Glossary

Terms

Adjacent conditioned space (ZTC). A building element boundary condition where the element separates two conditioned thermal zones. No fabric heat loss is attributed to ZTC elements because both sides are maintained at controlled temperatures. TP-05, TP-07.

Adjacent unconditioned space (ZTU). A building element boundary condition where the element separates a conditioned zone from an unheated space such as a garage, attic, or corridor. The buffering effect is represented by an additional thermal resistance added to the external surface coefficient rather than by modelling the unconditioned space as a separate zone. TP-03, TP-05, TP-07.

Advanced start. A control feature that activates the heating system before the scheduled heating period begins, allowing the zone to reach the target setpoint by the time the occupant-defined period starts. The look-ahead window is configurable in hours. TP-17.

Air mass. The relative path length of sunlight through the Earth's atmosphere, expressed as a multiple of the shortest path (when the sun is directly overhead). Used in diffuse radiation decomposition via the Perez model. TP-08.

Air permeability. The measured volume of air leaking through the building envelope per hour per unit envelope area at a reference pressure difference (typically 50 Pa), expressed in m3/(h.m2). Used to derive the envelope leakage coefficient for infiltration calculations. TP-06.

Areal heat capacity. The thermal mass of a building element per unit area, expressed in J/(m2.K). It quantifies the amount of energy stored per degree of temperature change per square metre. The value is distributed across the element's node model according to the mass distribution class. TP-05, TP-07.

Backup heater. An auxiliary heating element within or alongside a heat pump system that activates when the heat pump cannot meet the demand, either supplementing (top-up mode) or replacing (substitute mode) the compressor output. TP-12.

Bivalent temperature. The external temperature at which the heat pump's capacity equals the building's heat demand. Below this point, a backup heater may be required. Corresponds to test condition F in the EN 14825 dataset. TP-12.

Brightness coefficient. A pair of dimensionless parameters ($F_1$, $F_2$) used in the Perez anisotropic sky model to decompose diffuse solar radiation into circumsolar, sky dome, and horizon brightening components. Values are selected from lookup tables based on the clearness parameter. TP-08.

Building element. A discrete section of the thermal envelope, such as a wall, roof, floor, window, or door. Each element is characterised by its area, pitch, orientation, thermal resistance, U-value, and mass distribution class, and is represented by a discretised node model in the heat balance calculation. TP-04, TP-05, TP-07, TP-08.

Carnot CoP. The theoretical maximum coefficient of performance for a heat pump operating between a source and sink temperature, calculated as $T_{sink}/(T_{sink}-T_{source})$ in absolute temperature units. Used as a reference for the exergetic efficiency method. TP-12.

Charge control. A control type governing the charging of thermal storage devices (storage heaters and heat batteries). It determines the target charge level at each timestep, influenced by the control logic, room temperature, and external conditions. TP-13, TP-15, TP-17.

Circumsolar radiation. The component of diffuse solar radiation concentrated in the sky region immediately surrounding the solar disc. Modelled as part of the Perez anisotropic sky model and reassigned to the direct radiation component for shading calculations. TP-08.

Clearness parameter. A dimensionless index ($\epsilon$) characterising the sky condition, calculated from the ratio of direct and diffuse radiation. Used to select brightness coefficients in the Perez model. Values near 1 indicate overcast skies; values above 6 indicate clear skies. TP-08.

Coefficient of performance (CoP). The ratio of useful thermal energy output to electrical energy input for a heat pump. A CoP of 3 means 3 kWh of heat is delivered for every 1 kWh of electricity consumed. The value varies with source temperature, sink temperature, and part-load conditions. TP-12.

Cold water temperature. The temperature of mains water entering the dwelling's hot water system, varying with time of year. It affects the energy required to heat water to the target delivery temperature for all draw-off event types. TP-09, TP-11.

Combi boiler. A boiler that provides both space heating and instantaneous domestic hot water without a separate storage cylinder. Additional losses arise from rejected energy during draw-off and from keep-hot or storage facilities. TP-14.

Convective fraction. The proportion of a heat gain or emitter output delivered directly to the zone air node, as opposed to the radiative fraction distributed across internal surfaces. Default values from BS EN ISO 52016-1:2017 are 0.4 for internal gains, 0.1 for solar gains, and a system-dependent value for heating output. TP-04, TP-07, TP-13, TP-16.

