Glossary

HVAC and controls vocabulary, in plain English.

Every acronym our agent normalizes, defined in two sentences and tied back to what an operator actually sees. Search, follow cross-references, deep-link any term.

ACH aka Air Changes per Hour Air & ventilation

How many times the air in a room is fully replaced in an hour. Code minimums are usually 0.35–1; hospitals and labs run 10+. Higher ACH means more ventilation energy.

see also: IAQ, DOAS

AHU aka Air Handling Unit Systems

The box that moves and conditions air for a zone or a whole building: fans, coils, filters, sometimes humidification. Sits between the chilled-water plant (or DX coil) and the ductwork.

see also: DOAS, Chiller

BACnet Controls

The dominant open protocol for HVAC and building automation, defined by ASHRAE Standard 135. BACnet/IP runs over Ethernet, BACnet/MSTP over RS-485. Universally supported by name-brand BMSes.

see also: Modbus, BMS / BAS

BMS / BAS aka Building Management System · Building Automation System Controls

The supervisory layer that ties HVAC, lighting, access, and life-safety controls together. Schedules, alarms, trending, occupant-comfort overrides — usually a web UI on top of BACnet or proprietary buses.

see also: BACnet, Niagara

Chiller Systems

Central plant that cools water (or glycol), which is then pumped to AHUs and fan coils. Centrifugal, screw, scroll, or absorption types. Anything bigger than a mid-size office or a data center is almost certainly chilled-water.

see also: CRAC, AHU

COP aka Coefficient of Performance Efficiency

Heat moved per unit of electric energy consumed. A COP of 3 means 3 kW of heat for 1 kW of electricity. Higher is better. Heat-pump rated COP is usually 3–5 at design conditions, dropping as outdoor temperature drops.

see also: SCOP, EER

CRAC / CRAH aka Computer Room Air Conditioner · Computer Room Air Handler Systems

Precision cooling units for data centers and IT closets. CRAC has its own DX compressor; CRAH uses chilled water from a central plant. Tuned for sensible-heavy IT loads, not human comfort.

see also: Chiller, PUE

Demand response Controls

Temporarily reducing electric load when the grid signals constraint, in return for a utility incentive. HVAC is the largest curtailable load in most buildings.

see also: OpenADR

DOAS aka Dedicated Outdoor Air System Systems

A separate AHU that delivers tempered outdoor air directly to spaces, decoupling ventilation from sensible cooling. Common in modern offices and high-performance buildings.

see also: AHU, ERV

DX aka Direct Expansion Systems

Refrigerant evaporates inside the air-side coil that touches the supply air — no chilled water intermediary. Splits, mini-splits, RTUs, VRF indoor units are all DX.

see also: Chiller, TXV

EER aka Energy Efficiency Ratio Efficiency

Cooling capacity in Btu/h divided by electrical input in W, at a fixed test condition. A snapshot, not a season average.

see also: SEER, COP

ERV / HRV aka Energy Recovery Ventilator · Heat Recovery Ventilator Air & ventilation

Devices that transfer heat (HRV) or heat + moisture (ERV) between incoming and outgoing air streams, reducing the energy cost of ventilation. The basic enabler of low-energy buildings.

see also: DOAS

Fan coil unit aka FCU Systems

A small terminal unit with a chilled-water (or hot-water) coil and a fan, controlled per zone. The hotel-room version when the building has a central chilled-water plant.

see also: Chiller, AHU

Heat pump Systems

One box that can both heat and cool by reversing the refrigerant cycle. Replacing furnace-plus-AC pairs in much of the residential and small-commercial electrification push.

see also: COP, DX

HEPA aka High-Efficiency Particulate Air Air & ventilation

A filter standard requiring 99.97% capture of 0.3-micron particles. Used in cleanrooms, ORs, and aircraft cabins. Adds significant fan power, so rarely deployed building-wide.

see also: MERV

HSPF aka Heating Seasonal Performance Factor Efficiency

The heating-side counterpart to SEER. Total heating output over a season divided by electrical input. Watch the climate-region assumption — HSPF in Region IV is not the same as Region V.

see also: SEER, COP

IAQ aka Indoor Air Quality Air & ventilation

The umbrella term for CO₂, VOCs, PM2.5, humidity, and other indoor-air metrics. A real driver of how much outdoor air an HVAC system has to move and condition.

see also: DOAS, ACH

IPMVP aka International Performance Measurement and Verification Protocol Efficiency

