How to Use the Professional Radiator Calculator
How to Use the Professional Radiator Calculator
- 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
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
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
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
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
How accurate is the Radiator Pro heat loss calculation?
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.
What is the 3D visualization for?
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.
Why does Radiator Pro require premium access?
Radiator Pro är proffsverktyget: 5-stegsflöde, 3D-värmekarta, väderdata, rekommendationer, ROI och PDF-rapport. Detaljerade kostnader och PDF ingår i Thermopy Premium ($29.99/månad). Gratis AC Sizing och Cooling Comparison täcker snabba uppskattningar.
Can I size radiators for a heat pump with this calculator?
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.
What do the U-values mean and which should I pick?
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.
Can I share or download the results?
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.
Om Thermopy har hjälpt dig, överväg att dela din upplevelse på Trustpilot.