How Heatworx calculates heat loss
Heatworx takes the survey data you provide — room geometry, construction assumptions, ventilation evidence, temperatures — and runs a structured calculation for every room. This page explains the chain from survey to result, step by step.
The aim is transparency. If you understand how the number is built, you can judge which inputs matter most and where the uncertainty lies.
The calculation chain: survey to result
Every room follows the same sequence:
- Geometry — room dimensions and surface areas from a scan or manual entry
- Fabric heat loss — each surface: area times U-value times temperature difference
- Ventilation heat loss — air movement through the room, driven by survey evidence
- Total room heat loss — fabric plus ventilation, with an intermittent heating allowance if applicable
- Emitter comparison — corrected radiator output versus room heat loss
- Whole-house summary — all rooms summed, grouped by heating zone
Each step feeds the next. Change a wall construction assumption and the fabric heat loss updates, the total updates, and the emitter comparison updates — all the way through.
Room geometry and surface areas
The calculation starts with the physical shape of each room. Heatworx builds a room model from either a room scan or manual entry, producing wall lengths, wall heights, floor area, ceiling area, and the positions and sizes of windows and doors.
Each wall segment is classified: external wall, party wall, internal partition, or wall adjoining an unheated space. Only surfaces that separate the heated room from a colder space contribute to heat loss. A party wall shared with a heated neighbour at the same temperature loses no heat and is excluded.
Windows and doors are subtracted from wall areas. The net wall area — the solid part — gets the wall U-value; the glazed or door area gets its own U-value. Getting the geometry right matters because area is a direct multiplier in every fabric heat loss calculation.
Fabric heat loss: surface by surface
For each surface that separates the room from a colder space, Heatworx calculates:
Formula
Surface heat loss (W) = area (m²) × U-value (W/m²K) × temperature difference (K)
The temperature difference is the gap between the room's target temperature and the temperature on the other side — the design outside temperature for external surfaces, or an assumed temperature for unheated spaces like garages or lofts.
The U-value comes from the construction assumptions assigned to each surface. An uninsulated solid brick wall might have a U-value of 2.1 W/m²K. A modern insulated cavity wall might be 0.3 W/m²K. The difference in heat loss is roughly seven-fold for the same area and temperature difference.
The total fabric heat loss for the room is the sum of all individual surface losses — every wall segment, window, door, floor and ceiling.
Ventilation heat loss
Warm air escapes through gaps, vents, extract fans and general air leakage. Cold outside air replaces it, and the heating system has to warm that replacement air. The rate of heat loss depends on how much air flows through the room and the temperature difference.
Formula
Ventilation heat loss (W) = 0.34 × ventilation rate (m³/h) × temperature difference (K)
The 0.34 factor comes from the volumetric heat capacity of air. The ventilation rate is estimated from survey evidence — draughty windows, background vents, suspended floors, extract fans, airtightness — rather than taken from a single fixed assumption. In a leaky older house, ventilation losses can account for a third or more of the total.
Total room heat loss
The total design heat loss for a room combines both mechanisms:
Total room heat loss = fabric heat loss + ventilation heat loss
If the room is not heated continuously — and most domestic rooms are not — an intermittent heating allowance is added. This accounts for the extra capacity needed to bring the room back up to temperature after the heating has been off overnight or during the day. The size of the allowance depends on how long the heating is off, the building's thermal mass and its insulation level.
Heatworx calculates two variants: design heat loss using the outdoor design temperature (a cold day), and typical heat loss using a reference average temperature. Both are needed — design heat loss for emitter sizing, typical heat loss for assessing heat generator modulation range.
Emitter comparison
Once the room's heat loss is known, the next question is: can the installed radiator (or other emitter) deliver enough output to meet it?
Catalogue output is corrected to the system's actual flow and return temperatures. A radiator rated at 1,200 W at Delta T 50 might only deliver 650 W at the lower flow temperature of a heat pump. Heatworx applies this correction automatically and compares the result against the room's design heat loss.
The comparison shows whether the emitter is undersized (output less than heat loss), broadly matched, or oversized. This is the point where the heat loss calculation becomes practically useful — it tells you whether each room will actually be warm enough.
Whole-house and zone summaries
The room-level results are summed to produce whole-house totals, grouped by heating zone where zones are defined. This gives the total design heat loss for the dwelling — the figure used when considering boiler or heat pump capacity.
But the whole-house figure alone is not enough. A house with a total heat loss of 8 kW might have one room that is severely under-emitted and another that is heavily over-emitted. Only the room-by-room breakdown reveals those problems.
What Heatworx does not claim to do
Heatworx does not claim to measure the true real-world heat loss of a building. It produces a structured design estimate based on survey inputs, geometry, construction assumptions, design temperatures and recognised heat loss calculation principles.
It is not a full heating system design tool. It does not select pipe sizes, specify controls, design hot water systems, or replace the judgement of a qualified heating engineer. It does not perform dynamic thermal modelling or predict annual energy consumption.
What it does do is give you a transparent, editable, room-by-room heat loss estimate with emitter comparisons — the foundation that any competent heating design needs to start from.
Heatworx helps produce and review a structured heat loss estimate. It does not replace professional judgement, manufacturer requirements, regulatory obligations, or a full heating system design where one is required.
Frequently asked questions
What calculation method does Heatworx use?
Heatworx presents heat loss calculations in a transparent, room-by-room format aligned with the kind of inputs used in recognised UK domestic heating design guidance. Each room's fabric heat loss is calculated surface by surface (area times U-value times temperature difference), ventilation heat loss is derived from survey evidence, and emitter output is corrected to the system's actual flow and return temperatures.
Does Heatworx follow CIBSE?
Heatworx is designed around recognised UK domestic heat loss calculation principles, including CIBSE-informed assumptions where appropriate. It does not claim full conformance to a specific edition of any standard. The calculation inputs, formulas and structure are consistent with the approach described in domestic heating design guidance, but some inputs are simplified or estimated from survey evidence rather than taken from full engineering datasets.
How does Heatworx handle rooms it can't scan?
If a room cannot be scanned, dimensions can be entered manually. The calculation works the same way — the room model is built from the entered dimensions rather than from a scan. The key inputs (wall lengths, heights, window and door sizes, construction assumptions) are all editable regardless of how the room was initially captured.
Does Heatworx design a full heating system?
No. Heatworx produces room-by-room heat loss estimates and emitter comparisons. It does not design pipework, select controls, specify the heat generator, or produce a full heating system design. The heat loss estimate is one essential input to that process, but system design requires professional judgement and site-specific decisions beyond what a survey app can provide.