The calculation,
explained

Heat loss is a function of temperature difference — the greater the gap between inside and outside, the more heat flows through the building fabric. The outdoor design temperature is the lowest temperature the system must be capable of meeting at full output, representing an extreme but not exceptional cold spell for the location.

OpenHeatLoss uses CIBSE DHDG 2026 regional design temperatures for the UK, automatically selected by postcode area. These are the outdoor dry-bulb temperatures at which the heating system must be able to maintain internal design conditions without supplementary heat. The reference temperature (Te,ref) — used for the typical load calculation — is the annual mean outdoor temperature for the same region.

Internal design temperatures are set per room. Living spaces are typically designed to 21°C, bedrooms to 18°C, bathrooms to 22°C — though these are designer inputs, not fixed values. The temperature difference driving fabric heat loss (ΔT) is the difference between the room's internal temperature and the outdoor design temperature.

Fabric heat loss is the heat conducted through every element of the building envelope — walls, windows, doors, roof, and floor. It is calculated element by element for each room and then summed.

For each element, the area is the net area — window and door areas are deducted from the gross wall area they penetrate, so they are not double-counted. Each element has its own U-value, drawn from the RdSAP10 library or entered directly by the engineer.

The temperature difference used is normally the full room ΔT (internal temperature minus outdoor design temperature). For elements that adjoin unheated spaces — an unheated garage, a loft above an insulated ceiling, a party wall to an unoccupied dwelling — the designer specifies an assumed adjacent temperature, and the ΔT is calculated accordingly. This keeps the assumption explicit and auditable rather than hidden inside a temperature factor.

Thermal bridging occurs at junctions between elements — around window frames, at floor-to-wall junctions, at structural members — where heat flows more readily than through the centre of the element. In an existing dwelling, these bridges are difficult to quantify individually.

The CIBSE DHDG 2026 reduced method handles this with a blanket addition to each element's U-value (ΔU), selected per room from a lookup table based on construction type and insulation specification:

The default in OpenHeatLoss is 0.10 W/m²·K, appropriate for typical existing UK domestic construction. This can be adjusted per room. The thermal bridging addition is shown explicitly in the element table and in the PDF output — it is not folded invisibly into the U-value.

Ground floors behave differently from other elements because heat loss is not simply to outdoor air — it is through the ground, which has significant thermal mass and remains at a temperature closer to the annual mean than the winter design temperature. The steady-state heat loss therefore uses the annual mean outdoor temperature (Te,ref) rather than the outdoor design temperature, for both design and typical load calculations.

OpenHeatLoss distinguishes between two ground floor types: slab on ground, which uses Te,ref in both calculation columns, and suspended timber floor, which loses heat to a ventilated void at approximately outdoor design temperature and is treated as a normal external element.

The U-value for a ground floor slab is calculated using the BS EN ISO 13370 method, which accounts for floor dimensions (the B' characteristic dimension), edge insulation, and construction. A built-in calculator walks through this for the common case. Certified U-values from a building specification can be entered directly where available.

Ventilation heat loss is the heat carried out of the building by air movement — infiltration through the fabric, background ventilation through trickle vents and flues, and mechanical ventilation systems. Unlike fabric loss, which is calculated purely from element areas and U-values, the ventilation calculation begins with the whole-building air permeability and works down to individual rooms.

This is a meaningful difference from older methods. The CIBSE DHDG 2026 reduced method does not use assumed air change rates per room type — it builds up an estimated air permeability from physical characteristics of the building and distributes infiltration to rooms via their exposed envelope area. This produces more accurate results for well-sealed buildings and avoids the over-estimation that simple assumed-ACH approaches produce.

Stage 1 — Whole-building air permeability (q50)

If a blower door or pulse test has been carried out, the measured air permeability at 50 Pa (m³/h·m²) is entered directly. Where no test has been done, the SAP 10.2 estimation method builds up q50 from four additive components:

Stage 2 — Room leakage rate

The whole-building q50 is distributed to individual rooms via their exposed envelope area — the total area of external walls, walls adjoining unheated spaces, the floor area for rooms with a suspended ground floor, and the ceiling area for top-storey rooms. Background ventilation additions are made for trickle vents (54 m³/h each), intermittent extract fans (54 m³/h), and disused flues (109–435 m³/h depending on diameter). The result is then converted from a 50 Pa leakage rate to a leakage rate at typical pressure using a second shielding/height factor (CIBSE DHDG 2026 Table 2-17), giving the approximate room leakage rate in m³/h.

