What is Heating Load?
The heating load (also called design heat load) describes the thermal power a building requires at the coldest expected outdoor temperature to maintain a defined indoor temperature. It is the central parameter for sizing heating systems.
The Basic Concept
Imagine your building as a bathtub:
- Water drains out = Heat is lost (through walls, windows, ventilation)
- Water flows in = Heating system supplies heat
- Water level stays constant = Room temperature remains stable
The heating load answers the question: How much heat must the heating system supply to keep the interior warm when it is coldest outside?
The Standard: DIN EN 12831
Heating load calculation is standardised in the European norm DIN EN 12831:
| Part | Content |
|---|---|
| DIN EN 12831-1 | Room heating load (main standard) |
| DIN EN 12831-3 | Domestic hot water heating |
| DIN/TS 12831-1 Supplement 1 | National additions for Germany |
The standard defines standardised calculation methods, climate data and boundary conditions to ensure heating loads are comparable and reproducible.
The Two Components of Heating Load
The heating load comprises two main components:
1. Transmission Heat Losses (ΦT)
Heat that "flows" through the building envelope to the outside:
- Through external walls
- Through windows and doors
- Through roof and basement ceiling
- Via thermal bridges
Formula: ΦT = Σ (U × A × fT) × (θi - θe)
- U = Thermal transmittance (W/m²K)
- A = Component area (m²)
- fT = Correction factor (e.g. for ground contact)
- θi = Indoor temperature (°C)
- θe = Design outdoor temperature (°C)
2. Ventilation Heat Losses (ΦV)
Heat lost through air exchange:
- Infiltration: Uncontrolled air leakage
- Window ventilation: Manual airing
- Mechanical ventilation: Ventilation systems (with/without heat recovery)
Formula: ΦV = V × n × ρ × cp × (θi - θe)
- V = Room volume (m³)
- n = Air change rate (1/h)
- ρ × cp = 0.34 Wh/(m³K) for air
The Total Heating Load
The design heating load of a room is calculated as:
ΦHL = ΦT + ΦV + ΦRH
| Symbol | Meaning |
|---|---|
| ΦHL | Design heating load of the room |
| ΦT | Transmission heat losses |
| ΦV | Ventilation heat losses |
| ΦRH | Additional reheat capacity (optional) |
The building heating load is the sum of all room heating loads – though simultaneity factors may be applied here.
Why is Heating Load So Important?
The heating load fundamentally determines:
1. Heat Generator Sizing
| Heating Load | Recommendation |
|---|---|
| 5 kW | Small heat pump or gas condensing boiler |
| 10 kW | Medium-sized heat pump |
| 15+ kW | Large heat pump or cascade system |
Oversized heating: Cycles constantly, operates inefficiently, wears faster
Undersized heating: Building does not warm up during extreme cold
2. Radiator Sizing
Every room needs radiators capable of covering its individual heating load. The room heating load determines:
- Radiator type (Type 11, 21, 22, 33...)
- Radiator size (length × height)
- Number of radiators
3. Flow Temperature
The lower the specific heating load (W/m²), the lower the flow temperature can be:
| Specific Heating Load | Typical Flow Temperature |
|---|---|
| < 40 W/m² | 35–45°C (ideal for heat pump) |
| 40–60 W/m² | 45–55°C (low temperature) |
| > 60 W/m² | 55–70°C (conventional) |
Design Outdoor Temperature: The Design Case
The heating load is calculated for the coldest expected case. The design outdoor temperature is location-dependent:
| Region | Example City | Design Outdoor Temperature |
|---|---|---|
| Mild locations | Cologne, Düsseldorf | -10°C |
| Upland regions | Frankfurt, Stuttgart | -12°C |
| Alpine foothills | Munich, Augsburg | -14°C |
| Mountain regions | Oberstdorf | -16°C to -18°C |
Practical tip: You can find the design outdoor temperature in DIN/TS 12831-1 Supplement 1 or use our Heating Load Calculator, which automatically determines the correct values.
Heating Load vs. Heat Demand
These two terms are often confused:
| Term | Unit | Meaning | Analogy |
|---|---|---|---|
| Heating load | kW | Instantaneous power | Engine power |
| Heat demand | kWh/a | Annual energy | Annual mileage |
The heating load is a power – it indicates how powerful the heating system must be.
The heat demand is an energy quantity – it indicates how much energy is consumed over the year.
Factors Influencing Heating Load
The heating load depends on many factors:
Building Envelope
- Insulation standard (U-values of components)
- Window quality (glazing, frame)
- Airtightness (blower door value)
- Thermal bridges (junctions, penetrations)
Building Geometry
- Compactness (A/V ratio)
- Orientation (north/south side)
- Shading (neighbouring buildings, trees)
Usage
- Indoor temperature (20°C standard, bathroom often 24°C)
- Ventilation behaviour (mechanical/manual)
- Internal heat sources (people, appliances)
The Heating Load Calculator
Our Heating Load Calculator performs the complete calculation according to DIN EN 12831-1:
- Enter location → Climate data is determined
- Define building → Components and U-values
- Create rooms → Individual room heating loads
- Receive results → Room heating loads and building heating load
Calculate now: Determine the heating load of your building with our free Heating Load Calculator.
Further Reading
- Transmission Heat Losses – Heat losses through the building envelope
- Ventilation Heat Losses – The influence of air change
- The U-Value Explained – The most important component parameter
- Thermal Bridges – The hidden heat losses
Sources
- DIN EN 12831-1:2017-09 – Energy performance of buildings – Method for calculation of the design heat load
- DIN/TS 12831-1 Supplement 1 – National additions