What is a heat load calculation? Simply explained!

Heating load calculation is an important tool for designing heating systems. It is essential for the efficient operation of your heating system. Here you can find out how to carry out a heat load calculation, which data is required and what costs arise as a result.

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What is a heat load calculation?

A heat load calculation is a standardized procedure that determines the required heating output of a building. It ensures that your house stays warm even when outside temperatures are low. It is based on heat losses through walls, windows, ceilings and floors. This tool ensures the optimal sizing of your heating system.

When should I do a heat load calculation?

You need a heat load calculation if you want to ensure that your heating system works efficiently and is optimally dimensioned. This is particularly important in the following cases:

• Heating modernization: A heat load calculation is necessary before you replace your old heating system, e.g. when replacing an old oil or gas heating system with a heat pump.
• Energetic renovation: After measures such as façade insulation or window replacement, the building's heat demand changes. The heat load calculation helps to design a suitable heating system.
• Neubau: A heating load calculation is required by law for new buildings. It ensures the correct sizing of the heating system and energy efficiency of the building.
• changes of use: If the use of a building changes, for example when converting offices into living spaces, the heating load must be adjusted. There are other requirements for an optimal indoor climate here.

In all these cases, the heat load calculation ensures that your heating system is neither oversized nor undersized. This saves energy costs in the long term and increases living comfort.

How is the heating load calculated?

The process of Heat load calculation is carried out in accordance with DIN EN 12831 carried out. Various values and factors are included in the calculation, which are explained in detail in the national Annex D to the standard. The heat load is made up of the sum of the following variables:

• Transmission heat loss: Heat losses through building envelope components (e.g. walls, windows, roof).
• Ventilation heat loss: Heat losses that occur as a result of ventilation behavior.
• Additional heating capacity: Required heating capacity, which is required to quickly return to the desired room temperature after a heating break.

Can I create a heat load calculation myself?

You can also create a heat load calculation yourself, for example using online tools such as Heat load calculator from the German Heat Pump Association. These tools provide an easy way to estimate your single-family home.

Another option is using the following formula:

Heat load (in watts) = living area (in m²) X specific heat demand (in W/m²)

The following values are used for the specific heat requirement:

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For example, this would result in the following values for a single-family house of 150 square meters:

The Difference between a simple and detailed process (according to DIN 12831) lies primarily in accuracy: While the simple method uses standard values, the detailed process is based on specific, individually collected data.

What data is required for a heat load calculation?

In order for a heating installer or an engineering office to carry out a heat load calculation in accordance with DIN EN 12831, the required data must be provided in advance. The most important are:

• U-values of the building envelope: The heat transfer coefficient indicates how much heat is lost through a component. It is determined for exterior walls, windows and roof surfaces. These values are crucial for calculating transmission heat losses.
• Storage mass: The larger and more massive a building is, the longer it takes for the heating system to completely heat up the storage mass. It significantly influences the required heating capacity, as it describes the building's ability to store heat.
• Air exchange: Heat losses due to ventilation or leaky windows are taken into account in the calculation. The air exchange rate describes how much air is replaced per hour and is particularly important for ventilation heat loss.
• heating temperature: The desired room temperature and the outside temperature influence the heating output. For indoor use, there are standard temperatures depending on the room, such as 20 degrees Celsius for living rooms and 24 degrees Celsius for bathrooms. Outside temperatures depend on the location of the building and the time of year.

How much does a heat load calculation cost?

The costs for a heating load calculation in a single-family house are between 400 and 1,000 euros. The price depends on the size of the building and the effort involved. Complex calculations by energy consultants can result in higher costs.

Is there funding?

There are various government funding options available for heating load calculations:

• Funding under the Federal Funding for Efficient Buildings (BEG): Up to 50 percent of the costs of a heating load calculation are covered if this is part of subsidized energy planning or construction support.
• Hydraulic balancing support: The state subsidizes the implementation of a hydraulic adjustment, which usually also includes a heat load calculation, with up to 20 percent of the costs.
• Heater replacement: Anyone who installs a heat pump in an existing building can receive a subsidy of between 30 and 70 percent. The eligible costs often also include the heat load calculation. However, there is an upper limit of 30,000 euros for the total eligible costs.

What do I have to consider when calculating the heat load for a heat pump?

Avoid common mistakes when calculating heating loads and pay attention to the following points if you want to use a heat pump:

• Capture exact building data: Make sure that all relevant data on the building envelope, such as U values, window areas and insulation quality, is correctly recorded. Inaccurate data leads to incorrect calculations and an inefficient design of the heat pump.
• Taking room temperatures into account: Set the desired temperature for each room. These values are crucial because they determine the heat load in the different zones of a building.
• Include regional climate data: Use the specific climate data for your region, as these influence the heating output of the heat pump. These values are particularly important in colder regions.
• Take into account the air exchange rate: Pay attention to the correct design of ventilation heat losses. This includes how often the air in the rooms is replaced by ventilation or leaky windows.
• Additional services for peak loads: Consider additional heating outputs for particularly cold days. These can be covered by an additional electric heater without the need to oversize the heat pump.

Do you need an energy consultant to calculate the heat load?

An energy consultant is not absolutely necessary, but it is useful in many cases. Especially in the case of more complex buildings or extensive renovation measures, an energy consultant ensures that the heat load calculation is accurate and in accordance with standards. In addition, it can identify optimization potential.

If you claim federal funding for efficient buildings (BEG), then an energy consultant is often a prerequisite. In such cases, the consultant not only calculates the heat load, but also provides support in energy planning and applying for funding.

What is the difference between heat load and heating demand?

The heat load is an instantaneous value that describes the peak load. It is used to design the heating system. The heating demand is an annual value that shows the total energy consumption. This is important for calculating energy consumption and costs. Both values ensure efficient and tailored heating planning.

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