Cool-down loss. The thermal energy released when the contents of a pipe or emitter body cool from the operating temperature to the surrounding ambient temperature after flow ceases. Calculated from the fluid volume, volumetric heat capacity, and temperature difference. TP-09, TP-10, TP-16.

Cross-ventilation. A condition where airflow can pass through a dwelling from one facade to an opposing facade. Cross-ventilation increases the wind pressure coefficients applied to envelope openings, enhancing natural ventilation rates. TP-06.

DAHPSE method. The heat pump performance calculation methodology (CALCM-01) used in SAP 10.2, based on a draft of BS EN 15316-4-2:2017. It derives operating CoP and capacity from EN 14825 test data using exergetic efficiency ratios and Carnot CoP interpolation. TP-12.

Daylight correction factor. A multiplicative factor applied to lighting demand profiles to account for available natural daylight. It is influenced by glazing area, orientation, and solar conditions at each timestep. TP-02.

Decile banding. A method used in DHW event generation whereby the daily hot water volume places the dwelling into one of ten consumption bands, each with its own probability distributions for event counts, volumes, and timings. TP-02.

Degradation coefficient. A dimensionless factor (typically between 0.9 and 1.0 for water-sink systems) that quantifies the reduction in heat pump CoP during on/off cycling due to transient start-up and shut-down losses. Derived from EN 14825 test data. TP-12.

Degree hours. The product of time (in hours) and the temperature difference between a base temperature and the external air temperature, summed over a period. Used by HHRSH storage heater controllers to estimate next-day heating demand from past and forecast temperature data. TP-13, TP-17.

Design flow temperature. The maximum (design condition) flow temperature in a wet heating circuit, used as the nominal condition for emitter sizing and heat pump test data interpolation. Common values are 35, 45, 55, and 65 degC. TP-12, TP-16.

Design temperature difference. The temperature drop across the emitter system (flow temperature minus return temperature) at design conditions. Typical values are 5 K for underfloor heating and 10 K or more for radiators. TP-16.

Diffuse horizontal radiation. The component of solar radiation reaching the ground from the entire sky hemisphere (excluding the direct beam), measured on a horizontal surface. Provided as an hourly time series in the weather file. TP-01, TP-08.

Direct beam radiation. Solar radiation arriving in a straight line from the sun, measured perpendicular to the beam (normal incidence). Provided as an hourly time series in the weather file, with an optional conversion from horizontal to normal incidence. TP-01, TP-08.

Discharge coefficient. A dimensionless factor (typically 0.6 to 0.67) applied to theoretical airflow through an opening to account for flow contraction and friction losses. Used for windows, vents, and air terminal devices in the ventilation calculation. TP-06.

Draw-off. A discrete hot water usage event such as a shower, bath, or basin tap use. Each event is characterised by a start time, duration or volume, flow rate, and target delivery temperature. TP-02, TP-09.

Dwelling Emission Rate (DER). The annual CO2 emissions per unit floor area for the actual dwelling, in kgCO2/m2. Calculated from the regulated energy consumption, applying fuel-specific emissions factors. TP-02.

Dwelling Primary Energy Rate (DPER). The annual primary energy consumption per unit floor area for the actual dwelling, in kWh/m2. Calculated from regulated energy consumption using fuel-specific primary energy factors. TP-02.

EBV (Energy Balance Validation). An efficiency methodology for boilers that derives a gross efficiency curve as a function of return water temperature. The curve is calibrated against tested full-load and part-load efficiencies and adjusted for cycling and location effects. TP-14.

Ecodesign control class. A classification system (classes I through VIII) from the Ecodesign Regulation that categorises heating controls by their capability, ranging from simple on/off room thermostats to weather-compensated modulating controllers with room sensors. TP-16.

Emitter exponent. The exponent $n$ in the power-law characteristic equation of a heat emitter, relating heat output to the temperature difference between the emitter body and the room air. For radiators, typical values range from 1.2 to 1.4 (per BS EN 442). For underfloor heating, $n=1$. TP-16.

Equation of time. A correction factor (in minutes) that accounts for the eccentricity of the Earth's orbit and axial tilt, used to convert clock time to solar time. Expressed as a piecewise function of the day of the year. TP-08.

