Heating Load Calculator
Load, radiator sections, fuel use and running cost.
Quick answer
With the default inputs: Floor area 100 m² · Storey height 3 · Heat index 60 W/m² etc.Heating load 6.00 kW
Edit the inputs below to recalculate.Inputs
Result
Heating load
6.00kW
| Building volume | 300m³ |
| Temperature difference | 23K |
| Load per m² | 60W/m² |
| Volume method check | 3.79kW |
| In kcal/h | 5160kcal/h |
| Radiator sections at 120 W | 50pcs |
| Gas consumption | 0.67m³/h |
| Season gas use | 971m³ |
| Season cost | 2718CNY |
The area index is an estimate, not a load calculation. A proper calculation takes each envelope element — walls, windows, roof, floor — at its U-value and adds infiltration and ventilation. The index depends heavily on insulation: housing to a 65% energy standard needs only 40-50 W/m² in the same climate where 1980s stock needs 80-100. Retrofit cost hinges on that number.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Start with the area-index method: Q = floor area × heat index. Dwellings run 40-70 W/m² (high in severe cold zones, low in hot-summer/cold-winter zones), offices 60-80, schools 60-80, retail 80-120.
- Cross-check volumetrically: Qv = volume × 0.55 × (tn − tw). Agreement within 20% is fine; a wider gap means the index or the storey height is wrong.
- Apply orientation and position corrections: +15-20% for a top floor, +15% for an end unit, +10% for north-facing, another +10% for large glazed areas. Skip this and rooms in the same building end up 5°C apart.
- Radiator sections N = load ÷ output per section. A cast-iron four-column 760 gives about 130 W per section at standard conditions; steel panel radiators are rated per unit by the maker, with a correction for low water temperature.
- Gas rate V = Q × 3.6 ÷ (35.6 × efficiency). Natural gas has a net calorific value of 35.6 MJ/m³; conventional boilers run 0.85-0.90, condensing 0.95-1.05.
- Season consumption is the hourly rate times 8-12 equivalent full-load hours per day times the heating season length; running cost is that volume times the gas price.
Where it goes wrong
- Using structural area as floor area. They differ by 10-15% in multi-storey housing, which throws off the load and the boiler selection — and shows up as a boiler that runs constantly without warming the rooms.
- Estimating one radiator section per square metre. A cast-iron 760 gives 130 W per section, so a 20 m² room at 60 W/m² needs 9.2 sections. Twenty is waste; five leaves the room cold all winter.
- Skipping the orientation and exposure corrections. Without them, rooms in one building range from 16°C to 22°C, and the complaints all come from the coldest units.
- Using the gross calorific value. At 39.8 MJ/m³ it is 12% above net, so the gas estimate comes out low and the actual bill lands well above budget.
Material and cost
A 24 kW gas combi boiler costs 6000-15,000 CNY depending on origin and condensing capability; cast-iron radiators 60-120 CNY per section, steel panels 300-800 per unit. Gas is 2.5-3.5 CNY/m³; a 100 m² dwelling in north China burns 800-1200 m³ over a 120-day season, costing 2000-4000 CNY.
On site
- The heat index is a whole-building figure, not a room figure. For apartment-level metering or room-by-room control, calculate each room and never distribute the average.
- An oversized boiler short-cycles, which lowers efficiency and shortens life. 20-24 kW covers a 100 m² dwelling including domestic hot water; 28 kW is unnecessary.
- Radiators under windows create a warm curtain that blocks cold downdraught. Comfort is clearly better than side-wall mounting, and glazing condensation is reduced.
- When refurbishing an old house, look at the windows first. Replacing single glazing with double units cuts the load 15-20% — cheaper than adding radiator sections and it fixes condensation at the same time.
Basis
- GB 50736-2012 Design code for heating, ventilation and air conditioning of civil buildings, heating and envelope thermal parameters
- GB 50019-2015 Code for design of heating, ventilation and air conditioning of industrial buildings
- CJJ 34-2022 Standard for design of urban heating networks
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Heating Load Calculator. 2026-10-25. https://jianzhujisuan.com/en/heating-load
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.