Overview of the Method
One way to estimate a building’s heat load is by using the full-load hours of an existing heating system. This simplified method is defined in DIN EN 12831 and allows for a rough estimation of the heat load based on the building’s annual energy consumption.
Formula
Heat load (kW) =
(Useful heat output – Domestic hot water heating) / Full-load hours
Definitions:
Useful heat output = (Annual consumption × Efficiency of the system)
Domestic hot water heating (DHW) = ((14.9 kWh/day × 30 L × number of persons) / Efficiency of DHW system)
Full-load hours = Based on the postal code (region-specific)
Example Calculation (Postal Code 24306):
Given:
Annual consumption = 12,000 kWh
Heating system efficiency = 88%
4 residents
DHW system efficiency = 60%
Full-load hours = 2,199 h
Calculation:
((12,000 kWh × 0.88) – ((14.9 kWh × 30 × 4) / 0.6)) / 2,199 h = 3.44 kW
Extended Influencing Factors
To further refine the heating system design, the following elements are also considered:
Postal code → Defines the regional full-load hours
Type of previous heating system → Different technologies have different efficiencies
Year of installation of the old heating system → Older systems tend to be less efficient
DHW heating → Affects total load if hot water is provided by the same heating system
Number of residents → Relevant for sizing DHW demand, not the space heating load itself
Annual consumption of the last 3 years → Helps derive a realistic average
Conclusion
While the consumption-based heating load provides a useful initial estimate, a room-by-room heat load calculation is always necessary for accurate system dimensioning. Only a detailed calculation in accordance with DIN/TS 12831-1:2020-04 ensures compliance with standards and an efficient, needs-based heating system design.
Source: DIN EN 12831 Beiblatt 2, MZ-Energieberater, TGA-Planer
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