Enter dimensions, insulation, windows, and design temperatures for heat demand. Choose radiator type and water temps to match realistic output ratings. Add safety margin.
| Scenario | Area (m²) | Height (m) | Indoor (°C) | Outdoor (°C) | ACH | Design Load (W) | Rated at ΔT50 (W) | Units |
|---|---|---|---|---|---|---|---|---|
| Bedroom, average insulation | 16 | 2.6 | 20 | -2 | 0.5 | 1,650 | 2,050 | 2 |
| Office, good insulation | 24 | 2.8 | 21 | -5 | 0.4 | 2,150 | 2,600 | 2 |
| Workshop, leaky envelope | 30 | 3.0 | 18 | -5 | 1.2 | 5,100 | 6,400 | 4 |
Detailed method uses Q = Σ(U × A × ΔT), where U is the thermal transmittance (W/m²K), A is surface area (m²), and ΔT is the indoor–outdoor temperature difference (K).
Ventilation heat loss is estimated by Qvent = 0.33 × ACH × V × ΔT. Here ACH is air changes per hour, and V is room volume (m³).
Total heat loss is multiplied by distribution and safety factors: Qdesign = (Qtrans + Qvent) × (1 + L) × (1 + S), where L is pipe loss fraction and S is safety margin fraction.
Radiator output varies with mean water temperature. The correction uses CF = (ΔTactual/ΔTstandard)n, with ΔTactual = ((Ts + Tr)/2) − Troom. Required standard rating is Qrated = Qdesign/CF.
Start with a clear indoor setpoint, commonly 20–21°C for living areas and 18°C for bedrooms. Select an outdoor design temperature from local climate data, then keep it consistent across the project. The difference between indoor and outdoor temperatures drives the heat loss and the final emitter size.
Transmission loss is dominated by envelope area and U‑value. Typical ranges are 0.2–0.35 W/m²·K for well‑insulated walls, 1.2–2.8 W/m²·K for glazing, and 0.12–0.25 W/m²·K for roofs. Measuring wall, window, and floor areas carefully often improves accuracy more than tweaking any single assumption.
Fresh air is essential, but it also carries heat away. Many residential rooms fall between 0.5 and 1.0 air changes per hour, while leaky sites can exceed 1.5. Use tighter values for sealed construction and higher values for older buildings, large openings, or windy exposures.
Heat gains from people, lighting, and equipment can offset part of the loss, yet they vary by occupancy and use. On site, designers often apply a 10–15% allowance to cover uncertainty, intermittent door opening, and uneven insulation. Smaller zones can reduce oversizing and improve control.
Once the design load is known, match it to an emitter that can deliver the same watts at your operating conditions. If you also track BTU/h, use 1 W = 3.412 BTU/h for quick checks. Always compare the required output against the selected radiator’s published rating and size the nearest standard model upward.
Radiators are usually rated at a standard temperature difference, such as ΔT 50 K. Lower flow temperatures reduce output, especially with heat pumps. A correction factor uses CF = (ΔTactual/ΔTstandard)n, where n is often about 1.3 for panel units. This step prevents undersizing when you operate at 55/45°C or lower.
Panel, column, and convector styles behave differently in real rooms. Maintain clearance for air circulation, avoid deep covers, and consider cold‑down‑draft near glazing. Mounting height, wall reflectivity, and pipe entry positions all affect perceived comfort. In tight layouts, two smaller emitters can outperform one oversized unit.
After installation, balance the system so each radiator receives the intended flow. Pair thermostatic valves with room zoning and a sensible setpoint schedule to prevent overheating. Bleeding air, checking pump settings, and confirming return temperatures protects efficiency. Regular cleaning preserves convection and stable output over time.
Use comfort setpoints that match the room use: about 20–21°C for living spaces, 22°C for bathrooms, and 18°C for bedrooms. Consistency matters more than choosing a perfect number.
Start with typical guidance ranges for your construction type, then refine with insulation thickness and material data. When uncertain, choose the higher U‑value to stay conservative, and document the assumption.
A modest allowance, often 10–15%, helps with intermittent heating and unexpected drafts. Larger oversizing can cause short cycling and poor control, so prefer zoning and better insulation before adding big margins.
Condensing boilers often target lower returns, so 70/50°C may be reduced to 60/40°C or 55/45°C. Heat pumps typically run even lower. Enter your planned supply and return temperatures so the correction factor reflects reality.
Yes. Blocking the airflow can cut heat transfer noticeably and create cold spots. Keep a clear convection path above and below the radiator, and avoid full‑height covers unless they are specifically designed for airflow.
It can, but results depend on airflow and door habits. Temperature will usually be uneven. For predictable comfort and control, size emitters per room or use a designed transfer opening with careful balancing.
It accounts for real‑world uncertainty: varying wind, imperfect insulation, door opening, and installation tolerances. A small allowance can prevent complaints, but it should not replace proper envelope design and airtightness.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.