This is how a heat load calculation works (according to DIN 12831)

Efficient heating starts with well-founded planning. Calculation of heat load is an indispensable step for energy efficiency and comfort. But what role does DIN 12831 play and why is it so important? Here you can find out the basics and procedure for calculating the heat load.

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Why is a heat load calculation important?

One Heat load calculation Forms the basis for the correct sizing of your heating system. It ensures that your heating system works efficiently, that heating costs remain low and that the desired living comfort is guaranteed. It also contributes to environmental friendliness. Lower energy consumption reduces CO2 emissions and protects the environment.

Incorrect sizing has the following consequences:

• Oversizing: A heating system that is too large causes high purchase and operating costs. It works inefficiently, which unnecessarily increases energy consumption. In addition, the heating system switches on and off more often, reducing the life of the system.
• Undersizing: A heating system that is too small can adequately heat your home. This leads to inadequate living comfort and increased wear, as the heating system is constantly running at its limit.

Basics of heat load calculation in accordance with DIN 12831

The heat load calculation in accordance with DIN EN 12831 is a central method for determining the required heat output for buildings. Structural, climatic and user-related factors are taken into account. This standardized process creates the basis for precise heating planning and energy-efficient heat supply.

Construction factors

The U-value describes how well components such as walls, windows or roofs insulate. The lower the U value, the less heat is lost. Good insulation reduces transmission heat losses and thus lowers the required heating output. Windows and doors in particular play an important role, as they are often weak points. The airtightness of the building also influences the heat load. The denser a building is, the lower the ventilation heat losses.

The following is a comparison of the U-values for various components.

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Climatic factors

The standard outdoor temperature indicates how cold it can get at a location in winter. It is used to calculate the maximum heat load. According to DIN 12831, standard outdoor temperatures differ depending on the region in Germany. A location in the Alps, for example, has a significantly lower standard temperature than a town in the Rhineland. These climatic differences have a direct effect on the heating load.

User behavior

The target indoor temperature determines how warm a room should be heated. Standardised values specify that living rooms are heated to 20°C, bathrooms to 24°C and bedrooms to 18°C. In addition, the exchange of air through ventilation influences the heating load. Any ventilation results in heat losses, which are dependent on the air exchange rate. Controlled home ventilation with heat recovery can significantly reduce these losses.

Here is an overview of the standardized room temperatures and air exchange rates in accordance with DIN EN 12831:

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The steps of calculating the heat load

1. Determination of transmission heat losses

The transmission heat losses are caused by the flow of heat through the building envelope, such as walls, windows or roofs, when the indoor and outdoor temperatures differ.

The formula is:

It says:

• Qtr: Transmission heat losses in watts (W)
• A: Area of the component in square meters (m2)
• U: heat transfer coefficient (U value) in W/m²K
• ΔT: temperature difference between inside and outside in Kelvin (K)

To help you understand the calculation better, here is an example of a calculation for a living room of 20 m² (two outer walls, two inner walls):

• Room size: 20 m²
• Standard internal temperature: 20°C
• Standard outdoor temperature: -10°C
• external walls:
  • Area per outer wall: 3 m (height) × 4 m (width) = 12 m²
  • U-value of the outer wall: 0.24 W/m²K
• Interior walls: Negligible as they do not cause any transmission losses to the outdoor environment.

Bill for the Exterior Walls:

Since there are two outer walls:

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Outcome: The transmission heat losses for the living room are 172.8 W.

2. Determination of ventilation heat losses

Ventilation heat losses occur as a result of air exchange — intentionally through ventilation or unintentionally through leaks.

The formula is:

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It says:

• Qlue: Ventilation heat losses in watts (W)
• V: Room air volume in cubic meters (m3)
• c: heat capacity of the air (0.34 W/m³K at standard conditions)
• ΔT: temperature difference between inside and outside in Kelvin (K)

Calculation example for the living room:

• Room height: 2.5 m
• Air volume: 20 m² × 2.5 m = 50 m³
• Air exchange rate (n): 0.5 (living room, according to DIN EN 12831)

So that

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Outcome: The ventilation heat losses for the living room are 255 W.

3. Addition of transmission and ventilation heat losses

The total heat losses consist of transmission and ventilation heat losses.

Formula:

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So that

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Planner often a Safety surcharge. This compensates for fluctuations in user behavior, extreme weather conditions and heat losses due to aging buildings or thermal bridges. According to the standard, this supplement is not required, but planners often add 10%. As a result, the heating system provides sufficient power even under unexpected conditions.

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Outcome: The heating load for the living room, including safety surcharge, is approximately 471 W.

The example shows the calculation of the heating load for an individual room. In practice, experts calculate the heat load for the entire building. They consider each room individually, calculate transmission and ventilation heat losses and summarize the results into a total heating load.

Who carries out the heat load calculation?

Experts such as heating installers, energy consultants or TGA planners carry out the heat load calculation. They have the necessary expertise and software so that they can carry out the calculation precisely and in accordance with standards.

• Heating Installers and Specialist Companies: Calculate the heat load often as part of planning and dimensioning a new heating system. Especially when it comes to new buildings or renovations, they ensure that the heating system is optimally tailored to the requirements of the building. They also take over the installation of the system.
• Energy consultant: The heat load calculation is often carried out as part of state-sponsored renovation programs. They not only plan efficient heating systems, but also check the energy quality of the building. In addition, they provide information on suitable funding opportunities for energy measures.
• TGA Planners or Engineering Offices for Building Technology: For larger projects, such as apartment buildings or office buildings, specialized building services planners calculate the heat load. They take into account the complex requirements of such buildings and develop comprehensive concepts for heating and building technology.

How much does a heat load calculation cost?

In a single-family home, the costs for a heating load calculation are typically between 400 and 1,000 euros. The exact price depends on the size of the building and the effort required. If the calculations are particularly complex by an energy consultant, the costs can be higher.

How long does a heat load calculation take?

A heat load calculation in accordance with DIN EN 12831 usually takes 3 to 6 hours. Here, detailed data such as room sizes, U-values and ventilation losses are analyzed room by room. The use of software makes the process easier, yet precise calculation requires expertise and careful data collection.

What is the difference between a rough and detailed heat load calculation?

The Difference between a simple and detailed process Read in accuracy. The approximate calculation uses standard values and is only used as a guide. The detailed calculation in accordance with DIN EN 12831 is based on precise building data and is mandatory for new buildings and renovations. It provides precise results for heating planning and ensures that the legal requirements of the Building Energy Act (GEG) are met.

Funding only with heat load calculation in accordance with DIN EN 12831

A heat load calculation in accordance with DIN EN 12831 is a prerequisite for state heating subsidies as part of federal funding for efficient buildings (BEG). This includes the KfW subsidy No. 458 for the purchase and installation of a new, climate-friendly heating system and KfW Loan No. 358, 359 for individual measures for the energy-efficient renovation of residential buildings that have already been subsidized. Access to these subsidies is not possible without a heat load calculation.

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