Ventilation in Scandinavian buildings is commonly performed by means of a constant flow ventilation fan. By using a regulated fan, it is possible to make a seasonal adjustment of outdoor ventilation flow. Energy saving can be achieved by reducing the mechanical ventilation flow during the heating season, when natural ventilation driven by temperature differences between outdoor and indoor is relatively high. This ventilation principle has been called 'seasonally adapted ventilation (SAV)'. The aim was to study if a 25-30% reduction of outdoor ventilation flow during heating season influenced sick building syndrome (SBS) and the perception of the indoor environment. This was done in a 1-year cross-over intervention study in 44 subjects in a multi-family building. During the first heating season (November to April), one part of the building (A) got a reduced flow during the heating season [0.4-0.5 air exchanges per hour (ACH)] while the other part (B) had constant flow (0.5-0.8 ACH). The next heating season, part A got constant flow, while part B got reduced ventilation flow. Reduced ventilation increased the relative air humidity by 1-3% in the living room (mean 30-37% RH), 1-5% in the bathroom (mean 48-58% RH) during heating season. The room temperature increased 0.1-0.3 degrees C (mean 20.7-21.6 degrees C), mean carbon dioxide (CO2) concentration in the bedroom increased from 920 to 980 p.p.m. at reduced flow. The indoor air quality was perceived as poorer at reduced outdoor airflow, both in the bedroom and in the apartment as a whole. There was a significant increase of stuffy odor (P = 0.05) at reduced outdoor airflow and the indoor air quality was perceived as poorer, both in the bedroom (P = 0.03) and in the apartment as a whole (P = 0.04). No significant influence on SBS symptoms or specific perceptions such as odors, draught, temperature, air dryness or stuffy air could be detected. In conclusion, reducing the ventilation flow in dwellings to a level below the current Swedish ventilation standard (0.5 ACH) may cause a perception of impaired air quality. Technical measurements could only demonstrate a minor increase of indoor temperature, relative air humidity, and bedroom CO2 concentration. This illustrates that it is important to combine technical measurements with a longitudinal evaluation of occupant reactions, when evaluating energy-saving measures.
Practical implications: It is important to combine technical measurements with a longitudinal evaluation of occupant reactions, when evaluating energy-saving measures. Reduction of outdoor airflow in dwellings below the current ventilation standard of 0.5 ACH may lead to a perception of impaired air quality, despite only a minor increase of bedroom CO2-concentration.