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
To keep carbon dioxide concentrations below 1000 ppm, adequate room ventilation is required by supplying fresh outside air in classrooms during class time at a level of at least 4 l/sec per person. High levels of carbon dioxide impair the ability to concentrate and learn. As fresh air is supplied, the humidity level of the indoor air in the classrooms also increases. Because of the extra moisture that occurs when people breathe and sweat - 30 people produce about 1.5 l of water vapour per hour - the moisture must be removed by ventilation or dehumidification. The minimum amount of air in a classroom during classes should normally be above 120 l/sec. Proper ventilation can only be achieved with appropriate mechanical systems. These usually include mechanical supply and exhaust ventilation (two-way ventilation) and mechanical exhaust ventilation (one-way ventilation). Where two-way ventilation is not possible, one-way ventilation can be used and fresh air is supplied through specially designed sets of ventilation radiators. The air is filtered and heated in them. They provide adequate thermal comfort in classrooms.