60+ residential HVAC and building science terms with concise definitions, formulas where applicable, and the standards body that defines each. Organized by category and citation-ready for use in research or quote extraction.
IECC climate zones are defined by Heating Degree Days and Cooling Degree Days per ASHRAE Standard 169-2021. Heating design temperature is the 99% winter outdoor temperature (the temperature exceeded by 99% of winter hours); cooling design temperature is the 1% summer outdoor temperature. Your county-level zone is on the IECC climate zone map at codes.iccsafe.org.
How to Use This Glossary
This glossary is organized by category — efficiency metrics, capacity terms, equipment types, building science, psychrometrics, refrigeration, standards bodies, and controls. Each entry includes a concise definition (one to two sentences), a formula or numeric value where one exists, and the standards body or document that defines the term authoritatively.
For longer explanations of any term, follow the link to the relevant article or hub. Most glossary terms appear in context in one of the equipment hubs (/heat-pump/, /ac/, /furnace/), the building science hub (/building-science/), or the ACCA-methodology hubs (/manual-j/, /manual-s/, /manual-d/, /manual-t/).
Many of the terms defined here are climate-sensitive — BTU per square foot, design temperature, balance point — and the numbers shift substantially across the eight IECC climate zones. The chart below shows cooling BTU per square foot as a worked example of how a single sizing metric varies across climates.
Residential HVAC efficiency metrics with formulas and source standards
Term
Definition
Formula / value
Source
SEER
Seasonal Energy Efficiency Ratio — legacy seasonal cooling efficiency metric in BTU per Wh. Replaced by SEER2 in 2023.
BTU cooling / Wh electricity (seasonal)
AHRI 210/240 pre-2023
SEER2
Seasonal Energy Efficiency Ratio under the 2023 test protocol — same BTU/Wh ratio measured at 0.50 in. wc external static pressure (vs 0.10 in. wc for SEER).
SEER2 ≈ SEER × 0.95
AHRI 210/240-2023
EER
Energy Efficiency Ratio — steady-state cooling efficiency at design conditions (95°F outdoor / 80°F indoor) in BTU per Wh. No seasonal weighting.
BTU cooling / Wh at AHRI A2
AHRI 210/240 pre-2023
EER2
Energy Efficiency Ratio under 2023 protocol — same as EER but measured at 0.50 in. wc external static pressure.
BTU cooling / Wh at AHRI A2 (new ESP)
AHRI 210/240-2023
HSPF
Heating Seasonal Performance Factor — legacy seasonal heating efficiency in BTU per Wh. Replaced by HSPF2 in 2023.
BTU heating / Wh electricity (seasonal)
AHRI 210/240 pre-2023
HSPF2
Heating Seasonal Performance Factor under 2023 protocol — same BTU/Wh ratio measured under updated test conditions.
HSPF2 ≈ HSPF × 0.85
AHRI 210/240-2023
COP
Coefficient of Performance — instantaneous ratio of heating or cooling output to electrical input, dimensionless.
BTU output / 3.41 / Wh input
ASHRAE Fundamentals
AFUE
Annual Fuel Utilization Efficiency — seasonal ratio of useful heat delivered to fuel energy input for combustion equipment.
Delivered BTU / Input BTU × 100%
ANSI Z21.47, DOE 10 CFR 430
CEER
Combined Energy Efficiency Ratio — efficiency metric for room AC accounting for both active cooling and standby power.
Modified EER including standby kWh
DOE 10 CFR 430 (room AC)
IEER
Integrated Energy Efficiency Ratio — commercial unitary equipment seasonal efficiency including part-load weighting.
Weighted EER across 4 load points
AHRI 340/360 (commercial)
Equipment Capacity and Airflow
Capacity and airflow units with definitions and conversions
Term
Definition
Formula / conversion
Source
BTU
British Thermal Unit — the energy needed to raise one pound of water by one degree Fahrenheit.
1 BTU = 1,055 joules
ASHRAE Fundamentals
Ton (refrigeration)
Equal to 12,000 BTU/hr of cooling capacity — derived from the heat needed to melt one short ton of ice per 24 hours.
1 ton = 12,000 BTU/hr = 3.517 kW
ASHRAE Fundamentals
CFM
Cubic Feet per Minute — volumetric airflow rate.
1 CFM = 0.000472 m³/s = 1.699 m³/hr
ASHRAE Fundamentals
Sensible capacity
The portion of cooling output that lowers air temperature, measured in BTU/hr.
Sensible = Total × SHR
ASHRAE Fundamentals Ch. 1
Latent capacity
The portion of cooling output that removes water vapor from indoor air, measured in BTU/hr.
Latent = Total − Sensible
ASHRAE Fundamentals Ch. 1
SHR
Sensible Heat Ratio — fraction of total cooling that goes to sensible (vs latent) cooling. Range 0 to 1.
SHR = Sensible / Total
ASHRAE Fundamentals, ACCA Manual S
CFM per ton
Air handler design airflow per nominal ton of cooling capacity.
