Commercial HVAC Glossary for Facility Managers

Why does a shared vocabulary matter?

Facility managers approve work they did not diagnose, based on reports written by technicians. The more clearly both sides understand the terms, the easier it is to compare proposals, challenge vague findings, and spot repeat problems.

These definitions are written for that purpose: practical rather than textbook.

What do the equipment terms mean?

Equipment terms name the main hardware a technician's report refers to: rooftop units, split systems, air handlers, heat pumps, fan coils, make-up air units, evaporative coolers, and air curtains.

• RTU (rooftop unit): a packaged unit containing the compressor, condenser, evaporator, fan, and usually heat in one cabinet on the roof. See how a commercial rooftop unit works.
• Split system: a system with an outdoor condensing unit and an indoor air handler or furnace, connected by refrigerant piping.
• Air handler (AHU): the indoor section containing the fan, coil, and filters.
• Condensing unit: the outdoor section of a split system, holding the compressor and condenser coil.
• Mini-split / ductless: a split system whose indoor units deliver air directly to the room without ductwork.
• Water-source heat pump (WSHP): a heat pump that exchanges heat with a building water loop.
• Fan coil unit (FCU): a fan and water coil heating or cooling a zone with water from a central plant.
• Make-up air unit (MAU): equipment that brings in outdoor air to replace air removed by exhaust.
• Evaporative cooler: equipment that cools air by evaporating water, effective in dry climates.
• Air curtain: a fan over a doorway that creates an air barrier to reduce infiltration.

What do the component terms mean?

Component terms name the parts inside the equipment, from the compressor and coils to the economizer, heat exchanger, electrical parts, curb, and condensate drain.

• Compressor: the pump of the refrigeration circuit and usually the most expensive single component.
• Evaporator coil: the indoor coil where refrigerant absorbs heat from the air.
• Condenser coil: the outdoor coil where refrigerant rejects heat.
• Metering device: a valve or orifice that drops refrigerant pressure before the evaporator.
• Economizer: dampers and controls that use outside air for cooling when conditions allow.
• Heat exchanger: in a gas heat section, the metal chamber that separates combustion gases from building air.
• Contactor and capacitor: electrical components that switch and start motors and compressors; common wear items.
• Curb and curb adapter: the roof-mounted frame an RTU sits on, and the adapter that fits a new unit to an old curb.
• Condensate pan and drain: where moisture removed by the coil collects and leaves the unit.

What do the performance terms mean?

Performance terms describe capacity, airflow, pressure, and efficiency. Technicians use them to judge whether a unit is sized correctly, charged correctly, and running the way it was designed to.

• Ton: 12,000 BTU per hour of cooling capacity. See RTU tonnage.
• BTU: British thermal unit, a measure of heat energy.
• CFM: cubic feet per minute, a measure of airflow.
• Sensible and latent load: heat that changes temperature, and moisture that must be removed.
• Head pressure and suction pressure: high-side and low-side refrigerant pressures used to diagnose system health.
• Superheat and subcooling: temperature measurements used to judge refrigerant charge.
• Delta T: temperature difference, commonly across a coil, used as a simple performance check.
• Short cycling: a unit starting and stopping frequently, which wears components and limits dehumidification.
• Static pressure: resistance to airflow in the ductwork and unit; high readings point to restrictions.
• SEER2 / EER / IEER: efficiency ratings under different test conditions; useful for comparing equipment within the same category.

What do the maintenance and program terms mean?

Maintenance and program terms describe how service is planned, recorded, and approved, from preventive maintenance and Standard 180 to asset registers, deficiencies, and not-to-exceed limits.

• PM (preventive maintenance): scheduled inspection and service intended to prevent failures. Compare the two models in reactive vs preventive maintenance.
• ANSI/ASHRAE/ACCA Standard 180: a standard setting minimum inspection and maintenance practices for commercial HVAC. REE Mechanical Inc. builds its maintenance programs on it. Its checklists set minimum tasks and frequencies (ACCA and ASHRAE on Standard 180); see quarterly vs semiannual PM visits for how that plays out.
• Asset register: the list of every unit with its make, model, serial, capacity, age, and location. See building an HVAC asset register.
• Reactive maintenance: repairing equipment after it fails.
• Deficiency: a problem found during a PM visit that needs repair beyond the PM scope.
• NTE (not-to-exceed): a spending limit on a work order before additional approval is needed.
• IVR and work-order platform: the systems many multi-site operators use to dispatch and track service.

What terms show up in proposals and replacement scopes?

Proposal and replacement terms describe how equipment is swapped, lifted, started up, and sized, plus the temporary measures and lead times that shape a replacement schedule.

• Like-for-like replacement: replacing a unit with one of the same type and capacity, usually on the existing curb.
• Crane lift / rigging: the work of removing and setting rooftop equipment, which often requires a permit and a site plan for the crane.
• Start-up and commissioning: the checks performed when new equipment is first run, confirming it operates as designed.
• Temporary cooling: portable or trailer-mounted cooling used while equipment is repaired or replaced.
• Lead time: the time between ordering equipment and its availability for installation.
• Load calculation: the engineering calculation of heating and cooling requirements for a space, used to size equipment.
• Test and balance (TAB): measurement and adjustment of airflows and water flows to match design.

What do the controls terms mean?

Controls terms describe how equipment is monitored and scheduled, from the building management system and its communication protocol to setpoints, trends, and occupied hours.

• BMS / BAS / EMS: building management, building automation, or energy management system; central control and monitoring of HVAC.
• BACnet: an open communication protocol for building automation.
• Setpoint: the target temperature or value a controller maintains.
• Trend: logged data over time, used for diagnosis.
• Occupied / unoccupied schedule: the hours a site is controlled for comfort, and the hours it is set back.

This page sits inside the Learning Center, where each of these terms is covered in a fuller article.

Methodology: Written by REE Mechanical's team from field experience on multi-site commercial portfolios; standards cited link to the issuing body.