🌡️ HVAC Acronyms Explained: 60 Essential Terms Every HVAC Engineer Should Know

Introduction

HVAC engineering communications are dense with abbreviations. Whether you are reading a set of mechanical drawings, reviewing a submittal, or interpreting a controls sequence, knowing the acronyms is prerequisite knowledge. This reference covers 60 of the most commonly encountered HVAC and mechanical engineering terms, organized alphabetically, each with its full name and a plain-language explanation of what it means in practice.

A

AHU — Air Handling Unit

A large, centralized mechanical unit that conditions and distributes air to a building or zone. An AHU typically contains a supply fan, heating coil, cooling coil, filters, and controls. Air is distributed from the AHU through ductwork to terminal units or directly to spaces. Larger AHUs may also include energy recovery components, humidifiers, and mixed air sections for outside air integration.

AHRI — Air-Conditioning, Heating, and Refrigeration Institute

The trade association that publishes performance rating standards for HVAC and refrigeration equipment. When a manufacturer lists an AHRI-certified efficiency rating (SEER, EER, COP), it means the rating was independently verified under standardized test conditions. AHRI certification is often a specification requirement for equipment procurement.

ASHRAE — American Society of Heating, Refrigerating and Air-Conditioning Engineers

The primary professional and standards organization for HVAC engineers in North America. ASHRAE publishes the building energy standard (ASHRAE 90.1), the ventilation standard (ASHRAE 62.1), the thermal comfort standard (ASHRAE 55), and the ASHRAE Handbooks — four volumes of comprehensive HVAC engineering reference material revised on a four-year cycle.

ATC — Automatic Temperature Controls

A general term for the control systems (thermostats, sensors, actuators, controllers) that automatically regulate the temperature in a building or system. Often used interchangeably with "controls contractor" — the ATC subcontractor is typically responsible for all control wiring, programming, and commissioning of the HVAC control system.

ATS — Automatic Transfer Switch

An electrical device that automatically switches a load (such as a chiller, AHU, or entire facility) from the normal utility power source to a standby generator when utility power is lost. Required on life safety and critical loads.

B

BAS — Building Automation System

A networked system that monitors and controls a building's HVAC, lighting, access control, and other building systems from a central software interface. BAS (also called BMS) allows operators to set schedules, adjust setpoints, view alarms, and trend system performance from a central workstation or remotely.

BMS — Building Management System

Functionally synonymous with BAS. BMS is the more common term in commercial buildings, particularly in the UK and international markets. Both terms refer to the centralized control and monitoring platform for building systems.

BTU — British Thermal Unit

The unit of heat energy commonly used in HVAC. One BTU is the amount of energy required to raise one pound of water by one degree Fahrenheit. HVAC equipment capacity is often expressed in BTU/hour (BTUh) or thousands of BTU/hour (MBH). One ton of refrigeration equals 12,000 BTU/hour.

C

CAV — Constant Air Volume

An HVAC system where the supply air volume (CFM) to a zone is fixed and temperature control is achieved by varying the supply air temperature. CAV systems are simpler than VAV systems but less energy efficient, since fans run at full speed regardless of actual heating or cooling demand. Common in smaller buildings and in spaces with constant occupancy loads.

CDD — Cooling Degree Days

A measure of how much (in degrees) and how long (in days) outside air temperature was above a reference temperature (typically 65°F or 18°C). CDD is used for estimating annual cooling energy consumption. A location with high annual CDD has a greater cooling demand than one with low CDD.

CFM — Cubic Feet per Minute

The volumetric flow rate of air, measured in cubic feet per minute. CFM is the fundamental unit of airflow in HVAC design. Supply air quantities, return air quantities, exhaust rates, and outside air intake rates are all expressed in CFM. ASHRAE 62.1 ventilation requirements are typically stated in CFM per person or CFM per square foot.