Exergetic efficiency. The ratio of measured CoP to the theoretical Carnot CoP at the same operating temperatures. Used in the DAHPSE method to interpolate heat pump performance between test conditions. TP-12.

External conditions. The set of boundary conditions that drive the heat balance at each timestep, including air temperature, wind speed, wind direction, sky temperature, ground temperature, and solar radiation. TP-01, TP-03.

Fabric Energy Efficiency (FEE). An assessment metric that isolates the thermal performance of the building fabric by substituting idealised heating, cooling, and hot water systems. Expressed in kWh/(m2.year) and calculated as the sum of space heating and cooling demands normalised by total floor area. TP-02.

Fabric heat loss. The rate of heat transfer through the thermal envelope of a dwelling, expressed as a heat loss coefficient in W/K. It is the sum of contributions from all building elements (walls, roofs, floors, windows, and doors) based on their U-values and areas. TP-05.

Flow exponent. The exponent in the pressure-flow relationship for an airflow path. Values are 0.5 for windows and vents (orifice flow), and 0.667 for envelope leaks (intermediate between orifice and laminar flow). TP-06.

Flow temperature. The temperature of water entering the heat emitters from the heat generator, before any heat is released to the zone. May be fixed or modulated by weather compensation. TP-12, TP-14, TP-16.

Frame factor. The ratio of frame area to total window area ($F_f$). The glazed area through which solar radiation is transmitted equals the total window area multiplied by $(1-F_f)$. TP-05, TP-08.

Free-floating temperature. The operative temperature a zone would reach with zero heating or cooling input, calculated by solving the heat balance with ${\Phi }_{hc}=0$. Used to determine whether heating or cooling is required by comparison with the setpoint. TP-04.

g-value. The total solar energy transmittance of a glazing system, representing the fraction of incident solar energy that passes through the glazing into the zone interior (as both directly transmitted radiation and secondary heat emission from the glass). TP-05, TP-08.

Ground reflectivity. The fraction of solar radiation reflected by the ground surface towards building elements and PV panels, used in the ground-reflected component of diffuse irradiance. The default value is 0.2. TP-01, TP-08.

Ground temperature (virtual). An effective external temperature for ground floor elements derived from the periodic heat transfer method of BS EN ISO 13370:2017. It accounts for both steady-state and seasonal periodic ground heat flow. TP-03.

Heat balance. The simultaneous energy equation solved at each timestep for all thermal nodes in a zone. It accounts for fabric conduction, ventilation, solar gains, internal gains, thermal mass, and heating or cooling input. Formulated as a matrix equation $\mathbf{A}\cdot \mathbf{X}=\mathbf{B}$. TP-04, TP-07.

Heat battery. A thermal storage device that stores energy in phase change materials and releases it on demand for space heating and hot water. It charges electrically and discharges via a water circuit through an internal heat exchanger. TP-15.

Heat retention ratio. The fraction of stored energy remaining in a storage heater after 16 hours of standby at minimum output, derived from BS EN 60531. A value of 0.45 or greater qualifies the unit as a high heat retention storage heater. TP-13.

Horizon brightening. A component of diffuse solar radiation concentrated near the horizon, modelled in the Perez anisotropic sky model. It contributes to diffuse irradiance on tilted surfaces via the $F_2$ brightness coefficient. TP-08.

Immersion heater. A direct resistive electric heating element installed inside a hot water storage cylinder. It converts electrical energy to heat at effectively 100% efficiency and is positioned at a specific height within the tank. TP-11.

Infiltration. The uncontrolled movement of outdoor air into a dwelling through gaps, cracks, and imperfections in the building envelope. Quantified via pressurisation test data and modelled using pressure-dependent flow equations at notional leak paths. TP-06.

Internal gains. Heat released into the dwelling from occupants (metabolic), lighting, appliances, cooking, hot water use, and distribution system losses. Each source has a convective/radiative split that determines how the energy enters the zone heat balance. TP-02, TP-04.

Inverter efficiency. The ratio of AC power output to DC power input for a PV system inverter, varying with load fraction. Modelled as the minimum of three empirical curves fitted to residential-scale inverter performance data. TP-18.

