Reference vocabulary
HVAC glossary
Manuals assume vocabulary that nobody is taught. These definitions cover the terms that appear in service documentation and on this site, including the places where United States and United Kingdom usage differ.
A
- Air filter
- The filter that protects the coil and blower from dust. In ductless systems it is a washable mesh screen behind the front panel; in ducted systems it is a replaceable panel filter. A blocked filter is the single most common cause of reduced airflow.Also called: filter, MERV filter
- Air handler
- The indoor assembly containing the blower, the evaporator coil, the filter rack, and usually the drain pan. In a ducted system it moves conditioned air into the supply ducts and pulls room air back through the returns.Also called: AHU, fan coil unit, indoor unit
- Air-to-water heat pump
- A heat pump that transfers heat from outside air into a water circuit serving radiators, underfloor loops, or a hot water cylinder. This is the common domestic arrangement in the United Kingdom and much of Europe, where it usually replaces a boiler rather than a ducted system.Also called: hydronic heat pump, monobloc heat pump
- Auxiliary heat
- Supplementary heating that runs when a heat pump alone cannot meet demand, usually electric resistance elements in North America or an immersion heater and boiler in United Kingdom hydronic systems. Frequent auxiliary operation is expensive and worth investigating.Also called: aux heat, emergency heat, backup heat
B
- Balance point
- The outdoor temperature at which a heat pump's output exactly matches the building's heat loss. Below it, the system needs auxiliary heat or longer run times, which is why cold-snap complaints often have nothing to do with a fault.Also called: thermal balance point
- Blower motor
- The motor driving the indoor fan. Modern systems use variable-speed motors that adjust to maintain airflow, which means a failing blower may show up as reduced capacity or a protection code long before the fan stops entirely.Also called: indoor fan motor, fan motor
- Boiler
- An appliance that heats water for radiators, underfloor circuits, or a hot water cylinder, rather than heating air directly. Boilers dominate United Kingdom and European domestic heating, and are the equipment an air-to-water heat pump usually replaces or works alongside.Also called: combi boiler, system boiler, condensing boiler
- Branch box
- A distribution unit used on some multi-zone systems to split refrigerant and control signals between indoor units. Because it sits between the outdoor unit and several heads, a branch-box fault can present as a problem in one zone only.Also called: distribution box, branch selector
- BTU
- The British thermal unit, the heat needed to raise one pound of water by one degree Fahrenheit. Equipment capacity is usually quoted in BTU per hour in North America, while the same figure appears in kilowatts in the United Kingdom and Europe.Also called: BTU/h, British thermal unit
C
- C wire
- The common conductor that gives a thermostat a continuous 24-volt supply. Smart thermostats generally need one, and installations without it are a common source of thermostats that lose power, restart, or drop their connection.Also called: common wire
- Capacitor
- A component that stores and releases electrical energy to start and run single-phase motors. Capacitors hold a charge after power is removed, which is one reason opening an electrical compartment is technician work rather than a homeowner check.Also called: run capacitor, start capacitor, dual capacitor
- Commissioning
- The measured handover of a new system: charge verified, airflow set, controls configured, and readings recorded. A commissioning sheet is the reference that later service visits compare against when performance is questioned.Also called: handover, start-up
- Compressor
- The pump at the centre of the refrigeration circuit. It raises the pressure and temperature of refrigerant vapour so heat can be rejected at the condenser. Most protection codes exist to stop the compressor before it is damaged by pressure, current, or temperature outside its design range.Also called: compressor motor
- Condensate drain
- The path that carries water from the drain pan out of the building, by gravity or with a pump. A blocked drain is the usual reason an indoor unit leaks water, and many systems now include a float switch that stops the unit before water overflows.Also called: drain line, condensate pump, drain pan
- Condenser coil
- The coil that releases heat from the refrigerant to the surrounding air. In cooling it sits in the outdoor unit. A blocked, dirty, or restricted condenser raises operating pressure and is a common root cause behind high-pressure protection codes.Also called: outdoor coil
- Contactor