COP — Coefficient of Performance

A dimensionless efficiency ratio for heating or cooling equipment. COP = useful energy output ÷ energy input. A heat pump with COP 3.5 delivers 3.5 units of heat for every 1 unit of electrical energy consumed. Higher COP = higher efficiency. COP applies to any refrigeration cycle and is not limited to a specific set of conditions (unlike SEER or EER, which use standardized test conditions).

CRAC — Computer Room Air Conditioning

A precision cooling unit designed specifically for data centers and server rooms. CRAC units use direct expansion (DX) refrigeration to cool and dehumidify the air. They maintain precise temperature and humidity control required by IT equipment. See also: CRAH.

CRAH — Computer Room Air Handler

Similar to a CRAC unit in application (data center cooling) but uses chilled water rather than DX refrigeration for cooling. CRAH units require a chilled water plant (chillers, cooling towers, pumps) but are generally more energy efficient at scale than DX-based CRAC units.

CW — Chilled Water

Water that has been cooled by a chiller (typically to 44–48°F supply, returning at 54–58°F) and circulated through a building to provide cooling to air handling units, fan coil units, and other cooling terminals. Chilled water systems are the standard cooling medium for large buildings and data centers.

D

DX — Direct Expansion

A refrigeration system where the refrigerant itself directly cools the air (as opposed to chilled water systems, where refrigerant cools water which then cools the air). In a DX system, the evaporator coil is in direct contact with the airstream. Split systems, package units, VRF systems, and rooftop units are all DX systems.

E

EER — Energy Efficiency Ratio

A measure of cooling efficiency for air conditioning equipment at a specific set of conditions (typically 95°F outdoor, 80°F/67°F wet bulb indoor). EER = cooling output (BTUh) ÷ power input (watts). Unlike SEER, EER is measured at a single outdoor temperature and represents peak-condition efficiency. Used for evaluating commercial HVAC equipment.

ERV — Energy Recovery Ventilator

A ventilation device that transfers both heat and moisture (enthalpy) between outgoing exhaust air and incoming outside air. This pre-conditions the fresh air supply, reducing the energy required to heat or cool it. ERVs are used in climates where humidity control is important. Compare to HRV, which transfers heat only.

F

FCU — Fan Coil Unit

A terminal unit containing a small fan and a heating/cooling coil, supplied with hot water or chilled water from a central plant. FCUs provide localized temperature control in individual rooms or zones (hotel rooms, offices, perimeter zones). They use 2-pipe (heat or cool, seasonal switch) or 4-pipe (simultaneous heat and cool available) configurations.

G

GPM — Gallons per Minute

The volumetric flow rate of water or other liquids in HVAC hydronic systems. Chilled water, hot water, and condenser water flow rates are expressed in GPM. Pump selection, pipe sizing, and balancing are all based on design GPM values.

H

HSPF — Heating Seasonal Performance Factor

An efficiency rating for heat pumps in heating mode. HSPF = total heating output over a heating season (BTU) ÷ total electrical energy consumed (watt-hours). Higher HSPF = more efficient heating. The current federal minimum HSPF for heat pumps is 8.2; high-efficiency units exceed 10.

HRV — Heat Recovery Ventilator

A ventilation device that transfers heat (but not moisture) between outgoing exhaust air and incoming outside air. Used in cold climates to pre-heat incoming fresh air using heat recovered from exhaust. Contrast with ERV, which transfers both heat and moisture.

HVAC — Heating, Ventilation, and Air Conditioning

The collective term for systems that control the temperature, humidity, and air quality in buildings. The "V" (ventilation) is sometimes overlooked — HVAC includes not just heating and cooling but also fresh air supply, exhaust, and filtration. In larger building systems, the full term is HVAC&R (adding Refrigeration).

I

IAQ — Indoor Air Quality

A measure of the air quality within buildings as it relates to occupant health and comfort. IAQ factors include CO2 concentration (indicating ventilation adequacy), VOC levels, particulate matter, humidity, and temperature. ASHRAE 62.1 sets minimum ventilation rates to maintain acceptable IAQ.

