Heating Load Calculator
Load, radiator sections, fuel use and running cost.
Inputs
Result
Formula
| Load | area × heat index in watts |
| Volume check | volume × 0.55 × ΔT |
| Radiator sections | load ÷ output per section |
| Gas rate | Q × 3.6 ÷ (35.6 × efficiency) |
| Season use | hourly rate × hours/day × days |
| Units | 1 kW = 860 kcal/h = 3412 BTU/h |
100 m² at 60 W/m², 18°C indoors against −5°C outside over 120 days at 2.8 per m³: 6.00 kW (volume method 3.79 kW), 50 radiator sections, 0.67 m³/h of gas, 971 m³ for the season and about 2718 in cost.
Reference heating indices by building type
| Building type | Heat index W/m² | Note |
|---|---|---|
| Housing (65% efficiency) | 40-50 | Built in the last decade |
| Housing (50% efficiency) | 50-60 | Around 2010 |
| Older housing | 80-100 | No insulation, single glazing |
| Office | 70-100 | Including ventilation |
| School, nursery | 60-80 | Intermittent use |
| Retail | 80-120 | High fresh-air demand |
| Hospital | 80-120 | Runs 24 hours |
FAQ
Why does the index vary twofold?
Because insulation levels differ. U-values fall from 1.8 W/(m²·K) for old 240 mm brickwork to 0.4 with external insulation and double glazing — a fourfold gap. Windows are the weak point, losing 40-50% of a house's heat with single glazing, which is why replacing windows usually beats thickening walls.
What indoor temperature is right?
18-20°C is the comfort band for dwellings. Each degree adds 6-8% to energy, so lifting a 100 m² home from 18 to 22 costs an extra 600-800 over a season. Below 16 it is uncomfortable; above 24 the air is dry, occupants get headaches and consumption jumps. Hospitals and public buildings follow their own standards, with theatres at 22-25°C.
Radiators or underfloor heating?
Underfloor runs at 35-45°C supply, feels better with warm feet and cool head, frees the wall, but heats up slowly over 4-6 hours, raises the floor 60-80 mm and is hard to repair. Radiators run at 70-85°C, warm a room in 20 minutes and are easy to service, but take wall space and stir dust. Tight floor-to-ceiling or intermittent use favours radiators; continuous occupation favours underfloor.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.