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A heating system functions optimally when it’s properly designed and sized. This ensures the room is pleasantly warm during the heating season while also operating energy efficiently.
For example, if the system’s capacity is too small for the building and its conditions, the premises will experience insufficient power supply during peak hours. Always remember that at certain temperatures, moisture damage and mold growth can occur in the room and on the wall surfaces . Similarly, the heating system may be oversized, causing the burner to cycle due to a mismatch between supply and demand. Frequent cycling increases wear on the burner components.
To prevent this, it’s important to determine the building’s thermal load. There’s a standardized calculation for this. We’ll explain what a thermal load is, how it differs from heat demand, and what goes into calculating a thermal load.
Thermal load and its significance
The heat generator is selected based on the calculated heat load. Hot water heating for domestic use must also be taken into account. The heat load is specified in kW. The capacity must be high enough to compensate for heat loss, including through the building envelope, and maintain a comfortable temperature in the building throughout the heating season. Accordingly, it is important to maintain the set internal temperature. This requires a certain boiler capacity.
A standard heating load can be determined either for sizing a heat generator or for designing individual heating surfaces in each room. In existing buildings, boilers and heating surfaces are often oversized. Furthermore, small changes to the building envelope can affect the heating load. Even replacing windows can have a significant impact. This makes it even more important to determine the heating load when considering a heating system upgrade . The heating load should always be determined for new construction .
Heat load is not heat demand
In addition to the heating load, you’ll often hear the term “heating demand.” Be careful not to confuse them, as they are not the same thing. While a building’s heating load indicates how high the boiler output should be, the heating demand indicates the amount of energy required per square meter over the course of a year.
Thus, the heating demand, or energy demand of a building, is a criterion for describing a building’s energy performance. It is calculated in kilowatt-hours per year and compared to one square meter of net floor area. Heating demand cannot be used to determine boiler capacity.
Heat Load Calculation – Procedure and Influencing Factors
When designing a heating system for a new building, it is necessary to calculate the heat load. This calculation is also necessary during modernization projects, otherwise the system’s heating capacity will not match the building’s current conditions. Subsequent readjustment or balancing is virtually impossible. If the system is oversized or undersized, then beyond a certain point, balancing the hydraulic system will not help either. This makes it even more important to perform accurate calculations. This is not a job for an amateur. For modernization projects, contact your Viessmann sales partner , who will handle the calculation during the planning stage. In new construction projects, the calculation is the responsibility of the system design engineer. Some online calculators exist that calculate heat loads, but their results are only approximate and provide only an idea of the heat load at the preliminary planning stage.
Standardized procedure for calculating heat loads
The heat load calculation is based on DIN EN 12831 “Heating systems in buildings – Method for calculating the design heat load.” The heat load consists of the following elements:
Heat transfer losses : losses through building envelope components.
Heat losses through ventilation : airflow, leakages and minimum air change rates related to hygiene.
Additional heat output : The capacity of the heating system to provide heat for a short time for warming after it has been switched off.
When calculating the additional thermal power, the sum of heat losses during transmission and ventilation is added.
Building-specific influencing factors
The following variables and quantities have a significant impact on the heat load of a heating system:
Required temperature : The DIN standard specifies standard temperatures for indoor and outdoor areas.
U-values : This value indicates how much heat is lost through one square meter of the building envelope for a temperature difference of one degree. This applies to all relevant surfaces: exterior walls, windows, exterior doors, roof surfaces, ceilings, and floors of unheated spaces, as well as interior doors of less heated spaces.
Air change : This value shows how often the air volume in a room is changed using natural or mechanical ventilation.
Warehouse weight : Among other things, the weight of the building affects the efficiency of reheating the heating system after a long shutdown.
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