The main reason is that the requirements for proper school heating, cooling and ventilation are not understood to its full extent in the design and implementation phase.
The school building is very dynamic in terms of heating. Periodic use, empty rooms at night and on weekends and high and rapidly changing heat loads on weekdays, requires a rapid response from the heating system. Also, warming up classrooms after they have been empty must be done quickly.
Cold radiation in classrooms
Classrooms usually have large windows to make use of the outside light. The window surface is cooler during the heating season than the other surfaces of the classroom, resulting in an asymmetry of thermal radiation that students sitting near the window feel uncomfortable. Radiators underneath the windows compensate for this "cold radiation" and stop the cold air flowing through the window.
Classrooms are also challenging in terms of cooling. As mentioned above, the thermal load caused by students can be in the order of 3 kW, the direct solar radiation from the windows on the southern façade is up to 400 W per window square meter, that is, if a 10 m2 window is in the classroom. Instant window ventilation should be used if and when possible, for example during breaks. However, window ventilation does not provide sufficient cooling and ventilation effect in warm outdoor conditions. In this case, mechanical cooling is required during the lessons. The demand for cooling power is very high in the classroom, up to 200 W per m2 of floor space. The best conditions are achieved with sail-type suspended roof panels. They operate in a draft-free and silent manner. Fan coils are also used, but they have a problem with loudness and the risk of cold draught is high.
Ventilation in schools
Ventilation, or fresh outdoor air, is required in classrooms during the class to be at least 4 L / s per person to keep the carbon dioxide concentration below 1000 ppm. High levels of carbon dioxide impair the ability to concentrate and learn. Also, the humidity in the air rises with the humidity supplied by the outside air. Due to the humidity load caused by human breathing and sweating - 30 people produce about 1.5 L / h of water vapor - humidity should be removed by ventilation or dehumidification. The minimum amount of ventilation in the classroom during a class should typically be above 120 L / s. Adequate ventilation can only be achieved by mechanical systems. These include mechanical supply and exhaust ventilation (bidirectional ventilation) and mechanical exhaust ventilation (unidirectional ventilation). In cases where bidirectional ventilation is not possible, unidirectional ventilation can be used and fresh air supply is provided by ventilation radiators. The air is then filtered and pre-heated in the ventilation radiator.