Julkisten rakennusten ilmanvaihdon käyttöaikojen vaikutus työolosuhteisiin ja sisäilman laatuun: Työsuojelurahaston hanke 180094

The public sector employs about 30% of the total employment. In addition, the building users include just under a million children and students. Therefore, a good indoor climate and the proper use of ventilation systems are important. Night ventilation is used to improve indoor air quality in public buildings before the premises are used. A typical use has been to turn off the ventilation after using the facilities and restart it about 2 hours before reusing those facilities. Another option is to keep night ventilation running at minimum ventilation. The third option is to use night ventilation intermittently. Nowadays it is common to keep ventilation units running continuously to ensure good indoor air quality in all situations, even if it significantly increases the energy consumption of ventilation.

The goal of this research project of the Finnish Work Environment Fund during 2019-2021 was to find out how night-time ventilation in public buildings should be used wisely so that good indoor climate conditions can be maintained, and unnecessary energy consumption can be avoided. The study measured the effects of night-time ventilation on indoor air quality and thermal conditions in 11 public buildings with no previously reported indoor air problems. In addition, the potential for microbial growth in the ventilation duct was examined in the laboratory. The study compared a stopped, continuous, and intermittent night ventilation strategy. Daytime ventilation was used normally in all cases. Each test case lasted for 2 weeks. Indoor air quality was assessed by measuring TVOC, carbon dioxide, and microbial concentrations. The pressure differences were measured over the external wall of the building and the ventilation operation was monitored by measuring the pressure difference of the devices.

The results show that the average TVOC concentrations in the mornings were at the same level in all ventilation strategies. TVOC concentrations were higher during the day than at night. This indicates that the use of the facilities had the greatest effect on the TVOC concentrations. In the samples of settled dust, the microbial concentration was usually only a few percent of the corresponding outdoor air concentration. The used strategy of night ventilation did not have a systematic effect on indoor microbial concentrations. It has been hypothesized that the contaminants in ventilation ductwork can flow into the room when the ventilation unit is started. However, in the ventilation start-up test, the particulate matter levels were not increased in the supply air. In the laboratory test, no mold growth occurred in humid conditions during a 3-month measurement. The throw distance of a supply air terminal unit is usually poor at a low ventilation rate, and therefore, if the low airflow is used at night, the supply air temperature should be lower than the indoor air temperature because it mixes into space more efficiently than the isothermal supply air. In general, the natural variation of the measured physical quantities was greater during the test periods than could be observed with different night ventilation operating strategies. The working conditions at the measured buildings were at normal levels. The results show that 2 hours of ventilation before the premises are used is sufficient, and therefore, continuous ventilation at night is not necessary.