Air conditioning of nurseries and schools

Building and air quality requirements

Newest investigations show that the air quality in class rooms is responsible for the academic success. The times where the temperature and CO2 level of class rooms were controlled by opening and closing of windows is over. More airtight buildings require the use of mechanical and controlled air conditioning. This fact has to be taken in account at the planning of restructuring and building works of school facilities.

Class rooms have an average occupancy of 35 people. To guarantee a CO2 level below 1,000 ppm in the air of the class rooms, an outdoor air flow of 30 m³/h per person is required. This corresponds to an outdoor air flow of 1.050 m³/h so that a four- to fivefold air exchange is reached. The supply air system has to bring that air volume draft-free into the class rooms and to ensure the heating and cooling functions.

Nowadays an air conditioning which is not able to provide a comfortable climate because of drafts or different climate areas is not accepted. The focus at the equipment of nurseries and schools is to provide the room with conditions which doesn’t cause health or state problems and therefore ensures a complete physical and mental performance.

Indoor climate and energy efficiency


Picture 1: Indoor climate and energy efficiency

Picture 1 shows the tasks which result from the energetic requirements. Modern buildings get perfectly thermally insulated of which the result is that the buildings are almost perfectly airtight. Related to the transmission losses of the building, this is positive for the energy balance.

Airtight buildings do not offer air exchanges though. This means that the CO2 concentration of the breathing air is continuously rising. The students additionally generate approx. 50 g/kg steam so that the air humidity is also rising.

The consequences are that students ability to concentrate on the school work decrease and the increased humidity in the building can lead to mold on the walls. Airtight buildings tend to emit substances that evaporate or diffuse from building parts to the air. This fact can’t be prevented during the building phase.

It was the same with old buildings but untight buildings led to a permanent air exchange which didn’t let a concentration of pollutants happen. Primarily window air conditioning can be used to work against this. That lowers the CO2 concentration for a short time and brings the humidity out of the building.

Therefore the incoming outdoor air has to be heated up to the room temperature. This system requires a lot of energy and creates big temperature fluctuations as well as an uneven room temperature level in the room. This can only be solved by the application of a mechanic air conditioning.

An energy efficient air handling unit is almost covering the heat losses with the heat recovery system. Modern units use the indirect evaporation of water in the extract/exhaust air channel of the recuperator and provide comfort climate even in hot summer months. Thanks to the controlled mechanic air conditioning the efficiency stays high without harming the room climate.

Normative requirements

As in other buildings an airtight construction of schools is required for the energy efficiency of those. That design mandatorily requires a mechanic air conditioning. Besides the valid building regulations the norms of DIN EN 15251, DIN EN 13779 and VDI 6040 have to be considered. Superordinate the VDI 6040 specifies an operative class room temperature between 20° and 26° in dependence to the outdoor air temperature. Thereby it’s assumed that no student is exposed to the direct solar radiation. Furthermore a pollutant free room is implied and the exceedance of a CO2 concentration over 1,000 ppm is not allowed.

The following table helps for the interpretation of the outdoor air volume flow for ventilator supported air conditioning units by DIN EN 13779:

Category

description

Rising of the CO2 concentration toward to outdoor air (ppm)

Outdoor air volume flow (m³/h) per student

IDA 1

High room air quality

< 400

>54

IDA 2

Middle room air quality

400 - 600

36 - 54

IDA 3

Moderate room air quality

600 – 1,000

22 - 36

IDA 4

Low room air quality

> 1,000

>22

Table 1: Indoor room quality and outdoor air volume flow by DIN EN 13779

With a permitted CO2 concentration of 1,000 ppm a school falls under the IDA 3 section with an outdoor air volume of 22-36 m³/h per person. The interpretation of the outdoor air volume flow by DIN EN 15251 goes another path. In this guideline an area of 2 m² per student is taken as a basis. The following table is showing the interpretation of a low-pollutant building:

 

 

Air volume for pollutions by building emissions m³/(hm²)

 

Example for a low-emission building

2m²/person

Category

Air volume flow

m³/h (students)

 Very low-emission building

 low-emission building

not low-emission building

(1)

(2)

(3)

(4)

(5)

