Anyone planning a new heating system or would like to apply for funding will quickly come across the term “heat load calculation”. But not all methods produce the same results — and even small differences can have major consequences. In this article, you will learn the difference between consumption-based and room-by-room heating load calculation.

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What is consumption-based heating load calculation?

Consumption-based heat load calculation is a simplified method for calculating the heat load, which analyses the average energy consumption of previous heating seasons. The approximate heating load of a building is calculated from the annual consumption of heating energy (e.g. gas or oil) and a standardized climate factor.

The following context is used:

• Heating load = annual consumption x maximum temperature difference ÷ heating degree days × factor

The following is a Sample calculationHow to carry out a consumption-based heat load calculation. Assumptions:

• Old building with gas heating
• Annual consumption: 20,000 kWh of thermal energy
• Living space: 150 m²
• Heating degree days (HGT) for Germany: approx. 3,400 k·D (may vary regionally)
• Standard outdoor temperature: e.g. -12 °C (assumed)
• room temperature: 20 °C
• Heating season: approx. 180 days

Calculation using a simplified formula:

• Heating load = 20,000 kWh x (20 °C - (-12 °C)) ÷ 3,400 k·D × 24 h
• Heat load = 640,000 ÷ 81,600
• Heat load = 7.84 kW

The consumption-based heating load calculation is cost-effective and easy to carry out, but is not sufficient for standard-compliant planning. It is only suitable for rough estimates or as a preliminary stage for detailed calculations. Especially when renovating or using a heat pump, this method can lead to incorrect results.

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What is the room-by-room heating load calculation?

The room-by-room heating load calculation is a standards-compliant procedure to determine the required heating capacity for each individual room in a building. The basis is DIN EN 12831, which takes into account all relevant structural and climatic factors. The aim is to match heating surfaces (e.g. radiators or floor heating) and the heating source (e.g. heat pump) to the actual heat demand of each room.

This includes the following factors:

• Room size and use
• Exterior wall surfaces, window parts and insulation standard
• Temperature difference between interior and standard outdoor temperature
• Air exchange rate (e.g. for ventilation systems)
• Heat losses due to transmission and ventilation

The calculation is carried out by experts such as energy consultants or TGA planners using special software such as Autarc heat load calculation software.

The room-by-room heating load calculation is considered the most precise method for heating sizing. It enables an optimal design of all components, but requires more effort. It is particularly essential for new buildings, renovations or the use of heat pumps.

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What is the difference between consumption-based and roomwise heating load calculation?

The consumption-based method uses past consumption data, and the room-by-room calculation is based on building physics and standard requirements. The two methods differ significantly in terms of accuracy, effort and area of application. While the consumption-based estimation provides quick orientation based on historical heating costs, the room-by-room heat load calculation provides exact values for each room and is therefore the basis for standard-compliant heating planning.

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When is a consumption-based assessment sufficient?

A consumption-based assessment is sufficient if only a rough orientation of the heating load of a building is required, for example to compare offers. It is useful for existing buildings with stable use and existing consumption data over several heating seasons. However, it is not sufficient for an exact design or funding in accordance with BEG.

When is a room-by-room heating load calculation necessary?

A room-by-room heating load calculation is necessary if a new heating system is to be precisely designed, in particular for heat pumps, energy renovations or new buildings. It is a prerequisite for funding applications and serves as the basis for hydraulic balancing as well as the exact dimensioning of radiators or floor heating systems. Without them, there is a risk of bad planning, inefficient operation or loss of funding.

Why are there discrepancies between the two methods?

Deviations arise because both methods are based on completely different principles and therefore lead to different results. The consumption-based method looks at past user behavior, while the room-by-room calculation is based on building physics and determines actual demand independently of behavior.

The reasons for discrepancies between procedures are:

• user behavior: Consumption-based data reflect individual heating behavior — anyone who heats economically has lower consumption, regardless of actual demand.
• weather conditions: The heating consumption is based on the actual outdoor temperatures of recent years, while the room-by-room calculation is based on a standardized outdoor temperature (standard outdoor temperature).
• Building changes: Subsequent renovations (e.g. insulation or window replacement) change the heat load, but are often not included in old consumption data.
• Incomplete data: Consumption figures are susceptible to errors due to estimates, incomplete heating seasons or different levels of utilization (e.g. vacancy).
• Lack of space differentiation: The consumption-based calculation only provides an overall value for the building. Temperature differences or types of use of individual rooms are not taken into account.

Which method is better for heat pumps?

For heat pumps, the room-by-room heat load calculation is more suitable, as it determines the actual heat requirement of each room exactly in accordance with the standard. This is the only way to design heat pumps efficiently and precisely, which prevents overdimensioning or underdimensioning. Funding programs such as BEG also require a standard-compliant calculation as a requirement. The consumption-based method is not sufficient for this, as it does not provide sufficient accuracy.

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