HVAC-R Efficiency Glossary

Key takeaways

• Readings such as superheat, subcooling and approach temperature tell you how a system is performing now.
• Ratings such as kW/ton, EER, IEER and IPLV describe equipment under test conditions; compare your readings against them.
• Bill and rule terms (demand charge, 4CP, leak rate, M&V) decide what a fix is worth and how it is judged.
• ASHRAE's free Terminology database is the reference for formal engineering definitions.

This glossary defines the terms that come up when facility managers, energy managers and HVAC technicians talk about efficiency: the readings a technician takes, the ratings on a spec sheet, the line items on a utility bill and the rules that govern refrigerants. Definitions are written for a non-engineer and kept short. Where one of our guides explains a term in depth, the definition links to it.

For formal engineering definitions, ASHRAE maintains a free online Terminology database with more than 3,700 entries, and EPA, DOE and EIA publish their own definitions for the programs they run. Where a definition below includes a number or a rule, it comes from the sources listed at the end of the page.

A–C

• A2L refrigerant — A refrigerant in ASHRAE Standard 34 safety group A2L: class A means lower toxicity, and class 2L means lower flammability with a low burning velocity. Many newer low-GWP refrigerants, including some HFOs and blends, are A2L. See the HFC phasedown guide.
• AHRI — The Air-Conditioning, Heating, and Refrigeration Institute, the trade association whose test standards (for example AHRI 550/590 for chillers) and certification programs underpin published equipment ratings. See laboratory testing standards.
• Air handling unit (AHU) — The fan, coil and filter assembly that conditions and moves air through a building's ducts. Chilled water or refrigerant passes through its coil.
• Approach temperature — The difference between the temperature of the fluid leaving a heat exchanger and the refrigerant temperature inside it. On a chiller evaporator, it is leaving chilled water temperature minus saturated suction temperature. A rising approach usually means something is in the way of heat transfer: scale, oil, air or low flow. See diagnosing efficiency loss.
• ASHRAE — The American Society of Heating, Refrigerating and Air-Conditioning Engineers, which publishes many of the standards cited in this glossary. ASHRAE's free online Terminology database lists more than 3,700 terms.
• ASHRAE/ACCA Standard 180 — The standard practice for inspection and maintenance of commercial HVAC systems, which sets out maintenance tasks and frequencies by component. See commercial HVAC maintenance for efficiency.
• Baseline — The period of measured energy use, before a change is made, against which savings are calculated. A good baseline covers a full range of operating conditions and is adjusted for weather and occupancy. See baselines and weather normalization.
• Btu (British thermal unit) — The amount of heat needed to raise one pound of water by 1°F. One kilowatt-hour equals 3,412 Btu.
• Building automation system (BAS) — The control system that schedules equipment, holds setpoints and logs trends. Also called a building management system (BMS) or energy management system.
• Capacity — How much heat a system can remove, usually in Btu per hour or tons. Capacity falls when coils are dirty, charge is wrong or airflow is low, so the unit runs longer to meet the same load. See signs your HVAC is losing capacity.
• Charge (refrigerant charge) — The amount of refrigerant in a system. Both undercharge and overcharge reduce efficiency; charge is judged from superheat or subcooling against the manufacturer's chart, not from pressures alone.
• Chiller — A machine that cools water (or a water-glycol mix) that is then pumped to coils around a building. Chillers reject heat through a cooling tower (water-cooled) or fans and coils (air-cooled). See water-cooled chillers.
• Coefficient of performance (COP) — Cooling (or heating) output divided by energy input, in the same units. A COP of 4 means four units of heat moved for each unit of energy used. Higher is better. See kW/ton, EER and COP explained.
• Coincident peak (CP) — A customer's demand during the interval when the whole grid peaks. In ERCOT, four summer coincident peaks (4CP) set transmission charges for many larger customers.
• Commissioning and retro-commissioning — Commissioning verifies that a new system works as designed; retro-commissioning applies the same process to an existing building to find and fix controls and operating problems. See the retro-commissioning comparison.
• Compressor — The pump of a refrigeration circuit. It raises refrigerant pressure and temperature so heat can be rejected outdoors. Most compressors rely on oil for lubrication. See compressor wear and lubrication.
• Condenser — The heat exchanger where refrigerant gives up heat and condenses to liquid, to outdoor air or to cooling tower water.
• Cooling degree days (CDD) — A measure of how hot a period was, and so how much cooling a building needed. EIA uses a base temperature of 65°F: a day with a mean temperature of 80°F has 15 CDD. See hotter summers and cooling load.
• CRAC and CRAH units — Computer room air conditioners (with their own compressors) and computer room air handlers (using chilled water) that cool data center spaces. See data center cooling.
• Crankcase heater — A small heater on a compressor that keeps refrigerant from condensing into the oil during off cycles, which would dilute the oil and cause foaming at startup.