A minimum room leakage rate check is also calculated — if the approximate leakage rate falls below 0.5 ACH × room volume for habitable rooms, the tool flags that ventilation provision should be reviewed.

Stage 3 — Temperature-weighted factors

The room leakage rate is multiplied by both the design temperature difference and the reference temperature difference to produce design and typical temperature-weighted leakage factors. Where a continuous mechanical ventilation system is present, the supply rate is similarly weighted by the appropriate temperature difference (adjusted for heat recovery efficiency where applicable) to produce continuous ventilation factors.

Stage 4 — Three distinct heat loss outputs

The final step produces three separate figures from the leakage and continuous ventilation factors, each serving a different purpose in the design:

The distinction between the emitter sizing figure and the generator sizing component is significant. Emitters must cope with wind-driven infiltration from the worst exposed direction — hence the orientation factor of 2 on the leakage component. The generator serves the whole building simultaneously and uses average infiltration without the orientation uplift. In a naturally ventilated building, emitter ventilation heat loss can be substantially higher than the generator component for the same room.

The design load — the figure used to size the heat pump — represents the building's heat loss at the outdoor design temperature. This is a worst-case figure, representing a cold but not exceptional day. In practice, the heat pump operates at this output for only a small fraction of the heating season.

The typical load is the heat loss at the annual mean outdoor temperature (Te,ref). It represents the average demand across the heating season and is used to check the heat pump's minimum modulation capability — a heat pump that cannot modulate down to the typical load will short-cycle in mild weather, reducing efficiency and increasing wear.

The typical load calculation uses the same element areas and U-values as the design load, but with Te,ref substituted for the outdoor design temperature. Ground floor elements use Te,ref in both columns. Elements with fixed assumed boundary temperatures — party walls, unheated space boundaries — use the same assumed ΔT in both columns, since the adjacent condition does not vary with outdoor temperature.

Once the room heat loss is established, the emitters — radiators or underfloor heating — must be capable of delivering that heat output at the chosen flow and return temperatures. Heat pumps typically run at lower flow temperatures than gas boilers (35–55°C rather than 70–80°C), so existing radiators may be undersized for heat pump operation even if they were adequate for the boiler.

Radiators are sized using the EN 442 exponent method. A radiator's rated output is quoted at a standard condition (ΔT50 — mean water temperature 75°C, room at 20°C, giving a mean ΔT of 50K). At lower flow temperatures, output falls according to:

The n-exponent varies by radiator model and construction — panel radiators typically fall between 1.24 and 1.33, with lower values for fan-assisted emitters and higher values for traditional column types. The tool currently uses n = 1.3 as a reasonable default for panel radiators, which is appropriate for the majority of UK domestic heat pump installations. Per-model exponent selection from EN 442 test data is on the development roadmap. The radiator schedule shows the rated output, the derated output at design conditions, the room heat loss, and whether the emitter is adequate, marginal, or insufficient.

Underfloor heating output is calculated from EN 1264-2:2021, using the actual pipe spacing, floor construction (screed or timber), screed thickness, and flow and return temperatures. The standard gives a specific thermal output per unit floor area (q, W/m²), which is multiplied by the usable floor area to give total room output. UFH is not sized by flow rate — it is commissioned on temperature difference (ΔT) between flow and return.

The Seasonal Coefficient of Performance (SCOP) is the ratio of heat delivered to electrical energy consumed across the whole heating season. A heat pump with a SCOP of 3.0 delivers 3 kWh of heat for every 1 kWh of electricity consumed, on a seasonal average basis.

OpenHeatLoss estimates SCOP using the EN 14825:2022 Annex C bin hour method. The heating season is divided into outdoor temperature bins (approximately 3,000 heating hours total across the UK climate). For each bin, the heat pump's COP is estimated from EN 14511 manufacturer test points using a Carnot efficiency model — a mean Carnot efficiency η is derived by averaging across the entered test points, then applied bin by bin as COP = η × COP_Carnot(T_outdoor, T_flow). The engineer enters test point data from the manufacturer's datasheet, rather than relying on a model database.

The flow temperature for each bin is derived from the building's W/K coefficient — the ratio of the total heat loss to the temperature difference that drives it. This produces a weather-compensated heating curve specific to the building, not a generic or manufacturer preset. A well-insulated building with a low W/K will operate at lower flow temperatures for more of the season, improving SCOP.

A defrost penalty is applied to bins below 5°C outdoor temperature, scaling in proportion to how far below 5°C each bin falls. DHW contribution — daily reheat and periodic pasteurisation cycles — is included separately and added to the space heating SCOP to produce a whole-system figure.