(2)+(4)x2

I

36

1,8

3,6

7,2

43 m³/h

II

25

1,3

2,5

5,0

30 m³/h

III

14

0,7

1,4

2,9

17 m³/h

 

The DIN EN 15252 determines three categories of the indoor climate which specifies the degree of expectations of the user and allow the application of the categories for buildings with different ages or conditions. Category II applies for normal expectations for a new building, category III applies for moderate expectations for an existing building and category I applies for very high expectations and should be used for persons with physical disabilities or very little children only. The table itself shows the three categories of the pollution by the building. The given rates are shown per person and m² area. So for a new school construction with the category II the results of an outdoor air volume flow are:    

(2)  + 2 x (4)

25 m³/h + 2 x 2,5 m³/(hm²) = 30 m³/h/person

Besides the temperature, the humidity and the CO2 level, the acoustic of a classroom does have a significant influence on the performance of the students and teachers. Low noise levels make it possible for teachers to speak hassle-free and keep up the communication with the students. The acoustical perception of the students is mainly affected by sound reflection, reverberation time and background noises. The DIN 4109 prescribes a permitted noise pressure level of 35 dB(A). The limit of the noise pressure level for students with restricted hearing or the understanding of difficult and foreign-language texts is 30 dB(A).

For the execution of refurbishments and new constructions the valid ErP-guidelines and the F-gas regulation have to be applied. On the one hand the efficiency criteria for central and decentral air handling units lower the energy demands for the air conditioning and cooling but on the other hand the unit gets way bigger by fitting the bypasses and the heat recovery into the casing. The construction of engineering rooms and central ventilation stations should be big enough to fit the units in there without any struggle. Additionally the accessibility to the AHU for maintenance works and hygienic inspections has to be considered in the planning process of an engineering room.

Special attention should be paid on the cooling technology. In a modern building heat pumps and chillers find a huge range of application. The F-gas regulation which came into force on the 1st of January 2015 plays an important role in the future of school buildings. With the Phase Down scenario this regulation lowers significantly hydrofluorocarbon (HFC) which are available on the European market.


Picture 2: F-gas regulation Phase-down-scenario

In the planning process of engineering rooms the fact that future technologies need more space has to be taken into account. The operator obligation of the F-gas regulation for new installations as well as for already installed heat pumps and chillers is binding:

  • General emission reduction obligation (Article 3 section 1 and 2)
  • Repair obligation (Article 3 section 3)
  • Service check of density obligation (Article 3 section 1 and article 4 section 1)

This operator obligation will increase the training requirements for the technical staff of the schools and make the training more challenging.

Modern schools aren’t consisting of classrooms and staff rooms only. Most schools also have swimming pool halls, break rooms, auditoriums, kitchens, gyms, canteens, laboratories, science areas or technical rooms. These different areas set various operation requirements to the building technology.

Besides the general application of the VDI 6040 “ventilation and air conditioning in schools” the DIN EN 13779 and DIN EN 15251 for the determination of outdoor air amounts are valid. The additional areas are covered by single norms and regulations whose application is depending on the use.

The previously detected outdoor air volumes of 30 m³/h per person create air volumes in the class rooms of 900 to 1.100 m³/h. With multiple classrooms at one corridor a central air conditioning unit with single room controls makes sense. The classrooms can be provided with outdoor air and the necessary heating and cooling performance through the engineering room.

The maintenance is focused on works in the engineering room which can be done even during classes in an emergency case. A good heat insulation of modern buildings combined with a highly efficient heat exchanger enables the heating of the classroom by air.

The outdoor air volumes should be enough to cover the heating needs of the rooms. The central heating supply by the damper register reduces the investment costs as a static panel heating is not required anymore.

An energy efficient air handling unit usually has such a powerful heat recovery system that the air infiltration heat demand is reduced to a minimum. To avoid environmentally unfriendly F-gases the use of indirect, adiabatic cooling systems for the air conditioning of the rooms would make sense.

Adiabatic systems not only provide an environmentally friendly cooling capacity but they also reduce the energy needs for the cooling in substantial amounts and reduce the connected loads of the whole technical air handling installations.

Summarizing we can say that the energy efficient air handling with outdoor air not only lowers the energy needs of the school but also increases the achievement potential of the students.

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