D–F

• Delta T — The temperature difference across a coil or heat exchanger, for example return air minus supply air, or entering minus leaving chilled water. A shrinking delta T at the same flow means less heat is being moved.
• Demand charge — The part of a commercial electricity bill based on the highest rate of use (kW) in a billing period rather than total energy (kWh). Cooling on hot afternoons often sets it. See Texas demand charges and 4CP.
• Direct expansion (DX) — A system in which refrigerant evaporates directly in the air coil, as in rooftop units and split systems, rather than cooling water first.
• Discharge (head) pressure — The refrigerant pressure leaving the compressor. High head pressure means the compressor is working against more lift, often because the condenser is dirty, overcharged or short of airflow.
• Economizer — Dampers and controls that use cool outdoor air instead of mechanical cooling when conditions allow. Field studies have found high failure rates. See rooftop units.
• EER and EER2 — Energy efficiency ratio: cooling output in Btu per hour divided by power input in watts, at one test condition. EER2 is the version under DOE's updated test procedure for residential central air conditioners, effective 2023; the 2 ratings are not directly comparable with the older ones. See kW/ton, EER and COP explained.
• eGRID — EPA's Emissions & Generation Resource Integrated Database, the standard source of grid emission rates (lb CO2e per MWh) by region, used to convert electricity savings into emissions. See Scope 2 emissions and HVAC.
• Energy use intensity (EUI) — A building's annual energy use divided by its gross floor area, usually in kBtu per square foot per year. ENERGY STAR uses source EUI as the basis for its 1–100 score. See ENERGY STAR Portfolio Manager.
• Evaporator — The heat exchanger where refrigerant absorbs heat and boils, cooling the air or water passing over it. Oil that settles here can insulate the inside of the tubes. See how HVAC coils transfer heat.
• Expansion valve (TXV or EEV) — The metering device that controls how much liquid refrigerant enters the evaporator. A thermostatic expansion valve (TXV) responds to superheat mechanically; an electronic expansion valve (EEV) does so under control.
• Fault detection and diagnostics (FDD) — Software that analyzes building automation data to flag equipment and control faults, such as stuck dampers or simultaneous heating and cooling.
• Fouling — Any deposit that reduces heat transfer: dirt on the air side of coils, scale or biofilm on the water side of chiller tubes, or an oil film on the refrigerant side. See does coil cleaning restore efficiency.
• Fouling factor — The extra thermal resistance allowed for deposits when a heat exchanger is rated or designed. A clean exchanger outperforms its fouled rating; a heavily fouled one falls short.
• 4CP — ERCOT's four coincident peak method: transmission costs are allocated using demand during the interval of the ERCOT system peak in each of June, July, August and September. See Texas demand charges and 4CP.

G–L

• Global warming potential (GWP) — How much heat a gas traps relative to carbon dioxide over a set period, usually 100 years. The AIM Act phasedown and Technology Transitions rules are built around GWP.
• HCFC-22 (R-22) — An older ozone-depleting refrigerant. EPA's phaseout schedule ended U.S. production and import of HCFC-22 in 2020; EPA notes that some HCFC uses continue, including servicing existing refrigeration and air-conditioning equipment. See HFC phasedown and existing equipment.
• Heat exchanger — Any device that moves heat between two fluids without mixing them: evaporators, condensers, coils and chiller tube bundles.
• Heat pump — A refrigeration system that can run in reverse to heat as well as cool. See heat pumps.
• HFC and HFO — Hydrofluorocarbons (such as R-410A and R-134a) are the refrigerants being phased down under the AIM Act. Hydrofluoroolefins (HFOs) are newer, much lower-GWP compounds, several of them A2L.
• IEER — Integrated energy efficiency ratio, a weighted part-load efficiency rating for commercial unitary air conditioners such as rooftop units.
• IPLV and NPLV — Integrated part-load value: a single number that weights a chiller's efficiency at 100%, 75%, 50% and 25% load under AHRI 550/590 rating conditions. NPLV (non-standard part-load value) is the same calculation at a project's actual design conditions. See kW/ton, EER and COP explained.
• IPMVP — The International Performance Measurement and Verification Protocol, published by the Efficiency Valuation Organization (EVO). It defines four options for verifying savings: A and B (retrofit isolation), C (whole facility) and D (calibrated simulation). See IPMVP options explained.
• kW and kWh — A kilowatt (kW) is a rate of electricity use; a kilowatt-hour (kWh) is energy used over time. Utilities charge for both on most commercial bills.
• kW per ton (kW/ton) — Electric input in kilowatts per ton of cooling delivered. It is the usual chiller efficiency metric; lower is better. See kW/ton, EER and COP explained.
• Latent and sensible heat — Sensible heat changes air temperature; latent heat is the moisture removed from air. A system with low capacity often fails at humidity before it fails at temperature.
• Leak rate — The share of a system's full refrigerant charge lost over a year. Federal rules set leak-rate thresholds that trigger required repairs for many appliances.
• Lift — The pressure (or temperature) difference the compressor must overcome between evaporating and condensing conditions. Anything that raises condensing temperature or lowers evaporating temperature increases lift and energy use.
• Lubricity — How well an oil reduces friction and wear between moving surfaces. Oil diluted by refrigerant or degraded by heat and moisture loses lubricity. See compressor wear and lubrication.

