The following points should be borne in mind when planning filter installa­tions.

• Great care is required regarding the positioning and design of the air intake to avoid drawing in local impurities and rain or snow.

• The risk of microbial growth is low, but to minimize the risk, the plant should be designed so that relative humidity is always below 75% in all parts of the system.

• For reasons of hygiene, the inlet air should be filtered in two steps.9 The first filter in the air intake must be at least F5 (EU5) quality but preferably F7 (EU7). The second filter step should be effected by a filter of at least F7 (EU7) but preferably F9 (EU9) quality. If there is only one filter step, the minimum requirement is for F7 (EU7) quality.

• As regards recirculated air, at least F5 (EU5) quality must be used to cope with contamination of components in the system, but the minimum requirement is for F7 (EU7) if the environment in the room is to be improved.

• The exhaust air system must be protected from contamination by a filter of at least F5 (EU5) quality.

• Filters must not be installed directly after the fan outlet or across places where there is a big change in area or flow.

• The final pressure drop is dimensioned and selected with regard to permitted variations in flow, the filter’s life-cycle costs, and life-cycle analysis.

• Dust-holding capacity and test results from laboratory trials differ from performances in actual use.

• The efficiency must not deteriorate or fall below specific minimum values.

• The tightness and condition of the filter are checked regularly by visual inspection of the plant.

• Filter and aggregate must be clearly marked with the type and designation of the filter, date of installation, etc.

• In the case of more stringent requirements, in situ checks of the filter are carried out according to Eurovent 4/10.

• Filters must be replaced when the pressure loss reaches the dimensioned final pressure loss or when the following hygiene interval is reached, if this is earlier:

• The first step should be replaced after a maximum operating period of 8700 hours.

• The second step and filters in exhaust or recycled air systems should be changed after a maximum of two years’ continuous operation.

• For reasons of hygiene, the filter should be replaced after the pollen and spore season in the autumn. In the case of high demand, filters can also be changed in the spring after the heating season to eliminate odorous combustion products.

• Filters should be replaced carefully, using protective equipment so that the impurities trapped do not escape.

• It is a good idea to incinerate filters in well-filtered furnaces in order to burn trapped impurities, reduce quantities of waste, and recover some energy. The filters from normal ventilation systems could also be dumped at a landfill.