacturer's expected range is a fast check of airflow and charge.

A calibrated instrument that fits over a supply or return grille and directly reads the volume of air passing through it. It is the standard tool for verifying and adjusting room air quantities.

A U-shaped pipe arrangement on a drain line that holds water to block air from being pulled in or pushed out by the blower. It must be correctly sized and primed for drainage to work.

A component wired into a motor circuit that improves running torque and keeps current in phase during operation. A failed capacitor causes hard starts, overheating, and low current draw.

The EPA rule governing refrigerant handling that requires technicians to be certified, to recover refrigerant instead of venting it, and to follow evacuation and recordkeeping requirements.

A safety procedure in which energy sources are shut off, isolated, and tagged so equipment cannot be re-energized while someone is servicing it. Only the person who applied the lock removes it.

An area with limited entry and exit that is not designed for continuous occupancy, such as a crawlspace or mechanical pit. Entry may require testing for oxygen, combustible gas, and carbon monoxide.

A DOT-approved container used to hold refrigerant removed from a system. It must be filled only to its recovery limit, never overfilled, and labeled with the refrigerant type it contains.