Calculate building heat loss per BS EN 12831 / ASHRAE, size a Karnot heat pump, and compare annual costs vs gas boiler.
Enter your building dimensions, insulation levels, and climate data below. The tool computes design heat loss, estimates seasonal COP, selects the optimal Karnot Heat Pump, and produces a full annual savings report.
This calculator follows the BS EN 12831 / ASHRAE methodology: it sums fabric (transmission) losses through walls, roof, floor, windows, and doors, adds ventilation losses, then adds domestic hot water demand to determine the total design heat load. It then sizes a Karnot Heat Pump and estimates annual running costs using the Heating Degree Day method.
Define dimensions and U-values for walls, roof, floor, windows, and doors. Pick a building age preset or enter custom values.
Select your region to auto-fill design outdoor temperature and Heating Degree Days (HDD). These drive both peak and annual calculations.
Fabric loss + ventilation loss + DHW peak demand = total design heat load in kW. Each element is shown with its formula.
The tool sizes a Karnot iHEAT, estimates COP at design conditions, and calculates annual running cost savings vs your current fuel.
Our engineering team can perform a detailed site survey, building heat loss assessment, and Karnot Heat Pump specification tailored to your exact requirements.
Contact Karnot EngineeringBuilding heat loss is calculated per BS EN 12831 / ASHRAE by summing fabric (transmission) losses and ventilation losses. Fabric loss for each element (walls, roof, floor, windows, doors) = U-value × Area × Temperature Difference (indoor minus outdoor design temperature). Ventilation loss = Volume × Air Changes per Hour × 0.33 × Temperature Difference. The total design heat loss determines the minimum heat pump capacity required.
COP (Coefficient of Performance) depends on the temperature lift — the difference between the heat source (outdoor air) and the heat sink (flow temperature). At typical UK conditions (-4°C outdoor, 45°C flow), an R-290 Karnot iHEAT achieves a COP of approximately 2.8–3.2. Lower flow temperatures (e.g. 35°C underfloor) give higher COPs, while colder climates or higher flow temperatures reduce COP. The Karnot CO2 range excels at high-temperature applications above 80°C.
Flow temperature depends on your heat distribution system. Underfloor heating typically runs at 35°C, low-temperature radiators at 45°C, standard radiators at 55°C, and high-temperature systems at 65°C. Lower flow temperatures give significantly better heat pump efficiency (higher COP). If you are retrofitting, consider oversizing radiators to allow a lower flow temperature.
Heating Degree Days quantify how much heating a building needs over a full year by summing the daily temperature deficits below a base temperature (typically 15.5°C). The annual energy formula is: Annual kWh = (Design kW × HDD × 24) / Design ΔT. HDD accounts for mild days where little or no heating is needed, giving a far more accurate annual estimate than simply multiplying the peak heat loss by 8,760 hours.
R-290 (propane) heat pumps are ideal for heating up to 80°C with excellent COP. They use a natural refrigerant with a GWP of just 3. Karnot CO2 (R-744) heat pumps can deliver temperatures up to 110°C, making them suitable for domestic hot water, high-temperature radiators, and process heating. CO2 operates in a transcritical cycle that excels at heating water from low to high temperatures. R-290 is generally more efficient for space heating; CO2 is preferred when high-temperature output is required.