The HVAC Terms a Technician Will Use, Translated
A plain-language guide to the words that show up on invoices and in diagnoses, so you can follow the conversation.
Common HVAC terms include tonnage (cooling capacity, where one ton equals 12,000 BTU per hour), SEER2 (seasonal cooling efficiency), static pressure (resistance in the duct system), superheat and subcooling (measurements verifying refrigerant charge), and temperature split (the difference between return and supply air, normally 16 to 22 degrees).
Capacity and sizing
- Ton. A unit of cooling capacity. One ton equals 12,000 BTU per hour. It has nothing to do with weight — it originates from the cooling effect of melting a ton of ice in a day.
- BTU. British Thermal Unit, a measure of heat energy. Equipment capacity is expressed in BTU per hour.
- Manual J. The industry-standard residential load calculation that determines what capacity a house actually needs.
- Design temperature. The outdoor temperature a system is sized to handle — typically 108 to 110°F in the Valley.
Efficiency ratings
- SEER2. Seasonal Energy Efficiency Ratio 2 — cooling output per unit of electricity over a season, tested at realistic duct pressure. Replaced SEER in 2023; numbers run about 4.5–5 percent lower for identical equipment.
- EER2. Efficiency at a single high-temperature operating point rather than across a season. Arguably more relevant in Phoenix than SEER2.
- HSPF2. The heating equivalent for heat pumps.
- AFUE. Annual Fuel Utilization Efficiency — the percentage of fuel a furnace converts to usable heat. An 80 percent furnace sends 20 percent up the flue.
The components
| Term | What it is |
|---|---|
| Condenser | The outdoor unit. Rejects heat from the refrigerant to outside air. |
| Evaporator coil | The indoor coil. Absorbs heat from indoor air; where condensation forms. |
| Compressor | The pump at the heart of the refrigeration cycle. The most expensive component. |
| Air handler | The indoor unit containing the blower and coil, where there is no furnace. |
| Plenum | The box connecting the equipment to the ductwork. |
| Capacitor | Stores electrical charge to start and run motors. The most common failure point. |
| Contactor | The electrical switch that energizes the outdoor unit. |
| Reversing valve | On a heat pump, redirects refrigerant to switch between heating and cooling. |
| TXV / metering device | Controls refrigerant flow into the evaporator coil. |
| Float switch | Safety device that shuts the system off if the condensate pan fills. |
The measurements
- Superheat. How much the refrigerant vapor has warmed beyond its boiling point leaving the evaporator. Used to verify charge on systems with a fixed metering device.
- Subcooling. How much the liquid refrigerant has cooled below its condensing temperature. Used to verify charge on systems with a TXV.
- Temperature split (delta T). The difference between return air and supply air. Should be 16 to 22 degrees.
- Static pressure. Resistance the blower works against, measured in inches of water column. High static pressure means restricted ductwork.
- Amp draw. Current a motor is consuming, compared against the nameplate rating.
- Microfarads (µF). Capacitor capacitance. Measured against the rated value; more than about 10 percent low means replacement.
Terms that appear on quotes
- Changeout. Replacing existing equipment in the same location.
- Line set. The pair of copper refrigerant lines between indoor and outdoor units.
- Evacuation. Pulling a vacuum on the refrigerant circuit to remove air and moisture before charging. Measured in microns.
- Commissioning. Measuring and documenting performance after installation to verify the system works as designed.
- Matched system. An outdoor unit and indoor coil rated together as a pair. Mismatching costs efficiency and can void warranty.
- Whip and disconnect. The electrical connection and safety switch at the outdoor unit.
Refrigerants
R-22. The old standard, phased out for ozone depletion. Production ended in the United States in 2020. Systems using it have no economical future.
R-410A. The predominant residential refrigerant from the mid-2000s through the early 2020s. Being phased down for high global warming potential, but still available for servicing existing systems.
A2L refrigerants (R-454B, R-32). The current standard for new residential equipment. Lower global warming potential, mildly flammable classification, and new systems include leak detection.
Frequently asked
What does a "ton" mean in air conditioning?
A unit of cooling capacity equal to 12,000 BTU per hour. It has nothing to do with weight — the term comes from the cooling effect of melting a ton of ice over a day. A typical 2,000 square foot Phoenix home needs 3.5 to 4.5 tons.
What is superheat and subcooling?
Both are measurements technicians use to verify refrigerant charge. Superheat measures how far the vapor warmed past its boiling point leaving the evaporator; subcooling measures how far the liquid cooled below its condensing temperature. Which one applies depends on the metering device.
What is static pressure in an HVAC system?
The resistance the blower has to work against, measured in inches of water column. High static pressure means restricted ductwork, a dirty filter, or a blocked coil, and it reduces airflow while shortening blower motor life.