Building Design Days + Energy

The Building Design Days + Energy (BDD+E) method calculates hourly static balances of heat gains and losses in thermal zones, heat emission and absorption of room transmission systems for heating and cooling, as well as heat supply and removal from HVAC systems with heating and cooling coils. Finally, the energy concept components are calculated, which provide the required heat, cold, and electricity. Because all 8,760 hours of the entire year are included, the issue of part-load operation becomes visible and can be addressed through sustainable and reliable system sizing. In particular, the method takes into account and evaluates the hourly CO₂ emission intensity in the electricity mix. The goal of the BDD+E method is the fast and reliable sizing of heating and cooling systems, right down to the room-level systems and their modes of operation.

The calculation method of static hourly balances, using a predefined target room temperature, determines the heat gains and losses. The design criterion is that if the same predefined room temperature is reached at the end of the day, then the dimensioned capacity for heating and cooling is sufficient. This ensures that the energy can keep the room thermally stable on the coldest winter day as well as the hottest summer day. Even if many such extreme days occur consecutively, the room temperature at the end of each day will correspond to that at the beginning of the day.

The storage capacity of the room dampens changes in room temperature when partial capacity is missing in a given hour or when excess heat enters the room. Through the room’s storage capability, this capacity is converted into an hourly temperature change and accumulated, shown as a trend room temperature.

The Building Design Days + Energy method provides input and output values as time series with 24 hours per day over 365 days for each of the 8,760 hours of the entire year.

Distinction from dynamic simulation

This is not a dynamic simulation. In a dynamic simulation, the changed boundary conditions from the previous hour would be used as the new starting conditions for the next hour.

Calculation using the static balance is on the safe side, because for example, a drop in room temperature in winter due to insufficient heating capacity would result in a lower static heat loss through transmission and ventilation. However, the BDD+E method always calculates the losses based on the predefined target room temperature, which would be higher in such a case.

The BDD+E method thus serves as a bridge from statically calculated instantaneous values over the 24 hours of the design days for winter and summer, to the 8,760 hours and energy values of the simulation.

Advantages

The advantage is that a planner now only needs to size the heating and cooling capacity for the 24 hours of the two extreme days. The design criterion is that the room temperature at the end of the day is the same as at the beginning of the day. This ensures that the heating and cooling energy for the most extreme days of the year can be delivered and transferred.

Each individual hour is transparent and manually verifiable, since the boundary conditions are known and the calculation only determines the static balance for that specific hour. Cause and effect are presented transparently, forming the basis for decisions on system sizing.

A major advantage is that real measured efficiencies and part-load performance curves can be used for generation components, which then—based on the hourly heating and cooling load profiles—lead to annual performance factors and other derived values. Heat pumps in particular are very sensitive with regard to oversizing and part-load behavior.

Sustainability

In this way, the planner can sustainably reduce the required capacity while taking into account the operating modes of pre-heating and pre-cooling.

Operation and Automation

The planner can provide clear specifications for system operation and automation, and define certain control limits that allow the building to enter its first year of operation “with awareness.”

The supply temperature for heating and cooling of the room transfer systems can be designed depending on outdoor temperature and solar horizontal irradiation.

• For the heating case, this can mean, for example: lowering the supply temperature leads to a reduced required transfer capacity in the rooms during the day with solar radiation.
  • With less mass flow, pump power consumption and distribution losses can be reduced.
• For the cooling case, this can mean, for example: by additionally considering the solar horizontal irradiation on the roof, the supply temperature can be lowered specifically during those times.
  • The advantage is that the supply temperature needs to be lowered much less frequently, resulting in lower mass flow and reduced distribution losses.

Room air handling units are also calculated, including their energy demand for the heating and cooling coils after heat recovery, as well as latent dehumidification during pre-cooling of the supply air in summer. This increases transparency by showing the simultaneous energy demands of the various upstream components involved.

Monitoring

Today, energy monitoring generally only requires measuring the time series of power for heating, cooling, and electricity. However, these values merely document the effect. What is missing is the ability to derive the cause and thereby identify measures, for example, to reduce consumption.

The Digital Physical Twin of the Building Design Days + Energy method provides, through system sizing, the maximum expected daily profile of heating demand on the coldest winter day and cooling demand on the hottest summer day, as a benchmark for comparison with the monitoring values. These daily profiles can now be compared with the measured data — and the measured values should always be lower, at least in terms of the daily total.

A maximum power measured in monitoring should always be lower than the dimensioned capacity for the extreme design days. In any case, the measured daily energy should be lower than the daily energy determined for the design day, so that even on the most extreme day the building and its rooms can be maintained at the target temperature.

In addition, the daily profile of the previous day can be quickly reproduced using the input values. By adjusting the design parameters, those can be identified that come closest to the measured profile. This makes it possible to reveal causes related to usage and, if applicable, also effects of shading.

365 days with 24 hours each result in all 8,760 hours of the year.