The reference framework for proving that an energy-efficiency project actually saved energy. Four options (A–D) defining what you measure, model, and compare against baseline.

see also: M&V

KNX Controls

European open standard for home and light-commercial automation. Covers HVAC, lighting, blinds, intercom over twisted pair, IP, RF, or PL. Strong in EU residential and hotels.

see also: Smart home

M&V aka Measurement and Verification Efficiency

The discipline of proving claimed savings against a defensible baseline. IPMVP is the standard playbook. Mandatory for utility-incentive paydowns and most ESG audits.

see also: IPMVP

Matter Smart home

IP-based open smart-home standard (CSA, 2022). Targets cross-vendor interop so Apple, Google, Amazon, and Samsung hubs can all see the same thermostat. HVAC support is still maturing.

see also: Thread, Zigbee

MERV aka Minimum Efficiency Reporting Value Air & ventilation

A 1–16 rating for how well an air filter captures particles. MERV 13+ catches most virus-carrying droplets and is the common target for office and healthcare retrofit.

see also: HEPA, IAQ

Mini-split aka Ductless split Systems

A DX split system with one outdoor unit and one or more ductless indoor heads (wall or cassette). The cheapest path to zoned heating and cooling in a building without ducts.

see also: VRF

Modbus Controls

A simple, 40-year-old serial (RTU) and TCP protocol still everywhere in HVAC and industrial controls. No object model — you read holding registers and trust the device map.

see also: BACnet

Niagara aka Tridium Niagara Controls

A platform (Niagara 4 / N4) that normalizes BACnet, Modbus, OPC, LonWorks, and brand-cloud APIs into one programmable supervisory layer. The most common path to a vendor-agnostic BMS.

see also: BMS / BAS

OpenADR Controls

Open standard for demand-response signals from utility to building. Lets a BMS automatically curtail HVAC load when the grid is stressed, in exchange for tariff incentives.

see also: Demand response

PTAC aka Packaged Terminal Air Conditioner Systems

The single-room unit you see in hotel rooms, mounted through the wall under the window. Packaged DX with electric or heat-pump heat. Cheap to install, noisy, inefficient at scale.

see also: Heat pump

PUE aka Power Usage Effectiveness Efficiency

Total facility power divided by IT power, used in data centers. 1.0 is the unattainable theoretical floor (all power goes to IT). Hyperscalers operate around 1.1–1.2; legacy enterprise is 1.6+.

see also: WUE, CRAC

RTU aka Rooftop Unit Systems

A packaged HVAC unit installed on a building roof, common in light commercial: warehouses, big-box retail, strip malls. All-in-one DX cooling, gas or heat-pump heating, and a supply fan.

see also: DX, Heat pump

SCOP aka Seasonal COP Efficiency

European seasonal coefficient of performance for heat pumps, rated to EN 14825. The number on the EU energy label for heating mode.

see also: COP, HSPF

SEER aka Seasonal EER Efficiency

EER averaged over a defined cooling season. The US AHRI standard for rating residential AC and heat pumps. Higher number, lower bill — within the same climate zone.

see also: EER, HSPF

Setpoint / Setback Controls

The temperature the system is trying to hold (setpoint) vs. a relaxed target during unoccupied hours (setback). Aggressive setbacks are the single biggest no-cost saving lever in commercial HVAC.

see also: Demand response

Thread Smart home

A low-power IPv6 mesh radio (802.15.4-based) used as the transport for many Matter devices. Different layer from Matter — Matter rides on Wi-Fi, Ethernet, or Thread.

see also: Matter, Zigbee

VRF aka Variable Refrigerant Flow · VRV Systems

A multi-zone refrigerant-based system where one outdoor unit modulates refrigerant flow to many indoor units. The dominant choice for offices, hotels, and large residences. Daikin trademarked the term VRV; everyone else calls it VRF.

see also: Heat pump, DX

WUE aka Water Usage Effectiveness Efficiency

Liters of water per kWh of IT energy in a data center. The metric that exposes evaporative-cooling water use that PUE alone hides.

see also: PUE

Z-Wave Smart home

A proprietary low-power mesh radio at 908 MHz (US) / 868 MHz (EU). Smaller device catalog than Zigbee but more reliable mesh in some installations. Strong in alarm-system integration.

see also: Zigbee

Zigbee Smart home

A long-running low-power mesh radio for smart-home and light-industrial sensors. Predates Thread, still has the largest device catalog. Bridged to IP by a hub.

see also: Z-Wave, Thread