Heat Loss & Heat Pump Calculator

What it works out

The calculator follows a heat pump design from the building to the bill, in twelve steps. Each step uses the results of the ones before it, and every value can be changed, from U-values and design temperatures to your own heat pump datasheet and radiator catalogue outputs.

1. 01

   Property

   The postcode sets the climate location and its design outdoor temperature, from the CIBSE Guide A values the MCS heat load calculator uses, adjusted for altitude. Age band, EPC and ventilation fill in the defaults.

2. 02

   Fabric

   U-values for walls, floors, roofs, windows and doors, from about 80 typical UK constructions, your own figures, or a layer-by-layer calculator.

3. 03

   Floor plan

   Each floor drawn room by room, or traced over a plan you upload and scale.

4. 04

   Heat loss

   Fabric and ventilation losses for every room, checked against your annual fuel use and EPC, with what loft, wall and floor insulation, glazing and draught-proofing would save.

5. 05

   Heat pump

   Manufacturer models fitted to their published test data, generic units or your own datasheet, checked at the design temperature and at typical load, with weather compensation and hybrid operation alongside a boiler.

6. 06

   Emitters

   Radiators, underfloor heating and fan convectors at your chosen flow temperature, and which radiators would need replacing at each flow temperature from 35 to 55 °C.

7. 07

   Pipework

   Pipe sizes, pump head on the index circuit, system volume, expansion vessel and balancing flows.

8. 08

   Hot water

   Daily demand, cylinder size, reheat time and the legionella cycle.

9. 09

   Performance

   Seasonal efficiency (SCOP) and monthly energy, from an hour-by-hour simulation of a typical year at your location.

10. 10

    Costs

    Running costs on current tariffs, the Boiler Upgrade Scheme grant, solar PV self-use, and payback against gas, oil, LPG or direct electric heating.

11. 11

    Sound

    A noise check at neighbouring windows by the MCS 020 method.

12. 12

    Report

    A printable design summary, and the room, emitter and pipework schedules as CSV files.

How it calculates

• Fabric. U-value × area × temperature difference for each element, from internal dimensions, including heat flow between rooms kept at different temperatures, party walls to the neighbouring home, and an allowance for thermal bridging.
• Ground floors. BS EN ISO 13370, from the whole floor’s area and exposed perimeter, for solid and suspended floors.
• Ventilation. The reduced method of BS EN 12831-1:2017, as adopted in the CIBSE Domestic Heating Design Guide (2026). Each room is sized for the worst wind direction and the heat pump for the house as a whole, so the room loads add up to more than the heat pump load.
• Emitters. Catalogue output corrected to the mean water temperature, Q = Q₅₀ × (ΔT ⁄ 50)ⁿ. Underfloor heating is limited to a 29 °C floor surface, or 33 °C in bathrooms.
• Heat pumps. Capacity and efficiency across outdoor and flow temperatures, including minimum output and the losses from cycling when the load falls below it.
• Seasonal performance. An hour-by-hour simulation through a typical year, built from the location’s degree days, with the weather compensation curve.

Its heat loss figures match an independent hand calculation from the published formulae. Its generic radiator outputs are a median 1.9% below the Stelrad and Kartell catalogues, so they err low, and its generic heat pump model, fitted to 198 published Heat Pump Keymark and EN 14511 test points, has a median error of −3% to +1% at those points.