Room-by-room heat loss using CIBSE Domestic Heating Design Guide 2026 reduced BS EN 12831-1:2017 calculation methods. MCS-compliant design output to MIS 3005-D. Emitter sizing, pipe sizing, SCOP estimation & project management. Open source, for engineers and self-builders alike.
The tool is grounded in the standards MCS and industry leaders require. And the methodology is transparent enough that a technically capable self-builder or DIYer can follow every step. YouTube channel with instruction videos covering all the steps and technical guidance for design decisions.
Produce MCS compliant heat loss calculations and system design without opaque black-box calculations. Every calculation & formula traced back to a fully referenced standard and described method.
Understand exactly what heat pump your house needs — and why — before talking to any installer (or verifying a heat loss you've received). No black boxes.
Heat loss is the foundation to all designs. OpenHeatLoss takes you from survey data through U-values, Room-by-Room Heat loss to emitter sizing, pipe design, and SCOP estimation, complete with the set of MCS documentation — all in an auditable chain.
Room-by-room fabric and ventilation losses to CIBSE Domestic Heat Design Guide 2026 reduced BS EN 12831-1:2017 method. Thermal bridging, ground floor U-values per ISO 13370.
BS EN 12831-1:2017Regional outdoor design temperatures and reference temperatures by UK postcode area. Annual mean and Te,ref to calculate modulation and cycling risk at average temperatures.
CIBSE DHDG 2026244 RdSAP10 U-values across all UK regions and age bands A–M. Wall, roof, and exposed floor. Postcode-based region detection.
RdSAP10 June 2025Radiator sizing via EN 442 exponent method. UFH output per EN 1264-2:2021. Emitter status (new / retained / replace) per room.
EN 442 · EN 1264-2Hazen-Williams calculation across multiple pipe materials. Velocity limits, pressure drop, index circuit identification, pump selection.
MCS MIS 3005-DSeasonal performance from EN 14511 test points and EN 14825 bin hours - in addition to the standard MCS 031 Performance Estimate.
EN 14825:2022Heat loss report, radiator schedule, and pipe sizing report. MCS compliant design output.
MCS MIS 3005-DMCS 020 sound pressure level calculation for ASHP planning applications/permitted development. Integrated with project data.
MCS 020Materials list linked to emitter and pipe schedules. Quote builder with markup, category overrides, and snapshot history for Consumer Code compliance.
MCS QMSEvery formula, every standard reference, every default value is in the public repository. No black boxes, no locked or hidden methodology. Default values are alway over-rideable.
U-values trace to RdSAP10 (BRE/SAP open data). Bin hours trace to EN 14825:2022 Annex C. Every assumption is documented and made explicit.
If a figure looks wrong, you can read the source. The calculation chain from input to PDF is fully inspectable — for you, your client, or an MCS auditor.
Developed by a heating business (Mysa Heating) to address opacity and friction points in existing tools. Open source from the ground up and responsive to user feedback and change requests.
You can read, fork, and self-host. If you build on it, you share back. Network use triggers the licence — keeping derivative tools open.
// CIBSE DHDG 2026 s.2.5.4 — three outputs // Emitter, generator design, generator typical const SPECIFIC_AIR = 0.34; // Wh/m³·K // ×2 orientation factor on leakage — // worst-case wind direction, single room ventEmitter = ((leakageDesign * 2) + contVentDesign) * SPECIFIC_AIR; // No orientation factor — not all rooms // are simultaneously windward ventGeneratorDesign = (leakageDesign + contVentDesign) * SPECIFIC_AIR; // At Te,ref — feeds modulation check ventGeneratorTypical = (leakageTypical + contVentTypical) * SPECIFIC_AIR;
Each step produces an auditable output. The heat loss underpins every downstream decision — emitter sizing, flow temperature, SCOP, and the final MCS documentation.
Enter room dimensions, construction types, U-values, and internal design temperatures. The RdSAP10 library pre-fills U-values by age band and region — override with calculated or certified values where you have them.
Transmission losses calculated element by element, with thermal bridging (ΔU per room), ground floor treatment per ISO 13370, ventilation, building and room air infiltration assessment. Both design and typical loads are produced for maximum load and modulation assessment.
Each room's heat requirement is met by new or retained emitters. Radiators sized via the EN 442 exponent method; UFH via EN 1264-2:2021. The tool flags rooms where retained emitters are insufficient at lower flow temperatures.
Circuit-by-circuit Hazen-Williams calculation across copper, MLCP, or barrier pipe. Velocity limits enforced per material. Pressure drop and pump head derived from the index circuit.
Seasonal performance estimated from EN 14511 manufacturer test points and EN 14825 Annex C bin hours. The weather-compensated heating curve is derived from the building's W/K coefficient — building-specific, not a manufacturer preset. This is in addition to standard MCS 031 Performance Estimate.
Heat loss report, radiator schedule, and pipe sizing report generated as PDFs. MCS 020 noise assessment and MCS 031 Performance Estimate completed within the same project.
No subscription to start. Create an account to save projects, generate PDFs, and access company features. The full calculation is free for as long as we can make it work.
Heating engineer? Multi-project access is free during beta — email to request access