Steady-state vs. Dynamic Calculation — When is Which Enough?

The steady-state calculation in accordance with DIN V 18599 (monthly balance method) is the mandatory method for GEG verification and models energy demand with an accuracy of +-10 to 15 % for standard buildings. Dynamic simulation solves the energy balance hourly, calculates room temperatures, peak loads and comfort metrics, and is indispensable for summer thermal protection (DIN 4108-2), comfort analysis and HVAC sizing. In practice, both methods are frequently combined: steady-state for regulatory compliance, dynamic for optimisation.

The steady-state method in accordance with DIN V 18599

Monthly balance method

DIN V 18599 calculates the energy demand of a building on the basis of monthly energy balances. For each month, the heat losses (transmission and ventilation) are set against the heat gains (solar gains and internal gains). The heating energy demand $Q_H$ of a month is given by:

Here $Q_{l,month}$ denotes the monthly heat losses, $Q_{g,month}$ the monthly heat gains and $\eta_H$ the utilisation factor of the gains. The utilisation factor takes into account in a lumped way that not all heat gains are usable — a portion leads to overheating of the building.

The monthly transmission heat losses are calculated from the component areas, their U-values and the monthly temperature difference:

with the transmission heat loss coefficient $H_T$ (in W/K), the mean indoor and outdoor temperatures $\theta_i$ and $\theta_e$ and the monthly period duration $t_{month}$ (in hours).

Plant evaluation

DIN V 18599 evaluates not only the useful energy demand (heating energy, cooling energy) but also the entire system chain: delivery, distribution, storage and generation. This captures losses of the HVAC systems and calculates the final energy demand as well as the primary energy demand. This end-to-end accounting is a strength of the method and the basis for the GEG verification.

Strengths and limitations

The steady-state calculation is standardised, reproducible and efficient. Two calculations of the same building with the same input data produce identical results — independently of the software used. The effort is manageable, since no hourly profiles need to be defined.

The limitations lie in the temporal simplification: monthly mean values cannot represent peak loads, short-term overheating or the interaction between controls and thermal mass. The utilisation factor $\eta_H$ is an empirical quantity that describes the complex interplay of gains and losses only approximately.

The dynamic method

Hourly energy balance

Dynamic simulation solves the energy balance for each zone at each time step (typically 1 hour or finer). Heat conduction through building components is calculated in a transient manner, so that the thermal storage capacity is represented correctly in a physical sense. Solar gains are calculated hourly on the basis of the current solar position and cloud cover.

The zone energy balance at each time step reads:

Solving this equation for all zones and all time steps yields the time history of the room temperatures and the energy demand.

Strengths and limitations

Dynamic simulation represents actual building behaviour significantly more accurately. It can:

• Predict room temperatures hour by hour
• Determine peak loads for heating and cooling
• Quantify the influence of control strategies
• Represent interactions between zones
• Calculate the effect of night ventilation and thermal mass

The limitations lie in the higher effort: more input data are required, modelling takes longer and the results depend more strongly on the user’s assumptions (usage profiles, control strategies, climate data).

Comparison of the methods

Differences in accuracy

Numerous validation studies show that the steady-state calculation can model the heating energy demand of well-balanced buildings with a moderate window area proportion to an accuracy of ±10 to 15 %. For buildings with a high glazing proportion, large thermal mass or irregular usage, the deviation increases.

The differences are particularly large in the assessment of solar gains. The steady-state calculation treats solar gains as a monthly energy quantity and evaluates their usability via the utilisation factor. Dynamic simulation calculates for every hour whether solar gains contribute to covering the heating demand or lead to overheating. For a south-facing office with large windows, this can lead to significant differences in the calculated heating and cooling demand.

Another difference concerns the interaction between zones. The steady-state calculation treats each zone independently with a fixed indoor temperature. Dynamic simulation calculates the heat exchange between neighbouring zones and can show how a sun-flooded south room also warms the adjacent north room.

When is the steady-state calculation sufficient?

The steady-state calculation in accordance with DIN V 18599 is sufficient for:

• GEG verification: The legal verification under the German Building Energy Act requires the steady-state calculation. Dynamic simulation is not permitted here as an alternative.
• Energy performance certificates: Both the demand-based and the consumption-based certificates are based on the steady-state methodology.
• Residential buildings with standard geometry: For compact residential buildings with a moderate window area proportion (< 40 %) and conventional construction, the steady-state method delivers sufficiently accurate results.
• Early planning phases: When the building geometry is not yet fixed, a detailed dynamic simulation is not meaningful. The steady-state calculation provides a quick overview of the energy demand.
• Subsidy applications: Programs such as BEG (Federal Funding for Efficient Buildings) require proof on the basis of DIN V 18599.

When is dynamic simulation needed?

Dynamic simulation is required or recommended for:

• Summer thermal protection in accordance with DIN 4108-2 (§ 8.4): The simulation-based verification calculates over-temperature degree hours and is often the only way to provide the verification for demanding buildings.
• Comfort analysis: The evaluation of thermal comfort in accordance with ISO 7730 or DIN EN 15251 requires hourly room temperatures, which only dynamic simulation provides.
• HVAC sizing: Heating and cooling load calculations benefit from dynamic simulation, which delivers realistic peak loads taking storage effects into account.
• Buildings with a high glazing proportion: Façades with more than 50 % glazing require a differentiated evaluation of solar gains and the risk of overheating.
• Non-standard buildings: Atria, double-skin façades, buildings with phase-change materials (PCM) or thermally activated building systems (TABS) cannot be adequately represented with steady-state methods.
• Sustainability certification: DGNB, BREEAM and LEED require proof based on dynamic simulation for certain criteria.
• System comparison and optimisation: When different HVAC concepts (e.g. district heating vs. heat pump, passive cooling vs. active cooling) with different efficiency ratings are to be compared, dynamic simulation provides the more robust basis.

Combination of both methods

In practice, the two methods are often combined: the steady-state calculation in accordance with DIN V 18599 covers the legal verification, while dynamic simulation is used additionally for specific questions (comfort, summer thermal protection, HVAC optimisation). The input data partially overlap, so that the additional effort for the dynamic simulation is manageable when a steady-state model already exists.

Conclusion

Steady-state and dynamic energy demand calculations are not competing but complementary methods. The steady-state calculation in accordance with DIN V 18599 is the normative tool for the GEG verification and delivers sufficiently accurate results for standard buildings. Dynamic simulation goes beyond this and answers questions that the steady-state method cannot: How hot will it be in the office in August? Is night ventilation sufficient? How does the building respond to a heat wave? The choice of method depends on the question — and increasingly the answer is: both. With VICUS Buildings a tool is available that implements dynamic simulation efficiently and thus provides a sound complement to the normative verification.

Further reading: Dynamic Building Simulation — detailed look at the dynamic method, Summer Thermal Protection in Accordance with DIN 4108-2 — a practical application where dynamic methods are essential, Validation Standards for Building Simulation Software — how accuracy is verified for both approaches.

References and Standards

• DIN V 18599 — Energy efficiency of buildings — Calculation of the net, final and primary energy demand
• ISO 52016-1 — Energy performance of buildings — Energy needs for heating and cooling — Part 1: Calculation procedures
• DIN EN 12831 — Energy performance of buildings — Method for calculation of the design heat load