Minimum air exchange

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The minimum air exchange is generally understood to mean the exchange of air in rooms or buildings to ensure the indoor climate and the building's protection. The air exchange limits increased substance concentrations and moisture values ​​in the rooms / buildings.

Legal situation in Germany

This article or paragraph presents the situation in Germany . Help us to describe the situation in other countries.

In Germany, a distinction is made between two types of air exchange in building technology: the hygienic minimum air exchange, which is required according to EnEV and specified in DIN 1946-6, and the air exchange, which is set depending on the building, depending on the use and the substances occurring; this is mostly used in non-residential buildings.

Hygienic minimum air exchange

The hygienic minimum air exchange is required according to EnEV and is more precisely defined in DIN 1946-6. The hygienic minimum air exchange is required in apartments and similarly used room groups in order to prevent the room climate of people and room furnishings from being negatively influenced by substances such as CO ₂ , humidity, volatile organic substances (VOC) and odors. The substances that appear vary depending on the usage behavior, activity and equipment of the respective room.

Minimum air exchange in non-residential buildings

In contrast to the hygienic minimum air change, this minimum air change is not required by EnEV, but relates to the respective building or building. Room type and is anchored in the specific guidelines. The type of use plays a major role in the implementation. For example, different guidelines apply to places of assembly than to places of work. In general, it is important to ensure the quality of the air you breathe; in the case of meeting places, this is achieved through a minimum air exchange depending on the number of visitors. In the case of workplaces, on the other hand, the minimum air exchange is not only based on the substance loads entered by people, but also on the type of work performed there. It should also be noted that a certain level of indoor climate must be guaranteed for certain processes in workplaces, for example the humidity in food production.

The concentration of the substance loads in the air must also be taken into account, as this can have an adverse effect on the work safety of the employees. In particular, it must be ensured that the concentration of CO ₂ , volatile organic substances , formaldehyde or fibers from product production or storage is kept low.

The moisture concentrations must also be observed in order to avoid mold formation and the resulting health hazards for employees, but also economic damage from damage to products. Humidity loads from people and production must be compensated for by a minimum air exchange, and the heat loads caused by machines, lighting and people must also be dissipated in order to ensure a pleasant room climate.

Realization of the minimum air exchange

In order to understand why a minimum air change is required in buildings and how this can be implemented technically, a brief insight into general ventilation technology is necessary. Without our noticing it in our everyday life, there is a constant change of air in our buildings. This is mostly done with the help of cross ventilation . In the case of larger objects or very modern, airtight buildings, such free ventilation is not possible for structural or energetic reasons, because free ventilation can only influence the indoor air quality for a limited time. In such cases, mechanical ventilation is used, as this can ensure favorable indoor air quality regardless of the user.

The minimum air exchange in buildings must be guaranteed for various reasons. First of all, the well-being of the user must be guaranteed, so fresh air must be introduced regularly so that the CO ₂ concentration in the room air does not rise exorbitantly. During normal activities, an air change of 25 m ³ / h per person is expected to ensure pleasant CO ₂ concentrations. Furthermore, the building itself must be protected, because if the air in the room is not changed, both its temperature and its moisture content rise. The users carry heat and moisture into the room, which is exchanged for relatively dry fresh air through an air exchange, so that the relative moisture content in the room air drops. The exchange of air, i.e. the lowering of the CO ₂ concentration and the lowering of the relative humidity in the room, contribute to the comfort for the user.

Both types of ventilation are subject to the same physical basis; they are both based on the principle of conservation of mass, i.e. as much air as flows out of a room (regardless of whether it is natural or mechanical), as much air flows in as well. In special applications such as laboratories or operating rooms, it is also possible to create a negative pressure by cleverly arranging air outlets and air supply openings.

Free ventilation

Free ventilation is the simplest type of ventilation and is mostly used in simple residential buildings. However, the framework conditions must be right for the use of free ventilation (cross ventilation). The outside air around the building must not be heavily polluted so that it can be used unfiltered as fresh supply air. In addition, care must be taken to ensure that the environment is not permanently exposed to higher noise emissions, which would prevent permanent ventilation that may be required. Not every room is suitable for free ventilation, so the window surface must fit in relation to the room geometry, i.e. the supply air area in relation to the room volume, paying particular attention to the tilt angle of the window. When using free ventilation, it is also not possible to influence the state of the air, with the result that the comfort in the room cannot be influenced. User-dependent ventilation is also almost always possible, with free ventilation mostly used in residential properties or rooms that are constantly occupied during their use.