Although the planner only focuses on the two extreme days (winter and summer) with 24 hours each, and sizes the installed capacity accordingly, the process automatically generates time series of inputs and results for each of the 8,760 hours of the year. This makes it possible to calculate sums or averages for any desired time period. In addition, cumulative frequency curves of any variables clearly illustrate distributions and correlations, usually in relation to the outdoor air temperature. For example, they show the heating curve and cooling curve.

For economic feasibility calculations, such as those in accordance with VDI 2067 Part 1, all required quantities are now available — both with regard to the dimensioned capacity and component size, as well as annual energy and full-load hours. In this method, full-load hours are always an actual result, rather than just an estimate of how many hours a given capacity might be required during the year.

Balance Circuits

The Building Design Days + Energy approach can be broken down from thermal zones (room groups with similar thermal behavior) into individual rooms, heating circuits, cooling circuits, or any other grouping — for example, the rooms supplied by an air handling unit (AHU). Balance circuits can also be scaled up to the district level, thereby creating transparency of cause and effect down to the individual room.

Existing Standards

Existing standards always aim to prescribe the required inputs and calculation methods for a given result. Unfortunately, in Germany we have different inputs and calculation methods for determining heating load (DIN EN 12831), cooling load (VDI 2078), and energy demand for heating, cooling, and electricity (DIN 18599). In addition, summer comfort for limiting overheating in selected rooms is checked (DIN 4108 Part 2).

The inputs and boundary conditions for building and room calculations are required in different ways in the various standards.

• For heating load, only the loss per room from transmission through constructions and from ventilation heat is determined for a single extreme hour — without considering internal heat gains from lighting, equipment, or occupants, and without solar radiation.
  • The assumed conditions are: outside temperature is very cold, the room is dark, it is night, no one is present, all equipment is switched off, ventilation is occurring — and this state must remain constant for up to 36 hours with today’s building insulation in order for the static heat loss through the constructions to be established.
• For cooling load, the most extreme hour is identified through an extreme daily profile, which — repeated three times in succession — determines the peak cooling load. In this calculation, room geometry, constructions, a different air exchange rate, and of course all internal heat gains from occupants, equipment, and lighting are fully taken into account. Solar radiation is included as well, but attenuated by controlled shading calculated individually for each window.
  • The outdoor temperature and solar radiation for the extreme day are taken into account in accordance with the requirements of VDI 2078. After the third calculation day, the hour with the highest cooling load is used. The affected rooms are then evaluated in correlation with respect to their load peaks.
• For the annual energy demand for heating and cooling, a monthly calculation method is applied using the climate data set TRY 04 Potsdam for all of Germany. The result is used only as proof of compliance (permitted or not permitted according to the GEG) and does not provide any planning-relevant insights for the actual project location.
  • In addition, the GEG stipulates that for residential buildings, the electricity demand for appliances and lighting within the building may not be added to the final energy demand. For non-residential buildings, the electricity demand for appliances may not be added. This leads to calculated energy demands that are much lower than in reality, even though in practice these demands must actually be met.
  • For the calculation method to determine the electricity demand of air-to-water heat pumps, a table is provided in which the outdoor air temperatures are classified into ranges and linked with their frequency in hours per year.
• For summer thermal comfort in rooms, a simulation with 8,760 hours of the year is recommended, using predefined boundary conditions regarding internal heat gains.
  • This verification method looks only at summer and not at winter. Measures to comply include glazing with a lower g-value or a better shading system. However, the fact that a lower g-value also reduces light transmission in winter — thereby requiring more electricity for lighting — is not taken into account in this method.

Comparison and Transparency

The Building Design Days Energy (BDD+E) method is linked to transparent, easy-to-understand design days, whose inputs and results can be compared with hourly simulations and with monitoring data from the building and technical systems.

• Thanks to the static balance approach, each individual hour can be verified manually by calculation.

CO₂ emission intensity in the Power Mix

The CO₂ emission intensity in the electricity mix varies for each of the 8,760 hours of the year, depending on which energy sources were used for power generation at that time (see CO₂ in the electricity mix).

The Building Design Days + Energy method shows the CO₂ emission intensity in the electricity mix alongside the corresponding electricity demand. For this purpose, measured data from the EEX power exchange and the Fraunhofer Institute (FHG-ISE) have been processed.

Due to the highly volatile daily profiles, which so far have not been systematically converted into CO₂ design days, no solution is currently available. However, the original hourly electricity mix data and the corresponding energy sources used are available for the years 2018 to 2024.

Design Versions

In the Building Design Days + Energy (BDD+E) method [BDD+E, 2025], it is possible to define a design version by adjusting a combination of selected input values (while keeping the original inputs unchanged). Comparing the design versions then shows the impact on selected result variables. For example, the lowest outdoor air temperature can be set 2 °C colder, and one can observe how, on the winter design day, the room trend temperature with storage effect changes under an identically dimensioned heating capacity for the room heat transfer systems — and whether this poses a problem. This way, the causal link between an increased input value and a directly increased heating load is broken.