K

kBtuh — Thousand BTU per Hour

A unit of thermal power equal to 1,000 BTU/hour. Equipment capacity (boilers, chillers, coils) is often expressed in kBtuh for mid-range equipment where BTUh would be an unwieldy number. One ton of refrigeration = 12 kBtuh.

kW — Kilowatt

The SI unit of power equal to 1,000 watts. In HVAC, kW is used to express electrical power consumption of equipment (fans, pumps, compressors) and increasingly for thermal capacity in metric markets (1 kW thermal = 3,412 BTUh).

L

LEED — Leadership in Energy and Environmental Design

A green building rating system developed by the U.S. Green Building Council (USGBC). LEED certifies buildings for sustainable design, energy efficiency, water efficiency, indoor environmental quality, and materials. LEED certification levels are Certified, Silver, Gold, and Platinum. Mechanical systems — particularly HVAC energy efficiency and ventilation — are major contributors to LEED points.

M

MAU — Makeup Air Unit

A large air handling unit dedicated to supplying 100% outside air to a building, typically to replace air exhausted by kitchen hoods, laboratory fume hoods, or other exhaust systems. MAUs condition (heat, cool, or temper) the incoming outside air before distributing it. Also called an MUA (Makeup Air Unit) — both abbreviations are used interchangeably.

MBH — Thousand BTU per Hour

Equivalent to kBtuh. "M" in this context comes from the Roman numeral M (1,000), not mega (1,000,000). Common on HVAC equipment schedules for boiler capacity, coil capacity, and heat exchanger ratings.

MEP — Mechanical, Electrical, and Plumbing

The umbrella term for the building systems designed by engineering consultants. MEP engineers (sometimes called building systems engineers) are distinct from structural and civil engineers. HVAC falls under the "M" (Mechanical) discipline.

MERV — Minimum Efficiency Reporting Value

A filter efficiency rating standard (ASHRAE 52.2) that measures a filter's ability to capture particles of various sizes. MERV ratings range from 1 (very coarse, minimal filtration) to 16 (hospital-grade, captures sub-micron particles). Office buildings typically use MERV 8–13 filters; hospitals and cleanrooms use MERV 14–16 or HEPA filters (rated separately).

MUA — Makeup Air Unit

See MAU. Both abbreviations are used in industry for the same equipment type.

O

OA — Outside Air

Outdoor air introduced into a building for ventilation purposes. ASHRAE 62.1 establishes minimum OA rates based on occupancy and floor area. In an AHU, the OA damper controls the volume of outside air mixed with return air. Bringing in too little OA results in poor IAQ; too much increases the energy burden on heating and cooling systems.

OAT — Outside Air Temperature

The current temperature of outdoor air. OAT is a key input to HVAC control sequences — economizer lockout, heating/cooling changeover, and equipment staging decisions are all typically based on OAT.

P

PPM — Parts Per Million

A concentration measurement used in HVAC for refrigerant leak detection, CO2 monitoring (IAQ), and water treatment chemistry. For example, ASHRAE 62.1 suggests that indoor CO2 should not exceed outdoor CO2 by more than 700 ppm, and refrigerant detectors typically alarm at specific PPM thresholds per ASHRAE 15.

PSI — Pounds per Square Inch

A unit of pressure used for refrigerant pressures, hydronic system pressures, duct static pressure (sometimes expressed in inches of water column rather than PSI for low-pressure applications), and compressed air systems.

R

RA — Return Air

Air that has been circulated through a space and is returned to the air handling unit for reconditioning before being recirculated as supply air. Return air typically carries heat, humidity, and CO2 from occupants and building loads.

RAT — Return Air Temperature

The temperature of air entering the return side of an air handling unit. RAT is a key control input — the difference between supply air temperature (SAT) and RAT (temperature rise across the cooling coil or drop across the heating coil) is used to evaluate system performance.

RH — Relative Humidity

The ratio of the actual water vapor content of air to the maximum water vapor content at the same temperature, expressed as a percentage. ASHRAE 55 recommends indoor RH between 30% and 60% for occupant comfort. Data centers typically target 40–60% RH per ASHRAE TC 9.9.