Leakage coefficient. A derived parameter relating the volume flow rate through envelope leaks to the pressure difference, using the formula $q_v=C_{leak}\cdot |\Delta p{|}^{n_{leak}}$. Calculated from the pressurisation test result and distributed between facades and roof in proportion to their areas. TP-06.

Linear thermal transmittance ($\psi$-value). The rate of heat flow per unit length per degree of temperature difference through a linear thermal bridge (e.g. a wall-floor junction or window reveal), expressed in W/(mK). TP-05.

Load ratio. The fraction of a timestep during which a heat pump or boiler operates, calculated as the running time divided by the timestep duration. Determines whether the system runs continuously or cycles on and off. TP-12, TP-14.

Load shifting. The practice of moving the timing of appliance energy consumption (e.g. dishwasher, washing machine) to periods of lower cost or higher on-site generation availability. Implemented by ranking candidate timesteps using a weighting time series. TP-04, TP-17.

Mass balance. The requirement that the total mass of air entering a ventilation zone equals the total mass leaving it at each timestep. Solved iteratively to determine the internal reference pressure using Brent's method. TP-06.

Mass distribution class. A classification (I, E, IE, D, or M) describing where the thermal mass is located within a building element's construction. It determines how the areal heat capacity is distributed across the nodes of the element model. TP-05, TP-07.

Mean radiant temperature. The area-weighted average of the internal surface temperatures of all building elements in a zone. Combined with the air temperature to form the operative temperature. TP-03, TP-04, TP-07.

Minimum modulation ratio. The lowest firing rate of a modulating boiler, expressed as a fraction of rated output power. Below this ratio, the boiler cycles on and off rather than continuously modulating. TP-14.

Net-to-gross conversion factor. A fuel-specific factor used to convert between net (lower) and gross (higher) calorific values. Values are 0.901 for mains gas and 0.921 for LPG. TP-14.

Node model. A discretised representation of a building element's thermal structure. Opaque elements use five nodes, transparent elements use two nodes, and ground floor elements use a 3+2 arrangement. Nodes are connected by inter-node conductances and carry thermal capacities that enter the zone heat balance matrix. TP-05, TP-07.

Notional dwelling. A reference building with the same geometry as the actual dwelling but prescribed fabric performance, systems, and controls. Used to derive the Target Emission Rate and Target Primary Energy Rate against which the actual dwelling is compared. TP-02.

Operative temperature. The arithmetic mean of the zone internal air temperature and the mean radiant temperature, used as the basis for setpoint comparison in the heating and cooling demand calculation. TP-03, TP-04, TP-07.

Perez model. An anisotropic sky model that decomposes diffuse solar radiation into three components (sky dome, circumsolar, and horizon brightening) using empirically derived brightness coefficients. Used to calculate diffuse irradiance on tilted surfaces. TP-08.

Phase change material (PCM). A substance that stores and releases large amounts of thermal energy during its solid-liquid phase transition. Used in heat batteries to provide high energy density at a near-constant temperature. TP-15.

Pitch. The tilt angle of a building element or PV panel from the horizontal, measured in degrees. A pitch of 0 degrees denotes a horizontal surface facing upward; 90 degrees denotes a vertical surface; 180 degrees denotes a horizontal surface facing downward. TP-03, TP-05, TP-08, TP-18.

Point thermal bridge. A localised discontinuity in the thermal envelope (e.g. a fixing penetrating insulation) characterised by a single heat loss coefficient in W/K, as distinct from a linear thermal bridge measured per unit length. TP-05.

Primary energy factor. A fuel-specific multiplier converting delivered energy to primary energy, accounting for upstream generation, transmission, and distribution losses. Electricity uses time-varying half-hourly factors; other fuels use constant annual factors. TP-02.

Primary pipework. The pipework connecting a heat generator (boiler or heat pump) to a hot water storage cylinder. It carries water at flow temperature and incurs both steady-state conductive losses and cool-down losses between heating events. TP-10, TP-11.

PV diverter. A device that redirects surplus photovoltaic generation to an immersion heater in a hot water cylinder rather than exporting it to the grid, increasing self-consumption. TP-11, TP-18.

Radiative fraction. The proportion of a heat gain or emitter output distributed across internal surface nodes in the zone, as opposed to the convective fraction delivered to the air node. Equal to $(1-f_{conv})$. TP-04, TP-07.