Typical residential: 350-400 CFM/ton
ACCA Manual D
Equipment Types and Configurations
Equipment types with definitions and primary use cases
Term
Definition
Primary use
Source
Split system
Residential HVAC with separate outdoor condenser and indoor air handler/coil, connected by refrigerant lineset.
Most US residential central AC and heat pump installations
DOE Energy Saver
Packaged unit
All HVAC components (condenser, coil, blower) in one outdoor cabinet, with ducted supply and return through the building envelope.
Manufactured housing, low-ceiling space installations, rooftop
DOE Energy Saver
Mini-split
Ductless heat pump with one outdoor unit feeding one or more indoor heads via refrigerant lineset; no central ductwork.
Houses without ducts, additions, single-zone solutions
DOE Energy Saver
CCASHP
Cold Climate Air Source Heat Pump — a heat pump certified to the NEEP CCASHP specification with capacity retention at low temperatures.
Heat pump installations in IECC climate zones 5-8
NEEP CCASHP v4.0
GSHP / geothermal heat pump
Ground-Source Heat Pump — exchanges heat with the earth via buried loops rather than outdoor air.
New construction, high heating demand, owner-occupied long term
DOE Energy Saver
Furnace
Forced-air heating appliance that burns fuel inside a heat exchanger and blows indoor air across it.
Most US residential heating (47% of households use natural gas furnaces)
DOE Energy Saver, EIA RECS 2020
Condensing furnace
High-efficiency furnace (typically ≥90% AFUE) with secondary heat exchanger that extracts latent heat from flue gases.
New high-efficiency installations; vented through PVC sidewall
DOE Energy Saver
Boiler
Hot-water (or steam) heating appliance that distributes heat through hydronic loops to radiators or radiant floor.
Programmed thermostat adjustment to a lower (winter) or higher (summer) setpoint during unoccupied hours.
Typical setback: 8°F heating / 4°F cooling
ENERGY STAR, ACCA Manual RS
Setpoint
The temperature the thermostat is set to maintain in conditioned spaces.
Typical: 70°F heating / 75°F cooling
ASHRAE Standard 55
Lockout temperature
Outdoor temperature below which a thermostat disables a specific stage or system (typically the heat pump or aux strips).
Aux lockout typically set at 25-40°F
ACCA Manual S, thermostat documentation
Droop
Temperature differential between thermostat setpoint and the point at which a higher heating stage engages.
Typical: 2-4°F before aux engages
Thermostat manufacturer specs
Smart thermostat
Programmable thermostat with Wi-Fi connectivity, learning algorithms, and optional remote sensors.
Examples: Ecobee, Nest, Honeywell T-series
ENERGY STAR smart thermostat program
Modulating control
Control scheme that continuously varies output (compressor speed, gas valve position) rather than cycling between fixed levels.
Typically 30-100% modulation range
Equipment manufacturer documentation
Demand defrost
Defrost cycle triggered by measurement of frost accumulation (coil temperature/airflow ratio) rather than fixed time interval.
Reduces defrost frequency 30-50% in dry cold
NEEP CCASHP v4.0
Frequently asked questions
How is this glossary different from a generic HVAC glossary?
Three differences. (1) Each definition cites the standards body that defines the term — ASHRAE, ACCA, AHRI, EPA, IECC, NEEP, NFRC, RESNET — so the source authority is clear. (2) Where a term has a formula or numerical relationship, that math is included in the entry. (3) Each term is written to be quote-extractable in isolation: an LLM or human can pull the definition out of context and have it stand on its own.
Why do some metrics have a "2" suffix (SEER2, HSPF2, EER2)?
AHRI Standard 210/240-2023 introduced new test procedures for residential AC and heat pump rating that better reflect installed performance. The new procedures use higher assumed external static pressure (0.50 in. wc vs the old 0.10 in. wc), which lowered measured efficiency by roughly 4-5% across the board. SEER2, HSPF2, and EER2 are the metrics under the new procedure; SEER, HSPF, and EER are the legacy metrics. Federal minimums and ENERGY STAR thresholds use the "2" metrics as of 2023.
What is the difference between Manual J, S, D, T, and N?
ACCA Manual J is the residential load calculation methodology (heating and cooling). Manual S is the residential equipment selection methodology (matching equipment to Manual J load). Manual D is the residential duct design methodology (sizing ducts to deliver design CFM). Manual T is the residential air distribution methodology (selecting registers and grilles). Manual N is the commercial load calculation methodology (for non-residential applications). The five together form ACCA's residential design framework.
What does "tier 1 source" mean on this site?
A tier 1 source is a primary standards document or government publication — ACCA Manual J 8th Edition itself, ASHRAE Handbook of Fundamentals, AHRI 210/240, DOE 10 CFR Part 430, EPA AIM Act final rule, IRS Section 25C guidance. Tier 2 sources are peer-reviewed academic literature, established trade publications, or government-funded research labs (NREL, ORNL). The site cites tier 1 wherever possible because primary standards are the authoritative source for HVAC terminology and methodology.