- An electrically operated switch that connects mains power to the compressor and outdoor fan when the control system calls for them. Pitted or welded contacts are a common cause of an outdoor unit that will not start or will not stop.Also called: relay
- Control board
- The printed circuit board that reads sensors, drives outputs, and decides when to run or protect the system. It is also what generates the fault code you see, which means a code describes what the board detected rather than which part has failed.Also called: PCB, main board, controller board
- COP
- Coefficient of performance: heat delivered divided by electricity consumed at a specific operating condition. A COP of 3.5 means three and a half units of heat for each unit of electricity, and the figure falls as outdoor temperature drops.Also called: coefficient of performance
D
- Data plate
- The label on the equipment listing model number, serial number, electrical ratings, and refrigerant charge. It is the starting point for any accurate diagnosis, because code tables and service data are published per model rather than per brand.Also called: nameplate, rating plate, spec plate
- Defrost cycle
- A normal heat-pump routine that briefly reverses operation to melt frost from the outdoor coil. Steam from the outdoor unit, a pause in heating, and a short burst of cool air from indoor units are expected behaviour, not a fault.Also called: defrost, defrost mode
- Delta T
- The temperature difference between the air entering and leaving the indoor coil, sometimes called split. A delta T well outside the manufacturer's stated range points to an airflow or refrigerant problem, and tells you which direction to look.Also called: temperature split, ΔT
- Ducted split system
- A central system with an indoor air handler or furnace coil and an outdoor condensing unit, distributing conditioned air through ducts. Symptoms in ducted systems frequently trace to airflow through the duct network rather than to the equipment itself.Also called: central air, central heating and air
- Ductless mini-split
- A heating and cooling system with one or more indoor units connected to an outdoor unit by refrigerant pipes and a control cable, with no duct network in between. Because each indoor unit conditions the space it is mounted in, a fault often affects one room rather than the whole building.Also called: mini split, ductless system, split air conditioner
- Ductwork
- The supply and return duct network that carries air between the air handler and the rooms it serves. Undersized returns, crushed flexible duct, and leaking joints all reduce delivered capacity without producing an equipment fault code.Also called: ducts, duct system
E
- EER
- Energy efficiency ratio: cooling output divided by electrical input at one fixed operating condition, rather than across a season. It is useful for comparing equipment at a specific outdoor temperature, which is why it appears alongside SEER on North American data sheets.Also called: energy efficiency ratio
- EPA Section 608
- The United States certification required to handle refrigerants under the Clean Air Act. Like F-Gas in Europe, it makes opening a refrigerant circuit a licensed activity rather than a do-it-yourself task.Also called: EPA 608, Section 608
- Error code
- A short alphanumeric message a control board shows when it detects a condition outside normal limits. Codes are defined per product family, so the same characters can mean different things on two systems from the same manufacturer.Also called: fault code, alarm code
- Evacuation
- Pulling a deep vacuum on a system with a vacuum pump to remove air and moisture before charging, measured in microns or millibar. Skipped or shortened evacuation is a common origin of faults that appear months after installation.Also called: vacuum, pull down
- Evaporator coil
- The coil that absorbs heat from indoor air, causing the refrigerant inside it to boil from liquid to vapour. Because it runs below room temperature, it also condenses moisture out of the air, which is why every evaporator has a drain pan beneath it.Also called: indoor coil
- Expansion valve
- The metering device that drops refrigerant pressure before the evaporator and controls how much liquid enters it. Thermostatic expansion valves respond mechanically; electronic expansion valves are driven by the control board and report their own faults.Also called: TXV, EEV, metering device
F
- F-Gas regulation
- United Kingdom and European Union rules governing fluorinated refrigerants, including who may handle them, leak checking, and record keeping. Work on the refrigerant circuit requires certification, which is why refrigerant tasks sit outside homeowner checks.Also called: F Gas, FGas certification
- Float switch
