Heizlast

Heizlast, in building physics, denotes the maximum heating power a heating system must supply to keep indoor temperatures at a defined setpoint under design outdoor conditions. It is a rate, usually expressed in kilowatts (kW), and represents the peak load the system must handle. By contrast, Heizbedarf refers to the total amount of heat energy needed over a period (kWh).

The Heizlast is determined by heat losses and gains affecting a building. Key factors are heat transfer through the building envelope (walls, roof, windows) depending on U-values and surface areas, and ventilation or air infiltration losses. These losses are offset by solar gains through openings and internal gains from occupants, equipment, and lighting. The design outdoor temperature and the desired indoor temperature set the driving difference. Better insulation, tighter building envelopes, and reduced infiltration lower the Heizlast, while higher outdoor temperatures or large internal gains can reduce it.

Calculation methods range from simple steady-state estimates to dynamic simulations. A basic approach sums heat losses through the envelope and ventilation and subtracts anticipated gains to obtain a net heating load. More accurate methods use hourly simulations to determine the peak heating power required over a year. Standards and guidelines in various regions provide procedures to compute Heizlast for planning and equipment sizing.

Applications include sizing of boilers, heat pumps, and storage systems, as well as informing design choices to improve energy efficiency. Understanding Heizlast helps ensure thermal comfort while avoiding oversized or undersized heating equipment.