1. 1

    Set Room Geometry

    In Step 1 choose the room shape (rectangle, L-shape or custom) and enter length, width and height. The live 3D model and room volume update instantly as you type.

  2. 2

    Define the Thermal Envelope

    In Step 2 set the U-values for walls, ceiling and floor. Use the presets that match your building's age, or enter exact values from your EPC. Lower U-values mean better insulation and smaller radiators.

  3. 3

    Add Windows & Doors

    In Step 3 add each opening with its size, glazing (single/double/triple) and orientation. Windows are usually the biggest single heat-loss path, so accuracy here matters.

  4. 4

    Set Climate & Ventilation

    In Step 4 pick your location to pull design outdoor temperatures, set the indoor target temperature and the air-change rate. The temperature difference (ΔT) directly drives the heat loss result.

  5. 5

    Review System & Finance, then Generate the Report

    In Step 5 enter flow temperature, energy price and fuel type, then calculate. You get total heat loss, annual cost, CO₂, upgrade scenarios, matched radiator products and a downloadable PDF report.

Pro Tip: Use Ctrl+Enter to calculate quickly, or Ctrl+R to reset the form.

Frequently Asked Questions

Radiator Pro uses a room-by-room heat loss method based on EN 12831 principles: it sums transmission losses through walls, windows, ceiling and floor (area × U-value × ΔT) plus ventilation/infiltration losses. With correct U-values, dimensions and design temperatures, results are typically within 10-15% of a full professional survey — accurate enough to specify radiator output and compare upgrade scenarios.

The live 3D preview maps where your room loses the most heat — walls, windows, roof and floor are colour-coded from high to low loss. It updates in real time as you change insulation, glazing or dimensions, so you can instantly see the impact of each upgrade. You can also export the geometry as a CAD/STL file to hand to your installer.

Radiator Pro es la herramienta profesional: flujo de 5 pasos, mapa térmico 3D, datos climáticos, recomendaciones, ROI e informe PDF. Los costes detallados y el PDF están en Thermopy Premium ($29.99/mes). AC Sizing y Cooling Comparison gratuitos cubren estimaciones rápidas.

Yes. Set the flow temperature in the system step to a low-temperature value (35-45°C) typical of heat pumps. Because output drops at lower water temperatures, the calculator will recommend larger or additional radiators, or flag rooms where underfloor heating is a better match. This prevents undersizing when moving from a gas boiler to a heat pump.

The U-value is how fast heat passes through a structure (W/m²K) — lower is better. Use ~1.8-2.0 for solid uninsulated walls, ~0.6-1.0 for standard cavity-insulated walls, and ~0.15-0.30 for modern or retrofitted construction. Single glazing is ~5.0, double ~2.8, triple ~1.0. If unsure, pick the preset that matches your building's age; you can refine later.

Yes. From the results page you can email yourself a copy, download a full branded PDF report with charts and recommendations, export the 3D model as a CAD file for your installer, and view matched radiator products with quantities. Everything is generated from your inputs in your browser.

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