M–O

• Measurement and verification (M&V) — The process of measuring energy use before and after a change, adjusting for conditions such as weather, and calculating savings. See how HVAC energy savings are measured.
• MERV — Minimum efficiency reporting value, the ASHRAE Standard 52.2 rating of how well an air filter captures particles. Research on packaged units found better filtration sharply reduces the dust that reaches coils.
• Mineral oil — The traditional refrigeration lubricant used with older refrigerants such as R-22. It does not mix well with most HFCs, which use synthetic oils instead. See compatibility and safety.
• Non-baseload emission rate — An eGRID rate based on generators that typically ramp up and down with demand; often used to estimate the emissions avoided by reducing consumption.
• Oil circulation rate (OCR) — The share of the flow in the refrigerant circuit that is oil, by mass. Some oil always leaves the compressor; a high OCR means more oil reaching the coils. See how compressor oil migrates.
• Oil fouling — An insulating film of compressor oil on the inside surfaces of evaporator and condenser tubes, which reduces heat transfer and adds pressure drop. Coil cleaning does not reach it. See what is oil fouling.
• Oil logging — Oil accumulating in an evaporator or low point in the piping instead of returning to the compressor, which both starves the compressor and insulates the coil. See how compressor oil migrates.
• Oil return — The movement of circulating oil back to the compressor sump, which depends on refrigerant velocity, pipe sizing and slope. Long piping runs such as VRF systems make it harder. See VRF systems.

P–R

• Part load — Operation below full capacity, where most equipment spends most of its hours. Part-load ratings such as IPLV and IEER reflect this better than full-load numbers.
• Payback period — The project cost divided by annual savings, in years. Simple payback ignores the time value of money; NPV and IRR do not. See payback, ROI and NPV for HVAC.
• POE oil — Polyolester oil, the synthetic lubricant used with most HFC refrigerants. It absorbs moisture readily, so systems must be kept dry. See compatibility and safety.
• Portfolio Manager — The free ENERGY STAR tool for benchmarking building energy and water use and earning 1–100 scores. See ENERGY STAR Portfolio Manager.
• Power usage effectiveness (PUE) — Total data center energy divided by IT equipment energy. A PUE of 1.0 would mean no energy goes to cooling or other non-IT loads. See data centers.
• PVE oil — Polyvinyl ether oil, another synthetic lubricant used with HFC refrigerants, particularly in some VRF and ductless systems.
• Refrigerant — The working fluid that absorbs heat as it evaporates and releases it as it condenses. Refrigerants are designated by R-numbers under ASHRAE Standard 34.
• Refrigerant migration — Refrigerant moving to the coldest part of an idle system, often the compressor crankcase, where it can dilute oil. See how compressor oil migrates.
• Rooftop unit (RTU) — A packaged unit with compressor, coils and fans in one cabinet, the most common type of commercial cooling equipment. See rooftop units.

S

• Saturation temperature — The temperature at which a refrigerant boils or condenses at a given pressure. Technicians convert gauge pressures to saturation temperatures to calculate superheat, subcooling and approach.
• Scale — Mineral deposits from cooling tower or process water on chiller condenser tubes, which insulate the tubes and raise condensing temperature. Water treatment and tube brushing address it. See water-cooled chillers.
• Scope 2 emissions — Indirect greenhouse gas emissions from purchased electricity, steam, heat or cooling, as defined by the GHG Protocol. Electricity savings reduce Scope 2. See Scope 2 emissions and HVAC.
• SEER and SEER2 — Seasonal energy efficiency ratio, a seasonal rating for residential and small commercial air conditioners and heat pumps. SEER2 replaced SEER under DOE's 2023 standards and updated test procedure. See kW/ton, EER and COP explained.
• Slugging — Liquid refrigerant or oil entering the compressor, which can damage valves and bearings. See compressor wear and lubrication.
• Split system — A system with the condensing unit outdoors and the evaporator coil indoors, connected by refrigerant piping. See split systems.
• Static pressure — Resistance to airflow in ducts, filters and coils, measured in inches of water column. High external static pressure reduces airflow and raises fan power.
• Subcooling — How many degrees the liquid refrigerant leaving the condenser is below its condensing (saturation) temperature. Used to judge charge on systems with expansion valves. See diagnosing efficiency loss.
• Superheat — How many degrees the suction vapor is above its evaporating (saturation) temperature. High superheat suggests a starved evaporator; very low superheat risks liquid reaching the compressor. See diagnosing efficiency loss.
• Supply air temperature — The temperature of air leaving the cooling coil. Warmer supply air at full run time is a common early sign of lost capacity. See signs your HVAC is losing capacity.