To determine whether free ventilation in the residential building or in the respective usage unit is sufficient, the ratio of the infiltration volume flow, i.e. the volume flow generated by leaks in the building envelope, to the volume flow for moisture protection; the infiltration volume flow must always be greater than that Volume flow required for moisture protection. Since the volume flow for moisture protection is a user-independent variable that must be guaranteed under all circumstances, the following applies:

${\displaystyle {\dot {V}}_{\text{Infiltration}}>{\dot {V}}_{\text{Luftwechsel zum Feuchteschutz}}}$

If it is not possible to ensure moisture protection through infiltration, mechanical ventilation is required.

Free ventilation through windows and door openings

Residential buildings

In residential buildings, according to DIN 1946-6, free ventilation is divided into different levels:

Ventilation for moisture protection (LF)

This is the minimum air change required to prevent moisture damage in the building. This must be guaranteed permanently and independently of the user. Ventilation for moisture protection is usually ensured by the infiltration volume flow, i.e. by the volume flow through leaks in the building envelope. If this is not the case, mechanical ventilation is required.

Reduced ventilation (RL)

Reduced ventilation is the ventilation necessary to maintain hygienic conditions, i.e. H. CO ₂ concentration and the moisture content in the room, under normal conditions of use with partially reduced moisture and material loads, regardless of the user.

Nominal ventilation (NL)

Necessary ventilation to guarantee the hygienic requirements as well as the building protection. The nominal ventilation is required during normal operation, i.e. when the users are present, and window ventilation, which is partially carried out by the user, is required.

Intensive ventilation (IL)

Intensive ventilation is ventilation that temporarily provides an increased volume flow; it is required to reduce load peaks in the room.

Extract air system with central fan and outside air supply through windows and door openings

Non-residential buildings

The requirements for a ventilation system in non-residential buildings or in workplaces are presented in the Workplace Directive. In the ASR 3.6, free ventilation is divided into two different systems:

System I.

System I describes a one-sided ventilation with inlet and outlet air openings in an outer wall, whereby a common opening for inlet and outlet air is permissible. This describes the simplest form of free ventilation, usually window ventilation. The big advantage is that the ventilation can be determined by the users themselves.

System II

System II describes a cross ventilation with openings in opposite outer walls or in an outer wall and the roof surface.

Which of the two system types is used differs from building to building, however, it must always be ensured that the necessary opening areas for air exchange are guaranteed. When designing windows as opening areas, particular attention should be paid to the tilt angle. If windows are used for continuous ventilation, it is assumed that they are opened on "tilt", the opening area is composed as follows.

Clear explanation for the calculation of air intake openings for tilted windows.

${\displaystyle A_{\text{kipp}}=a\cdot (B+H)}$

With forced ventilation, it is assumed that the user opens the window completely; thus the full window area is regarded as the opening area:

${\displaystyle A_{\text{Stoß}}=B\cdot H}$

If the minimum opening area for continuous ventilation cannot be achieved, mechanical ventilation is necessary.

Mechanical ventilation

If the above requirements for free ventilation cannot be met, mechanical ventilation is used. This is especially the case in many modern buildings that are built in accordance with the EnEV standard, as they no longer achieve the necessary infiltration volume flow due to their high degree of impermeability.

Table of air exchange rate and minimum air volume flow per person in rooms and buildings