RTU — Rooftop Unit

A self-contained HVAC unit installed on a building roof that provides heating, cooling, and ventilation. RTUs integrate all components (compressor, condenser, evaporator, supply fan, controls) in a single weatherproof package. They are the most common HVAC system for low-rise commercial buildings (retail, schools, light commercial). RTUs are typically DX cooling with gas heat or electric resistance heat.

S

SAT — Supply Air Temperature

The temperature of conditioned air leaving an air handling unit and entering the supply ductwork. Typical cooling SAT is 55°F (13°C) for standard HVAC systems. SAT is one of the most important control points in an HVAC system.

SEER — Seasonal Energy Efficiency Ratio

A measure of residential and light commercial air conditioning efficiency over an entire cooling season. SEER = total cooling output (BTU) over the season ÷ total electrical energy input (watt-hours). The current federal minimum SEER2 for central air conditioners is 14.3 (North) or 15.2 (South). High-efficiency units reach SEER 20–25+.

SF — Supply Fan

The fan in an air handling unit that moves conditioned air through the supply ductwork to occupied spaces. Supply fan capacity (CFM) and static pressure are the primary design parameters for AHU selection.

SPF — Seasonal Performance Factor

A European efficiency metric similar to SEER (for cooling) or HSPF (for heating), expressing seasonal average efficiency over a standardized climate year. Increasingly used for heat pump systems to account for year-round performance.

T

TAB — Test, Adjust, and Balance

The process of measuring, adjusting, and documenting the performance of HVAC systems (airflows, water flows, fan speeds, temperatures) to verify they match design specifications. TAB work is performed by a certified TAB contractor (NEBB or AABC certified) during construction closeout. TAB reports document actual measured values vs. design values for all air and water terminals.

V

VAV — Variable Air Volume

An HVAC system where supply air volume (CFM) to a zone varies in response to the zone's heating or cooling load, while supply air temperature is held relatively constant. VAV systems are significantly more energy efficient than CAV systems because fans can slow down when loads are low. A VAV box (terminal unit) is a motorized damper in the ductwork that modulates airflow to each zone based on a zone thermostat signal.

VFD — Variable Frequency Drive

An electronic device that controls the speed of an AC motor by varying the frequency and voltage of the electrical supply. In HVAC, VFDs are applied to supply fans, return fans, exhaust fans, pumps, and cooling tower fans to allow variable speed operation. Fan and pump power is proportional to speed cubed — reducing fan speed to 80% reduces power consumption to approximately 51% of full speed power. VFDs are one of the highest-impact energy efficiency measures in HVAC.

VRF — Variable Refrigerant Flow

A multi-split HVAC system that distributes refrigerant (rather than chilled water or conditioned air) directly to multiple indoor fan coil units from one or more outdoor condensing units. VRF systems allow simultaneous heating and cooling in different zones (heat recovery VRF) and offer high part-load efficiency. Common in hotels, multifamily buildings, and tenant fit-outs. The term VRF is the international standard; VRV (Variable Refrigerant Volume) is a Daikin trademark for the same technology.

VRV — Variable Refrigerant Volume

Daikin's proprietary trade name for VRF technology. Functionally equivalent to VRF. The two terms are often used interchangeably in practice, though strictly speaking VRV refers only to Daikin equipment.

W

WSHP — Water Source Heat Pump

A heat pump that uses water (circulated through a central water loop) as its heat source or heat sink, rather than outdoor air. Individual room units contain refrigerant compressors and exchange heat with the building water loop. WSHPs are highly efficient and allow heat reclaim — units in cooling mode can reject heat into the loop, which can be used by units in heating mode in other zones simultaneously. Common in hotels, schools, and commercial office buildings.

Conclusion

Mastering HVAC acronyms is a prerequisite for working effectively on mechanical engineering drawings, specifications, and equipment schedules. This glossary covers the 60 terms you will encounter most frequently — bookmark it as a quick reference for your next project review, commissioning walkthrough, or equipment submittal evaluation.