Rev 1.0 May 2024 Author: Sarah Hanson


ABV: Automatic bleed valve

Device fitted to vessels, piping, or radiators, to release any air build-up automatically.

ABV: Automatic bypass valve

A subset of PRVs installed in emitter circuits to maintain some circuit flow at zero or very low emitter flow, to reduce the risk of mechanical damage to circulation equipment (e.g. pumps).

ASHP: Air source heat pump

Equipment that transfers thermal energy from a cold source (external air) to a warm sink (space heating and DHW) by means of a refrigeration cycle.

BMS: Building Management System

Network of hardware and software that enables centralized monitoring and control of various HVAC and security systems.

BSH: Booster heater

DHW tank electrical immersion heater.

BUS: Boiler Upgrade Scheme

Department for Energy Security & Net Zero initiative to incentivise installation of energy saving equipment (e.g. PV panels/inverters/batteries, heat pumps) though a system of grants. Replaced DRHI on 1st April 2022.

BUH: Back-up heater

An independent boiler which supplements a heat pump for space heating and/or DHW.

CH: Central heating

Network of emitters (e.g. radiators, UFH), circulating pump, and distribution piping. Synonymous with space heating.

COP: Coefficient of Performance

Instantaneous ratio of useful energy produced in a device to the energy required to operate that device. In heat pumps, by convention includes the Outdoor Unit heat generated and power consumed, but excludes power required for circulating pumps(s) and the heat pump controller. For comparison purposes, values at “standard” conditions (7degC ambient temperature, 35degC LWT) are often cited by Outdoor Unit vendors. See also SCOP.

CT: Current transformer

Non-invasive (typically clip-on) instrument that measures current in a cable used (with known voltage) to estimate load power.

DRHI: Domestic Renewable Heating Incentive

Predecessor to BUS (scheme ended 31st March 2022).

DHW: Domestic hot water

Hot water distribution system, by convention includes hot water tank coil with isolation valve(s), and piping to consumer outlets.

EEV: Electronic expansion valve

An integral part of the refrigerant circuit in the Outdoor Unit that reduces pressure from the required condensation (compressor discharge) pressure to the required evaporation (compressor suction) pressure.

EPC: Energy Performance Certificate

Rating of a property’s energy performance by an independent accredited assessor.

EV: Expansion vessel

Small tank containing some air (separated by a diaphragm) that maintains constant pressure in a CH piping system by absorbing liquid thermal expansion/contraction through diaphragm movement.

GSHP: Ground source heat pump

Equipment that transfers thermal energy from a cold source (underground) to warm sink (space heating and DHW) by means of a refrigeration cycle.

HVAC: Heating Ventilation and Air Conditioning

Collective term for equipment which modifies a building’s temperature and/or humidity.

LLH: Low loss header

A vessel that separates two pumped systems and allows each to operate with minimal pressure or flow interference (e.g. a buffer tank between the heat pump and emitter circuits, or between UFH and radiator loops, which may require different flows).

LMTD: Log Mean Temperature Difference

Effective temperature driving force for heat transfer in a heat exchanger when one or both fluids are non-isothermal. Used for rating heat exchanger performance from Q = OHTC × Area × LMTD for countercurrent exchangers.

LS: Lockshield valve

Radiator valve, normally left in a preset position, used for adjusting flow to balance heat output from multiple radiators.

LWT: Leaving water temperature

Temperature of the circulating fluid serving CH and DHW systems leaving the Outdoor Unit. (Note - not necessarily pure water, may be glycol solution or brine for example.)

MCS: Microgeneration Certification Scheme

The (independent, part charitable) organisation that administers the BUS, sets standards, ensures competency of practitioners (manufacturers, vendors, installers), and issues the certification required for obtaining installation grants.

MID: MCS Installation Database

Information about every MCS certified small scale, low carbon installation in the UK since 2010, access normally restricted to MCS-approved installers.

MID: Measuring Instruments Directive

EU Commission Implementing Decision on standards harmonisation for measuring instruments.

MVHR: Mechanical ventilation with heat recovery

System for extracting stale household air and recovering its heat to warm up fresh incoming (filtered) air.

ODP: Ozone depletion potential

A measure of the damage caused to the ozone layer by release of a refrigerant (accidental loss or fugitive emissions).

OHTC: Overall heat transfer coefficient

A term that combines conductive, convective and radiation coefficients when heat is transferred from one fluid to another.

P&ID: Process and Instrument Diagram

A diagram which shows the interconnection of process equipment and the instrumentation used to control the process, using an agreed set of symbols, intended to convey engineering requirements to contractors for detailed equipment/piping/instrument specification and purchase.

PFD: Process Flow Diagram

Simplified version of the P&ID, intended to convey purpose rather than engineering requirements (e.g. inclusion of operating conditions, control loops etc.).