Return temperature. The temperature of water leaving the heat emitters and returning to the heat generator. It affects heat pump CoP and boiler efficiency. Calculated iteratively from the flow temperature, emitter characteristics, and zone air temperature. TP-12, TP-14, TP-16.

Roughness coefficient. A dimensionless correction factor ($C_R$) that adjusts meteorological wind speed for the aerodynamic roughness of the terrain surrounding the building site. Determined by terrain class and height above ground. TP-03, TP-06.

Roughness length. A parameter ($z_0$) characterising the aerodynamic roughness of a terrain type, expressed in metres. Ranges from 0.01 m for open water to 1.0 m for dense urban areas. TP-03, TP-06.

Runge-Kutta method (RK45). An adaptive numerical integration scheme used to solve ordinary differential equations governing emitter thermal dynamics and storage heater state of charge evolution. It adjusts the internal step size to maintain accuracy. TP-13, TP-16.

Setback temperature. A reduced heating setpoint applied outside the main heating periods to prevent the dwelling from cooling excessively. When specified, the heating system maintains this minimum temperature during off-periods rather than switching off entirely. TP-02, TP-17.

Setpoint temperature. The target temperature for a thermal zone, as determined by the heating or cooling control schedule. The zone heat balance determines whether heating or cooling energy is required to maintain this temperature. TP-02, TP-04, TP-17.

Shading reduction factor. A dimensionless factor (0 to 1) quantifying the fraction of solar radiation that reaches a surface after accounting for obstruction by shading objects. Separate factors are calculated for direct ($F_{sh,dir}$) and diffuse ($F_{sh,dif}$) radiation. TP-08.

Shelter factor. See shield class.

Shield class. A classification (Open, Normal, or Shielded) describing the exposure of a building to wind, used to select wind pressure coefficients for ventilation and infiltration calculations. TP-06.

Simulation timestep. The fixed time interval at which the model advances, typically 0.5 hours (30 minutes). All calculations, including the heat balance, ventilation, and plant models, are evaluated at this resolution. TP-01.

Sky view factor. The fraction of the hemisphere above a surface that is occupied by sky rather than ground, calculated from the element pitch as $(1+\cos \beta )/2$. It determines the longwave thermal radiation exchange between the surface and the sky. TP-03, TP-08.

Solar absorption coefficient. The fraction of incident solar radiation absorbed by the external surface of an opaque building element, with the remainder reflected. Applies at the external surface node in the heat balance. TP-04, TP-08.

Solar altitude. The angle between the solar beam and the horizontal plane, calculated from the solar declination, latitude, and hour angle. When the sun is below the horizon, the solar altitude is zero. TP-08.

Solar azimuth. The horizontal angle of the sun measured from south, with east positive and west negative. Used alongside solar altitude to determine the angle of incidence on tilted surfaces and facade direction for wind pressure coefficients. TP-08.

Solar declination. The angle between the plane of the Earth's equator and a line from the centre of the Earth to the centre of the sun. It varies throughout the year and determines the path of the sun across the sky. TP-08.

Solar time. Clock time corrected for the equation of time and the difference between the site longitude and the standard meridian for the time zone. Used to calculate the solar hour angle. TP-08.

Solar transmittance. See g-value.

Specific fan power (SFP). The electrical power consumed by a ventilation fan per unit of airflow rate, expressed in W/(l/s). It includes in-use factors and is used to calculate fan energy consumption. TP-02, TP-06.

Standing loss. The heat lost from a hot water storage cylinder to its surroundings when no draw-off is occurring, derived from the manufacturer-declared 24-hour heat loss at reference test conditions per BS EN 12897:2016. TP-11.

State of charge (SOC). A dimensionless quantity (0 to 1) representing the fraction of a storage device's capacity currently held as stored energy. Used for storage heaters (thermal), heat batteries (thermal), and electrical batteries. TP-13, TP-15, TP-18.

State of health (SoH). A dimensionless factor (0 to 1) representing the remaining capacity of an electrical battery relative to its nominal capacity, degrading linearly with age at 4% per year. TP-18.

