Space heating demand in districts

How is the space heating demand determined?

The heat balance for a building is characterized by four heat flows:

• \(Q_T\): Transmission heat losses due to heat conduction (through walls or windows)
• \(Q_V\): Ventilation heat losses as a result of air exchange (through leaks and ventilation)
• \(Q_i\): Internal heat gains (operation of electrical devices, lighting or body heat)
• \(Q_s\): Solar heat gains (solar irradiance through windows)

According to EN 832, the heating energy demand \(Q_h\) is calculated as follows: $$Q_h = (Q_T+Q_V)-\eta\cdot(Q_i+Q_s)$$ Here, \(\eta\) denotes the degree of utilization of heat gains, which ranges from 0.9 (light construction) to 1 (heavy construction).

Simplified methods

To simplify matters, calculations often assume a linear relationship between the heating demand and the temperature difference between the room and the outside air: $$Q_h \propto (T_i - T_a)$$ Accordingly, the maximum heating load occurs at minimum outdoor air temperature (design temperature). The heating load is often specified in W/m². The design can also be based on the full load hours. For older residential buildings, these are around 1800 h/a in Germany. For residential buildings with a high insulation standard (passive house or low-energy house), the full load hours are around 1000 h/a. The full load hours \(\tau_V\) are calculated as follows: $$\tau_V = Q_h^{tot} / Q_h^{max}$$ Here, \(Q_h^{tot}\) denotes the annual heating demand in kWh and \(Q_h^{max}\) the peak heat load in kW.

Weather adjustment of the space heating demand

The energy required to provide space heat is directly dependent on the weather. In years with severe, cold winters, heating demands are higher than in years with mild winters. winter. To determine an average annual heating demand, measured consumption data can be subjected to a weather adjustment, which is usually based on degree day figures.

Space heating demand for different types of buildings

Table 1 lists the area-specific heat demands for 4 building types as examples. While old buildings have an annual heating demand in the order of 250 kWh/m², buildings with a high insulation standard much lower annual demands. Low-energy houses, for example with the German building standard KfW 55, have a heating requirement of around 35 kWh/m². An office built in 1995 or later has a space heating requirement of around 70 kWh/m² in Germany. Hospitals with an average building standard have a heating requirement of around 140 kWh/m² in Germany.

Sources

1. Leitfaden Energienutzungsplan, Bayerisches Staatsministerium für Umwelt und Gesundheit, 2011.
2. Fachbuch: Recknagel - Taschenbuch für Heizung + Klimatechnik 2017/2018.