- A safety switch in the drain pan or condensate pump that interrupts operation when water rises above a set level. A system that stops in cooling but runs in fan mode, with no fault beyond a water symptom, often has a tripped float switch.Also called: safety switch, overflow switch
- Flow temperature
- The water temperature leaving a boiler or air-to-water heat pump toward the emitters, central to United Kingdom and European system design. Lower flow temperatures raise heat-pump efficiency but require emitters sized to suit them.Also called: flow temp, leaving water temperature
- Frozen coil
- Ice forming on the indoor coil, usually because airflow has dropped or the refrigerant charge is low. Running a frozen system risks liquid refrigerant reaching the compressor, so the standard response is to stop cooling and let it thaw before diagnosing.Also called: iced coil, coil icing
H
- Heat pump
- A system that moves heat rather than creating it, and can run in both directions: taking heat out of a building in cooling and drawing heat in from outside air, ground, or water in heating. The same hardware provides both functions, which is why one fault can affect heating and cooling together.Also called: ASHP, air source heat pump, reverse cycle
- HSPF and HSPF2
- Heating seasonal performance factor, the heat delivered over a season divided by the electricity used, with HSPF2 the current United States test procedure. It is the heating counterpart to SEER and is not comparable with European SCOP figures.Also called: HSPF, HSPF2
I
- Inverter compressor
- A compressor whose speed varies with demand instead of switching fully on and off. Inverter systems hold temperature more steadily and use less energy at part load, and they report a distinct family of drive and module fault codes.Also called: variable speed compressor, DC inverter
- Inverter module
- The power electronics that vary compressor speed, often called the IPM or intelligent power module. Inverter-module protection codes usually mean the module measured current, voltage, or temperature outside limits and stopped before damage occurred.Also called: IPM, inverter board, power module
L
- Line set
- The pair of insulated copper pipes connecting an indoor unit to an outdoor unit: a smaller liquid line and a larger suction line. Length, insulation, and the quality of the flare connections all affect performance and are frequent sources of leaks.Also called: refrigerant lines, pipe run
M
- MERV rating
- A scale from 1 to 16 describing how effectively a filter captures particles, used mainly in North America. A higher rating traps more, but also adds resistance, so a filter must be matched to what the blower and duct system can handle.Also called: MERV
- Multi-zone system
- One outdoor unit serving several indoor units, each controlled separately. A fault reported on one indoor head can originate in the shared outdoor unit or in the branch pipework, so a multi-zone diagnosis has to establish whether other zones behave normally.Also called: multi split, multi-split
P
- Protection code
- A code that reports the system stopping itself deliberately, for example on high pressure, high current, or module temperature. It records that a limit was reached rather than that a part failed, and clearing it without finding the cause simply repeats the event.Also called: protection fault, limit fault
- PTAC unit
- A packaged terminal air conditioner: a self-contained heating and cooling unit installed through an exterior wall, most often in hotels, care homes, and apartments. All components sit in one chassis, so service usually means removing the unit rather than working on separate indoor and outdoor sections.Also called: PTAC, through-the-wall unit
R
- R-32
- A single-component refrigerant with a lower global warming potential than R-410A, now standard on many new systems in Europe and increasingly in North America. It is mildly flammable, classified A2L, which changes installation and service requirements.Also called: R32, A2L refrigerant
- R-410A
- A refrigerant blend widely used in split systems and heat pumps from the 2000s onward. It operates at higher pressures than the R-22 it replaced, and equipment, gauges, and recovery equipment must be rated for it.Also called: 410A, Puron
- Refrigerant
- The working fluid that carries heat around the circuit by evaporating and condensing at useful temperatures. It is not consumed in normal operation, so a system that is low on refrigerant has a leak rather than a need for a routine top-up.Also called: gas, coolant
- Refrigerant charge
- The mass of refrigerant a system is designed to hold, stated on the data plate and adjusted for pipe length at installation. Both undercharge and overcharge reduce capacity and can trigger protection codes rather than an obvious fault.Also called: charge, gas charge
- Refrigerant leak