Stratification (tank). The vertical temperature layering within a hot water storage cylinder, with hotter water at the top and cooler water at the bottom. Modelled by dividing the tank into equal-volume horizontal layers, with a mixing algorithm enforcing monotonically non-decreasing temperatures from bottom to top. TP-11.

Surface resistance. The thermal resistance at the interface between a building element surface and the adjacent air (internal or external), accounting for convective and radiative heat transfer. Internal surface resistance depends on the direction of heat flow. TP-05.

Target Emission Rate (TER). The annual CO2 emissions per unit floor area for the notional dwelling, in kgCO2/m2. The actual dwelling's DER must not exceed the TER. TP-02.

Target Primary Energy Rate (TPER). The annual primary energy consumption per unit floor area for the notional dwelling, in kWh/m2. The actual dwelling's DPER must not exceed the TPER. TP-02.

Temperature spread correction. A multiplicative factor applied to the heat pump CoP to account for the difference between the emitter temperature spread in operation and the temperature spread under standard test conditions. TP-12.

Terrain class. A classification of the land surrounding a building site (OpenWater, OpenField, Suburban, or Urban), characterising its aerodynamic roughness. It determines the terrain factor, roughness length, and minimum height used in wind speed correction. TP-03, TP-06.

Thermal bridge. A localised area or junction in the building envelope where the insulation layer is interrupted, creating a path of increased heat flow. Modelled as either linear (per unit length, in W/(mK)) or point (in W/K) contributions to the zone heat loss. TP-05.

Thermal mass. The capacity of a building element to store and release heat, characterised by its areal heat capacity. High thermal mass dampens temperature swings and shifts peak heating loads. TP-07.

Thermal resistance. The resistance of a material layer or surface to heat flow, expressed in m2.K/W for planar elements or K.m/W for cylindrical elements (pipes, ducts). The reciprocal of thermal resistance gives the heat transfer coefficient. TP-05, TP-10.

Thermal time constant. A parameter characterising the rate at which a system responds to changes in boundary conditions. For storage heaters, it governs the on/off cycling penalty. For the warm-up procedure, it determines convergence speed. TP-07, TP-13.

Thermal zone. A discrete volume within a dwelling that is treated as a single well-mixed air space in the heat balance calculation. Typical zones are "living room" and "rest of dwelling". Each zone has its own operative temperature, setpoint, and heat balance matrix. TP-02, TP-04.

Total floor area (TFA). The sum of all zone floor areas in a dwelling, in m2. Used as a normalisation factor for emissions, primary energy, and fabric energy efficiency metrics. TP-02.

U-value. The overall thermal transmittance of a building element, expressed in W/(m2.K). It represents the steady-state rate of heat transfer per unit area per degree of temperature difference between inside and outside. Derived from the total construction resistance and surface resistances. TP-05.

Ventilation cooling. The use of increased natural ventilation (e.g. opening windows) to reduce the zone operative temperature when it exceeds the cooling setpoint, before resorting to mechanical cooling. TP-04, TP-06.

Ventilation effectiveness. A dimensionless factor ($E_v$) representing how well ventilation air mixes with zone air. A value of 1.0 (perfect mixing) is assumed for residential single-zone models. TP-06.

Ventilation heat loss coefficient ($H_{ve}$). The rate of heat loss through ventilation per degree of temperature difference between the zone air and the external air, expressed in W/K. Calculated from the effective air change rate, zone volume, air density, and specific heat capacity of air. TP-04, TP-06.

Virtual ground temperature. See ground temperature (virtual).

Warm-up procedure. An initialisation step performed before the first simulation timestep. All node temperatures are iterated under steady-state conditions using a large timestep (one full year) until convergence within a relative tolerance of $10^{-8}$, establishing realistic starting temperatures. TP-04, TP-07.

Weather compensation. A flow temperature control strategy (Ecodesign classes II, III, VI, VII) that modulates the heating circuit flow temperature based on the outdoor air temperature. As external temperature rises, the flow temperature is reduced to improve heat pump CoP or boiler efficiency. TP-02, TP-16.

Wind pressure coefficient ($C_p$). A dimensionless factor relating wind dynamic pressure to the static pressure on a building facade, dependent on facade direction (windward, leeward, or neither), height, shield class, and whether cross-ventilation is possible. TP-06.

Zone. See thermal zone.

Acronyms