Room / building	Air exchange rate 1 [1 / h]		Minimum air volume flow per person 2 in [m 3 / (h · person)]	Technical regulations 3
	Min	Max
Residential buildings / living spaces	0.5	3.6		DIN 13779 (no longer valid) / EN 12831
Residential building with mechanical ventilation	> 1			DIN 13779 (no longer valid) / EN 12831
Residential buildings> 3 floors	> 2			DIN 13779 (see above) / EN 12831
Kitchens	1.5			DIN 13779 (see above) / EN 12831
bathroom	1.5			DIN 13779 (see above) / EN 12831
Lecture halls		6-8		DIN EN 16798 part 3 ASR 3.6
Classroom	2			DIN EN 16798 part 3 ASR 3.6
offices	1	4-8		DIN EN 16798 part 3 ASR 3.6
movie theater		5-8		VDI 2082
Restaurants		4-8	40-60	VDI 2082
Salesrooms		4-8		VDI 2082
gym		4-6
workshops	4-6	10-20		VDI 2082
Commercial kitchens		15-30	6-60	VDI 2052
Fish and meat preparation			25th	VDI 2052
Vegetable and salad preparation			25th	VDI 2052
Cold kitchen			6-10	VDI 2052
storage			6 m³ / (h · m²)	VDI 2052
Distribution rooms			60	VDI 2052
Swimming pool				VDI 2089
entrance				VDI 2089
Showers / sanitary facilities		15-25		VDI 2089
laboratory	2-5	8-15		VDI 2051
Chemical rooms		5		VDI 2051
Pressurized gas cylinder rooms		2		VDI 2051
hospital		5-8	activities	DIN 1946-4
OP	-			DIN 1946-4
OR with gas application			40+ 150 per patient	DIN 1946-4
Adjacent rooms			40	DIN 1946-4
Patient room			40+ 100 per patient	DIN 1946-4
Ice sports facility with spectators / multi-purpose hall		4-6	30-40	VDI 2075
Ice rink without spectators	1.5		30-40	VDI 2075
have a shower		15-25		VDI 2075
Lounges / meeting rooms				VDI 2075
Changing rooms		6-8	50	VDI 2075
Conditioning rooms		4-6		VDI 2075
Slope bar			50	VDI 2075
gastronomy			50	VDI 2075

1 values ​​recommended in practice

² According to DIN EN 1946-2

³ Contains exact calculation bases

Calculation example

In order to support the above-mentioned theoretical principles with numerical values, the following calculation should be assumed:

The minimum air exchange should be achieved in a restaurant with a dining room. The building has a floor area of ​​160 m ² (D = 8 m and L = 20 m) and a room height of 3 m. It is assumed that there will be a maximum of 120 people in the restaurant. Special attention should be paid to the indoor air quality so that the maximum CO ₂ concentration is 1000 ppm.

Calculation method 1: CO ₂ balance according to ASR A3.6 and the resulting outside air volume flow

To ensure good indoor air quality, the CO ₂ concentration in the indoor air must be kept low. According to ASR A3.6 , the upper limit is 1000 ppm.

On the basis of this regulation, a CO ₂ balance is created on the assumption that each person enters 15 l / h CO ₂ into the room and the outside air supplied (SUP) has a CO ₂ concentration of 400 ppm (averaged value for the outside air in German cities), the result is the following:

${\displaystyle C_{\text{CO2 Ausstoß pro Person}}\cdot n_{\text{Personenanzahl}}+{\dot {V}}_{\text{Zuluft}}\cdot C_{\text{CO2 Außen}}={\dot {V}}_{\text{Abluft}}\cdot 1000\,\mathrm {ppm} }$

${\displaystyle {\dot {V}}_{\text{Zuluft}}={\dot {V}}_{\text{Abluft}}}$

${\displaystyle {\dot {V}}_{\text{Abluft}}={\frac {15l/(h\cdot Person)\cdot 10^{-3}\cdot 120\,{\text{Personen}}}{(1000-400)\cdot 10^{-6}}}}$

${\displaystyle {\dot {V}}_{\text{Abluft}}=3000\,\mathrm {m^{3}/h} }$

Calculation method 2: outside air volume flow according to DIN EN 13779

In order to meet the requirement of ≤1000ppm, according to DIN EN 13779 Tab. A.10 the indoor air quality according to category IDA 3 is to be used, Tab. A.11 the category corresponds to an outside air volume flow of 6 l / s · person, and thus results from DIN EN 13779 following volume flow ${\displaystyle C_{CO_{2}}}$

${\displaystyle {\dot {V}}_{IDA3}=n_{\text{Personen}}\cdot {\dot {V}}_{\text{Abluft pro Person}}}$

${\displaystyle {\dot {V}}_{IDA3}=\ 120\,{\text{Personen}}\cdot 6l/(s\cdot {\text{Person}})\cdot 3{,}6\,\mathrm {(m^{3}\cdot s)/(l\cdot h)} }$