- A loss of refrigerant through a joint, a corroded coil, or damaged pipework. Because refrigerant is not consumed, repeated loss of capacity between seasons points to a leak that needs finding rather than a system that needs recharging.Also called: gas leak
- Return air
- Room air drawn back to the air handler to be filtered and conditioned again. Blocked returns, closed doors in single-return houses, and dirty filters all starve the system of return air and show up first as reduced capacity.Also called: return, return duct
- Reversing valve
- The valve that redirects refrigerant flow so a heat pump can switch between heating and cooling. A valve that sticks part-way through a changeover produces the classic symptom of a system that cools when heat is called for, or the reverse.Also called: four-way valve, changeover valve
S
- Saturation temperature
- The temperature at which a refrigerant changes between liquid and vapour at a given pressure. Because the relationship is fixed for each refrigerant, a pressure reading can be converted into a temperature and compared with what the system is actually doing.Also called: sat temp
- SCOP
- Seasonal coefficient of performance, the European measure of heat-pump efficiency across a whole heating season and climate profile. It is the figure quoted on United Kingdom and European product labels and is more realistic than a single-point COP.Also called: seasonal COP
- SEER and SEER2
- Seasonal energy efficiency ratio, the cooling output over a season divided by the electricity used. SEER2 is the current United States test procedure, measured under higher external static pressure, so SEER2 figures read lower than older SEER values for the same equipment.Also called: SEER, SEER2
- Sensor fault
- A code raised when a temperature or pressure sensor reads outside its expected range. The sensor itself is only one possibility: the connector, the harness, and the board input all sit in the same measurement path.Also called: sensor error
- Service manual
- The manufacturer document written for technicians, containing wiring diagrams, code tables, pressure data, and diagnostic sequences. It is more specific than the owner's manual and is the source that settles what a code means on a given model.Also called: technical manual, installation manual
- Short cycling
- Repeatedly starting and stopping after only a few minutes of running. It wastes energy, wears the compressor, and usually points to a control, sizing, airflow, or refrigerant problem rather than to normal thermostat behaviour.Also called: cycling on and off
- Static pressure
- The resistance the duct system presents to the blower, measured in inches of water column or pascals. High static pressure reduces airflow, raises energy use, and can cause coil freezing even when the equipment itself is healthy.Also called: external static pressure, ESP
- Subcooling
- How many degrees the liquid refrigerant has been cooled below its condensing temperature. Together with superheat it indicates whether the charge and the metering device are working as designed, which is why both are recorded during commissioning.
- Superheat
- How many degrees the refrigerant vapour has risen above its boiling point at the measured pressure. Technicians use it to judge whether the evaporator is being fed correctly, since too little superheat risks liquid returning to the compressor.
T
- Thermistor
- A temperature sensor whose electrical resistance changes predictably with temperature. Sensor fault codes normally mean the board measured a resistance that is open, shorted, or out of range, which can be the sensor, the plug, or the wiring to it.Also called: temperature sensor, NTC sensor
- Thermostat
- The control that measures room temperature and calls for heating or cooling. Its wiring determines what it can control, which is why a thermostat swap can change how a heat pump stages its auxiliary heat.Also called: room stat, programmable thermostat
- Ton of refrigeration
- A capacity unit equal to 12,000 BTU per hour, roughly 3.5 kilowatts. It comes from the cooling once provided by melting a ton of ice in a day, and is still standard shorthand for equipment size in North America.Also called: ton, tonnage
V
- VRF and VRV systems
- Variable refrigerant flow systems, sold by Daikin under the VRV name, use one outdoor unit to serve many indoor units and vary refrigerant flow to each. They are common in commercial buildings and report faults with both a code and the address of the unit that raised it.Also called: VRF, VRV, variable refrigerant flow
W
- Weather compensation
- A control strategy that varies flow temperature according to outdoor temperature so the system delivers only the heat the building currently needs. It is standard on European heat-pump installations and materially affects running cost.Also called: load compensation, outdoor reset