PHE: Plate heat exchanger

A type of compact heat exchanger comprising several gasketted (or welded) plates with the two heat exchange fluids flowing in alternate channels. True countercurrent flow, so high thermal and space efficiency. Often used in Outdoor Units for the refrigerant condenser, and the refrigerant evaporator in GSHPs and WSHPs.

PID: Proportional-Integral-Derivative

An algorithm used to define setpoint rate-of-approach and overshoot in a control loop. Also known as 3-term control.

PRV: Pressure relief valve

Spring-loaded valve which prevents system overpressure by routing fluid to a lower pressure sink at a preset pressure. As a safety-critical device, it is subject to several design/inspection/testing requirements and constraints.

PRV: Pressure reducing valve

Spring-loaded valve which responds to an instrument signal to control a process condition (e.g. the upstream or downstream pressure).

PWM: Pulse width modulation

A method for electrical load control by voltage-chopping the power supply at varying time intervals. Often used to control circulation pump speed in order to obtain a desired ΔT between LWT and RWT in the heat pump condenser.

R125: Refrigerant 125

Pentafluoroethane, C2HF5.

R134a: Refrigerant 134a

1,1,1,2-tetrafluoroethane, CH2FCF3.

R290: Refrigerant 290

Propane, C3H8

R32: Refrigerant 32

Difluoromethane, CH2F2.

R410a: Refrigerant 410a

50/50wt% mixture of R32 and R125.

RWT: Return water temperature

Temperature of the circulating fluid serving CH and DHW systems as it re-enters the Outdoor Unit.

SCOP: Seasonal Coefficient of Performance

As COP, but time-averaged over a longer period (e.g. weekly, monthly, annually) to better reflect overall performance (e.g. taking into account changing seasonal duty loads, or minimising the effect of unusual extreme loads).

TPI: Time Proportional Integral

A type of algorithm used in (usually wireless) room thermostats that reduces setpoint overshoot and speeds convergence to it through 2-term control (cf. PID control).

TOU: Time of Use

A type of electrical energy tariff that varies with time of day. Used to minimise utility costs by pre-programming the timing of significant loads.

TRV: Thermal relief valve

A type of PRV that prevents overpressure due to thermal expansion of blocked-in fluid.

TRV: Thermostatic radiator valve

A radiator valve that controls flow of circulating fluid based on local temperature, and thus radiator heat output.

UFH: Under floor heating

A network of pipework installed below floor level, containing warm circulating fluid which heats rooms by convection; an alternative/supplement to convection radiators.

WAH: Warm air heating

A form of room heating consisting of a heat exchanger which takes heat from a boiler or other heat source, and transfers warm air to rooms using a fan and ducting.

WC: Weather compensation

A heat pump controller algorithm that adjusts its operation to achieve a desired heat output over a range of ambient temperatures, normally by varying the LWT, to recognise that house heat losses increase at lower ambient temperatures. Set up by the installer/user, this can be a key element in running cost minimisation, but does not usually take into account factors such as wind speed/direction and air humidity which can also affect heat loss.

WSHP: Water source heat pump

Equipment that transfers thermal energy from a cold source (a body of water) to warm sink (space heating and DHW) by means of a refrigeration cycle.


Buffer tank (aka Decoupler):

A circulating fluid reservoir that separates a primary heat pump circulation loop (flow and temperatures) from a secondary loop supplying the emitters. Normally has four process nozzles (heat pump flow and return, emitter flow and return), plus drain/ABV. A type of low loss header.

Cycling: Repeated stop-starts.

Undesirable consequence of mismatch of system controller hysteresis with system thermal inertia.

Deadband: See Hysteresis

Design output: Performance at design conditions.

Typically the vendor-specified maximum under ideal or stated conditions e.g. radiator heat output at ΔT=50°C (e.g. radiator at 70degC, room at 20degC). cf. Rated output.

Economiser: Additional heat exchanger downstream of condenser.

Technique to improve heat pump COP by evaporating part of the refrigerant condensate at an intermediate pressure and subcooling the remainder.

Emitter: Radiator and/or under floor heating system

Heat Meter: Device for measuring heat pump energy production.

Consists of a precision flowmeter in the circulation fluid and temperature sensors in the heat pump flow and return piping, and calculates heat flow from Q = M × Cp × ΔT (Typically pre-programmed for water, so recalibration needed for glycol solution due to differing Cp and ρ values.)

Heat pump: ASHP or GSHP or WSHP

Device that uses electrical energy to drive a refrigeration cycle, thus transferring heat from a cold source (ambient air, ground, water) to a warm sink (living space) in winter for space heating, and in some designs, the reverse in summer for space cooling.

Hysteresis (aka Deadband)

The change required in a process variable to initiate a change in operation. For example, in a roomstat, the room temperature difference between demand on and demand off signals.

Indoor Unit: Circuit components located inside the building

A compact unit offered by some vendors, typically comprising DHW tank, circulation pump (serving CH emitters and DHW coil, with diverter valves), expansion vessel, and the heat pump controller circuit board(s). Some vendors offer a remote user interface/display, which can be located in a more convenient place, and may incorporate a roomstat.

