Radiator Size Inputs
Use the advanced heat-loss model and optional sizing outputs.
Example Data Table
| Parameter | Example Value | Unit |
|---|---|---|
| Room length | 5.0 | m |
| Room width | 4.0 | m |
| Room height | 2.7 | m |
| Outdoor design temperature | -3 | °C |
| Indoor design temperature | 21 | °C |
| Window area | 3.2 | m² |
| Wall U-value | 0.35 | W/m²K |
| Window U-value | 1.60 | W/m²K |
| Ventilation rate | 1.0 | ACH |
| Output per section | 120 | W/section |
Formula Used
1. Floor area: Afloor = L × W
2. Room volume: V = L × W × H
3. Estimated external wall area: Awall = ((2(L + W) / 4) × Nwalls × H) − Awindow
4. Transmission loss: Qtrans = ΔT × Σ(U × A)
5. Ventilation loss: Qvent = 0.33 × ACH × V × ΔT
6. Net room heat load: Qnet = Qtrans + Qvent − Qgains
7. Safety adjusted load: Qdesign = Qnet × (1 + Safety%)
8. Mean water temperature: Tm = (Tflow + Treturn) / 2
9. Actual radiator delta T: ΔTactual = Tm − Troom
10. Catalog correction factor: CF = (ΔTactual / ΔTcatalog)n
11. Required catalog output: Qcatalog = Qdesign / CF
12. Sections needed: N = ceil(Qcatalog / Output per section)
This method sizes the radiator against actual water temperatures, which is essential for lower-temperature systems and catalog comparison.
How to Use This Calculator
- Enter room dimensions to establish floor area and room volume.
- Set indoor and outdoor design temperatures for your heating scenario.
- Choose the number of external walls or enter a manual wall area.
- Add window area, U-values, and exposed ceiling or floor losses when relevant.
- Enter air changes per hour to account for ventilation heat loss.
- Subtract internal gains if occupants, equipment, or solar gains offset heating demand.
- Enter radiator flow and return temperatures to correct catalog output for real operating conditions.
- Optionally add section or per-metre output ratings to estimate sections or radiator length directly.
Frequently Asked Questions
1. Why does the calculator ask for flow and return temperatures?
Radiators are often rated at a catalog reference delta T. Real systems may run cooler, so actual output drops. Using flow and return temperatures corrects the sizing to match realistic operating conditions.
2. What does air changes per hour mean?
Air changes per hour estimates how frequently room air is replaced. Drafty or mechanically ventilated spaces lose more heat, so a higher ACH increases the required radiator capacity.
3. Should I use manual wall area or estimated wall area?
Use the manual wall area when you know the true exposed wall surface. The automatic estimate is convenient for early sizing, but manual values are better for detailed design.
4. Why include a safety factor?
A safety factor adds margin for intermittent door opening, uncertain inputs, and small modeling errors. Moderate margins are common, but oversizing too much can reduce control quality.
5. What is the purpose of internal gains?
People, appliances, lighting, and sun can add useful heat. Internal gains reduce net heating demand, so including them can prevent selecting a radiator larger than necessary.
6. Can this calculator size low-temperature radiators?
Yes. The correction factor adjusts catalog performance when mean water temperature is lower than the catalog reference. That makes the tool helpful for condensing boilers and heat pumps.
7. What if I know watts per section or per metre?
Enter those values in the optional fields. The calculator then converts the required catalog output into an estimated number of radiator sections or the total radiator length needed.
8. Is this suitable for final construction documents?
It is useful for engineering estimates and preliminary selection. Final design should still verify local weather data, detailed fabric areas, manufacturer ratings, and project-specific standards.