T–Z

• Technology Transitions rule — EPA's AIM Act rule restricting the use of higher-GWP HFCs in new equipment by sector, with compliance dates that EPA revised in 2026. See HFC phasedown and existing equipment.
• Ton of refrigeration — A rate of cooling equal to 12,000 Btu per hour, originally the cooling from melting one ton of ice over a day.
• Utility incentive (custom or prescriptive) — A rebate from a utility program for an efficiency measure. Prescriptive incentives pay a set amount per item; custom incentives are calculated from verified savings. See custom utility rebates and M&V.
• Variable frequency drive (VFD) — An electronic motor controller that varies fan or pump speed to match load, cutting energy at part load. See the VFDs and controls comparison.
• Variable refrigerant flow (VRF) — A multi-zone DX system that varies refrigerant flow to many indoor units from one outdoor unit, with long piping runs. See VRF systems.
• Weather normalization — Adjusting energy data for differences in weather (usually degree days) so that periods can be compared fairly. See baselines and weather normalization.
• Wet-bulb temperature — The temperature read by a thermometer with a wetted wick, reflecting both heat and humidity. It limits how cold a cooling tower can make water.

How to use these terms with your service contractor

Three groups of terms do most of the work in a conversation about efficiency. The first is the readings: superheat, subcooling, approach temperature, delta T and head pressure. Ask for them on every service visit and keep them in a log, because a single reading means little and a trend means a lot. The second is the ratings: kW/ton, EER, IEER and IPLV describe equipment under test conditions, so compare your measured performance with them to see how far a unit has drifted. The third is the money and rules vocabulary: demand charges, 4CP, M&V, payback and leak rate, which shape whether a fix is worth paying for and how its results will be judged.

CryogenX4 is relevant to one of the losses defined here, oil fouling. The company describes its product as a one-time treatment that lifts oil film from internal heat-exchanger surfaces and returns it to the compressor. If your readings point that way, the 25-point efficiency self-check is a practical next step.

Frequently asked questions

What is the difference between approach temperature and delta T?

Delta T is the temperature change of the air or water passing through a coil. Approach is the gap between the leaving fluid and the refrigerant inside the coil. Delta T tells you how much heat is being moved; approach tells you how easily it is moving through the heat exchanger walls.

Is a lower kW/ton better?

Yes. kW/ton is energy in per unit of cooling out, so lower numbers mean more efficient operation. EER, COP and IPLV expressed as ratios work the other way: higher is better.

What does A2L mean on a refrigerant?

Under ASHRAE Standard 34, A means lower toxicity and 2L means lower flammability with a low burning velocity. Many newer low-GWP refrigerants carry this classification.

Where can I find official definitions?

ASHRAE's free online Terminology database covers engineering terms. EPA, DOE, EIA and EVO define the terms used in their own programs, such as eGRID, ENERGY STAR, CBECS and IPMVP.

Sources

1. ASHRAE Terminology — ASHRAE
2. Update on New Refrigerants Designations and Safety Classifications (fact sheet) — ASHRAE / UNEP OzonAction
3. Degree days explained — U.S. Energy Information Administration
4. British thermal units (Btu) — U.S. Energy Information Administration
5. Purchasing Energy-Efficient Water-Cooled Electric Chillers — U.S. DOE Federal Energy Management Program
6. 2023 Energy Conservation Standards for Central Air Conditioners: FAQ — U.S. Department of Energy
7. International Performance Measurement and Verification Protocol (IPMVP) — Efficiency Valuation Organization
8. What is energy use intensity (EUI)? — ENERGY STAR
9. 16 Tex. Admin. Code §25.192: Transmission Rates for Export from ERCOT — Legal Information Institute (Texas Administrative Code)
10. Phaseout of Class II Ozone-Depleting Substances — U.S. Environmental Protection Agency