Operating map: Permissible system parameters programmed into heat pump controller.

For example, the range of ambient temperatures against which the heat pump manufacturer guarantees his specified energy performance.

Outdoor Unit: Refrigeration circuit components located outside the building

The refrigerant circuit comprising compressor, a condenser (the heat exchanger with the CH/DHW circulating fluid), an expansion valve, and an evaporator (the heat exchanger with the heat source fluid – air [ASHP] or ground [GSHP] or water [WSHP]), and for ASHPs the pipework/valving to allow reverse flow for evaporator defrosting.

P-H diagram: Pressure-enthalpy diagram

Phase diagram useful to convey thermodynamic conditions in a cyclic system such as heat pump Outdoor Units.


Weight fraction vaporised in a two-phase fluid (ranges from zero at bubble point to unity at dew point).

Rated output: Performance at off-design conditions

Expected outcome, usually related to Design output through a theoretical or empirical equation. For example, predicted radiator heat output at a ΔT of 20°C (radiator at 40degC, room at 20degC) based on the known (nameplate) heat output at the Design ΔT of 50°C.

Roomstat: A room thermostat

Device for controlling/programming CH. May be wired to the heat pump controller (typically simple hysteresis-driven on-off), wireless (typically TPI control q.v.), or “smart” (TPI plus a learning facility that anticipates heating on-off requirement based on the user’s recent CH programming/setpoint demands)

Setback: The night-time setting of the roomstat

Strategy to reduce heat pump energy consumption at times of lower heat demand.

T-S diagram: Temperature-entropy diagram

Phase diagram useful to convey thermodynamic conditions in a cyclic system such as heat pump Outdoor Units.

Thermal store: A large vessel used to store hot water as a heat source

For example, to provide heat for heat pump defrost cycles or DHW. Typically, charged during low tariff periods or from PV generation for later, more convenient, consumption.

Turndown: Minimum capacity (as a ratio or percentage) of design (maximum) capacity

May be limited by accuracy (e.g. flowmeters), or controllability (e.g. compressor speed due to surge onset), or vendor guarantee (e.g. onset of unpredictable performance).


A circulating fluid reservoir that adds volume to the heat pump circulation inventory primarily to absorb flow and temperature changes between the heat pump and emitters. Normally has two process nozzles, plus drain/ABV.

Water Law: Samsung terminology for weather compensation

Weather compensation: User-enabled option to set a target LWT based on ambient temperature

Typically, controller uses linear interpolation between two user-defined points on the ambient/LWT map to control heat pump output, often by compressor speed control. Intent is to recognise that higher emitter temperature will be required at lower ambient temperature (due to increased heat loss), but is simplistic as it takes no account of wind speed/direction which can affect heat loss significantly.

Zone: Separately controlled section of central heating system

Typically, radiators in house sections (e.g. upstairs/downstairs) or UFH.


A: Heat transfer area (m2)

  • For radiators, face area (height x width), irrespective of any finning.

  • For tank coils, external surface area.

  • For airfin exchangers inclusive of fin area (unless otherwise stated).

  • For PHEs, height x width x no. of plates.

Cp: (Isobaric) specific heat of a substance (kJ/kg/°C)

Used to calculate fluid heat flow where there is no change of phase. Basic equation Q = M × Cp × ΔT (consistent units)

H: Enthalpy (kJ/kg)

Heat content of a fluid.

M or m: Mass flow rate (kg/s)

By convention, capitalised for hot fluid, lower case for cold fluid.

P: Pressure (kPa or bar)

By convention, absolute pressure is used for refrigeration circuits (hence kPaa or bara) whilst gauge pressure is used for emitter circuits (hence kPag or barg).

Q: Heat duty (W or kW)

May be heat generated or transferred or consumed or lost.

S: Entropy (J/K)

Measure of the irreversibility of a process.

U: Overall heat transfer coefficient (W/m2/°C or kW/m2/°C)

Used to predict heat exchanger performance. Basic equation Q = U × A × Δtm (Fourier’s Law).

V or v: Volumetric flow rate (dm3/s or l/min or m3/h)

By convention, capitalised for hot fluid, lower case for cold fluid.

W: Electrical power (W or kW)

ΔT or Δt: Temperature difference (°C)

Referenced fluids should be explicitly defined (e.g. LWT/RWT, or emitter inlet/outlet temp, or mean radiator temp/room temp). By convention, capitalised for hot fluid, lower case for cold fluid.

Δtm: LMTD (°C)

Effective temperature driving force for heat transfer. Varies according to flow regime (countercurrent flow, crossflow, cocurrent flow) and phase (single phase/non-isothermal, or phase change/isothermal for pure components).

ρ: Fluid density (kg/m3)

Used to convert volumetric flow to mass flow (or vice versa).

μ: (Dynamic) fluid viscosity (Pa.s or cP)

Important factor in fluid flow friction loss calculations.