CalcHeizlast Calculator (Heat Load)
Last updated: 2026-09-18
How to Use This Calculator
This Heizlast calculator helps you estimate the required heating power for a room or building. To use it, simply enter the room volume in cubic meters, select the appropriate insulation quality, input the total window area, and specify the indoor-outdoor temperature difference. The calculator then computes the required heating power in kilowatts, which you can use to select radiators or a heat pump. It is designed for quick estimates but can be a valuable tool for preliminary planning.
Formula and Methodology
The calculation is based on the standard heat load equation: Q = Q_transmission + Q_ventilation. Q_transmission accounts for heat loss through walls and windows, calculated as the sum of U-values multiplied by areas and the temperature difference. Q_ventilation accounts for heat loss due to air exchange, calculated using the air change rate, room volume, air density, specific heat capacity, and temperature difference. The U-values and air change rates are derived from typical values for different insulation qualities.
Practical Examples
For example, a room with a volume of 75 m³, medium insulation, 8 m² of windows, and a ΔT of 30°C requires approximately 2.5 kW. If the insulation is poor, the required power might increase to 4.0 kW. Conversely, with good insulation, it could drop to 1.8 kW. These examples illustrate how insulation and window area significantly impact heating requirements.
Tips and Best Practices
Always use the actual window area and consider the orientation and glazing type. For existing buildings, verify insulation levels with an energy audit. When sizing heat pumps, add a safety margin of 10-20% and consider domestic hot water needs. Remember that this calculator provides a simplified estimate; for precise sizing, consult a heating engineer and perform a room-by-room calculation according to DIN EN 12831.
FAQ
What is Heizlast?
Heizlast is the German term for heat load, which is the amount of heating power required to maintain a desired indoor temperature on the coldest design day. It is typically expressed in kilowatts (kW) and is used to size heating systems such as radiators, underfloor heating, or heat pumps.
How accurate is this Heizlast calculator?
This calculator provides a simplified estimation based on typical U-values and air change rates. For precise sizing, a detailed room-by-room heat load calculation according to DIN EN 12831 should be performed by a qualified engineer.
What insulation quality should I choose?
Choose 'Poor' for uninsulated old buildings (pre-1970s), 'Medium' for standard buildings from the 1980s-1990s, 'Good' for modern buildings complying with current energy codes, and 'Passive' for passive house or ultra-low energy buildings.
Can I use this for heat pump sizing?
Yes, the calculated required kW is a good starting point for heat pump selection. However, heat pump sizing also considers domestic hot water demand, defrost cycles, and bivalence point. Always consult a professional for final sizing.
What temperature difference should I use?
For heating, use the difference between your desired indoor temperature (e.g., 20°C) and the local design outdoor temperature (e.g., -12°C in Germany), which gives a ΔT of 32°C. Check local building codes for the exact design temperature.