The new Theodor-Heuss-School in Wetzlar demonstrates how future-proof educational buildings operate. The sustainable hybrid construction combines modern aesthetics with sophisticated building technology. The air-based concrete core cooling ventilation system CONCRETCOOL supplies the classrooms with hygienically fresh outdoor air. In addition to providing a comfortable indoor climate, the maximum utilization of free cooling ensures high energy efficiency.

In Wetzlar, the largest school construction project in the history of the Lahn-Dill district was realized, including the Theodor-Heuss-School, which opened in summer of 2023. The project was planned by the joint venture (ARGE) Diehl Freischlad Schmees-Wagner architects. The new building complex consists of four structures housing various subject areas. Serving as a connecting element is a three-story atrium. Constructed with steel and fully glazed outer walls, it is an optical highlight and a focal point for the various access paths leading to over 200 rooms. Overall, the new school, spanning approximately 12.000 m², accommodates around 1.400 students.

Sustainable Hybrid Construction
The architects aimed to design a modern learning environment while simultaneously creating a holistically resource-efficient educational facility, starting with the remnants of the demolished existing structure. These remnants were repurposed as construction debris for the new foundation. The new school building is designed as a hybrid construction following Passive House principles: the foundation and structural elements such as columns, floors, and staircases are constructed from solid concrete, while the exterior and interior walls are realized in cross-laminated timber (CLT) construction using solid wood, some of which remains visible. The natural material creates indoors, particularly in harmony with the expansive views of the green surroundings a comfortable learning atmosphere. Externally, the building ensemble is unified into an optical whole by a ventilated wood facade system. Thanks to the high degree of prefabrication of floors and walls, as well as short transportation distances, the architects and planners achieved a rapid and resource-efficient implementation of the project with the hybrid construction method. Consequently, the new building attains Platinum standard according to the "Cradle to Cradle" (C2C) approach. C2C aims for an idealized, closed-loop raw material cycle. In the construction industry, this means selecting products and materials with a focus on their purity for disassembly and recyclability during the design phase.

Energetically designed with concrete core cooling ventilation system
A closer look behind the facade reveals a well-thought-out HVAC (heating, ventilation, and air conditioning) concept where both materials – wood and concrete – play to their strengths. Wood, being a natural insulator, offers excellent thermal protection in the solid CLT (cross-laminated timber) structure, inherently providing high insulation for cold months due to its low transmission heat loss. In contrast, during the summer, the high thermal mass of the solid construction helps mitigate overheating within the interior.

The goal was to meet the high architectural standards of a modern, aesthetic, and sustainable school building with a maximally energy-efficient HVAC concept. Rempe Polzer Engineers from Gießen were responsible for the planning of the air conditioning technology. The engineers combined modern standards with technical design, seamlessly integrating it into the building's overall look.

On the school's green sloped roof, a photovoltaic system with 416 modules is installed, generating a total power of 156 kWp. The heating is realized by an underfloor heating. For the planning of the complex ventilation and cooling system, Rempe Polzer Engineers once again enlisted the expertise of Kiefer Klimatechnik. Having successfully collaborated on several projects, including the nearby Goetheschool, the engineers were impressed by the high technical competence in execution planning and the reliable delivery service. Therefore, it was quickly decided that the innovative concrete core cooling ventilation system CONCRETCOOL, would be implemented in Wetzlar as well.

Its operation is as simple as it is highly efficient: CONCRETCOOL leverages free cooling, combining building activation with ventilation functions for optimal thermal comfort and energy savings of up to 50 percent. As air diffusers and cooling pipes are integrated into the concrete ceiling, this clever ventilation system also provides full flexibility in ceiling design.

Approximately 4.300 meters of cooling pipes are installed in a conditioned floor area of A = 6.065m² within the reinforced concrete slabs of the buildings. The cool outside air, with an optimal supply air volume flow for CONCRETCOOL ranging from 6 to 7.5 m³/hm², flows through the cooling pipes within the concrete ceiling and warms up to nearly ceiling temperature. The concrete mass serves as a cold or heat storage and is charged during the night to support the temperature control of the rooms during the day. Throughout the day, the thermal capacity of the concrete leads to only a slight increase in room temperature. The energy in the room is then used for reheating the supply air. This allows the stored heat to be released at a later, energetically more sensible time, such as during the night or early morning hours. The process is self-regulating and nearly fluctuation-free, ensuring high temperature stability.

Marcus Auer, Project Manager and Sales at Kiefer, explains: "The use of fresh outdoor air as an energy source in this system is ideal for new school buildings. For concentrated work and learning – and also to reduce the potential concentration of virus-laden aerosols – classrooms always require fresh air. Fresh, cool air is available for free and up to 70 percent of the year with temperatures below 14 °C.” Through the continuous exchange of indoor air, an increase of the CO2 concentration is prevented. A special feature of the ventilation system is the supportive adiabatic, indirect cooling of the supply air during particularly hot summer days. Active cooling is generally not permitted in school buildings. In this case, the exhaust air is sprayed with finely misted water. The evaporative cooling – but not the humidity – is transferred to the supply air through a rotary heat exchanger.

During winter, students generate more heat than what escapes through the well-insulated building envelope. It causes a further reduction of the heating demand for the classrooms. The heating is provided indirectly by the occupants and CONCRETCOOL. In this scenario, the radiant heating warms the room initially to the desired starting temperature and is then significantly lowered when the classrooms are occupied. In conjunction with Free Cooling, this results in double energy savings. The introduction of supply air for the CONCRETCOOL system is realized through air ducts installed in the floor. The internally ripped cooling pipes run from the floor in the reinforced concrete ceiling to the concreted plenum boxes of the Kiefer ceiling swirl diffusers GLS 400. These diffusers introduce supply air into the rooms without creating drafts, meeting the hygienic fresh air requirements and creating a comfortable environment for studying and learning.