${\displaystyle {\dot {V}}_{IDA3}=2592\,\mathrm {m^{3}/h} }$

Calculation method 3: outside air volume flow according to DIN EN 13779 predecessor DIN 1946-2

According to the predecessor of DIN EN 13779, DIN 1964-2, which is cited from time to time, the outside air volume flow is assumed to be 40 m ³ / h per person. This results in the following volume flow

${\displaystyle {\dot {V}}=n_{\text{Personen}}\cdot {\dot {V}}_{\text{pro Person}}\,\mathrm {m^{3}/(h\cdot Personen)} }$

${\displaystyle {\dot {V}}=120\,{\text{Personen}}\cdot 40\,\mathrm {m^{3}/(h\cdot Person)} }$

${\displaystyle {\dot {V}}=4800\,\mathrm {m^{3}/h} }$

Calculation method 4: Outside air volume flow using air exchange rates from empirical values

Experience from practice can be found in various literature. With these values, the room volume is subjected to a certain air exchange based on its use. The so-called air exchange rate LWR, the product of room volume and air exchange rate, gives the required volume flow. The value for the restaurant or its dining rooms is 8 1 / h, and this results in the following outside air volume flow:

${\displaystyle {\dot {V}}=LWR\cdot V_{\text{Raum}}}$

${\displaystyle {\dot {V}}=8\,\mathrm {1/h\cdot 480m^{3}} }$

${\displaystyle {\dot {V}}=3840\,\mathrm {m^{3}/h} }$

Comparison of the calculation methods

Percentage deviation of the minimum volume flows in relation to the CO ₂ balance

Decision, free ventilation or mechanical ventilation?

If one now assumes the outside air volume flow calculated according to ASR 3.6 and a room geometry of (T = 8 m and L = 20 m) with opposite windows, then according to ASR 3.6 Tab. 3, free ventilation is initially permitted. Assuming standard windows of size B = 1 m and H = 1.2 m with an opening gap of a = 11 cm and calculating the necessary number of windows: ${\displaystyle 3000\,\mathrm {m^{3}/h} }$

${\displaystyle A_{\text{Notwendig}}=n_{\text{Personen}}\cdot 0{,}2\,\mathrm {m} ^{2}}$

${\displaystyle A_{\text{Notwendig}}=120\cdot 0{,}2\,\mathrm {m} ^{2}}$

${\displaystyle A_{\text{Notwendig}}=24\,\mathrm {m} ^{2}}$

${\displaystyle A_{\text{kipp}}=a\cdot (B+H)}$

${\displaystyle A_{\text{kipp}}=0{,}11\,\mathrm {m\cdot (1\,m+1{,}2\,m)} }$

${\displaystyle A_{\text{kipp}}=0,242m^{2}}$

${\displaystyle n_{\text{Fenster}}={\frac {A_{\text{Notwendig}}}{A_{\text{Fenster}}}}}$

${\displaystyle n_{\text{Fenster}}=\mathrm {\frac {24m^{2}}{0,242m^{2}}} }$

${\displaystyle n_{\text{Fenster}}=99{,}2\thickapprox 100\,{\text{Fenster}}}$

Even if you increase the size of the windows significantly:

Ex. (W = 5m, H = 2m and a = 20cm) ${\displaystyle A_{\text{Fenster}}=\mathrm {0{,}2\,m\cdot (2m+5m)} =1{,}4\,\mathrm {m} ^{2}}$

${\displaystyle A_{\text{Notwendig}}=\mathrm {0{,}2\,m\cdot (2\,m+5\,m)} }$

${\displaystyle A_{\text{Notwendig}}=1{,}4\,\mathrm {m} ^{2}}$

${\displaystyle n_{\text{Fenster}}=\mathrm {\frac {120\cdot 0,2\,m^{2}}{0{,}2m\cdot (5\,m+2\,m)}} }$

${\displaystyle n_{\text{Fenster}}=17{,}2\thickapprox 18\,{\text{Fenster}}}$

Plausibility: Given the room geometry and the arrangement of the windows, free ventilation is not feasible and mechanical ventilation is necessary.

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• Workplaces Directive 3.6
• DIN EN 13779 replaced by DIN EN 15251
• DIN 1946-2
• DIN 1946-6
• EnEV 2014 (unofficial version)
• Recknagel - Pocket book for heating + air conditioning 77th edition 2015/16