Mechanical positive input and extract ventilation are common strategies employed in English houses, generally because they provide adequate indoor air quality and specifically because they are effective at minimizing mould growth and its associated negative health consequences. Air is either exclusively supplied or extracted (never both) by a mechanical system at a prescribed airflow rate designed to ensure adequate indoor air quality. The remaining airflow, required to balance mass through the building, occurs through the fabric, through a mix of purpose provided and adventitious airflow paths. However, there is an important need to reduce adventitious airflow during the heating season in order to save energy and this is highlighted by the building standards of many countries who advocate increasing the airtightness of their housing stocks. Increasing a fabric’s airtightness also increases its resistance to airflow and so a positive displacement or extract ventilation system must balance the corresponding increase in the fan power required to overcome this resistance with the need to provide an adequate supply of fresh air. Accordingly, the total energy consumption attributable to these ventilation systems during the heating season is a function of the dwelling’s infiltration rate and heating system efficiency, and the fan’s hydraulic power and efficiency.
This paper explores the possibility of an optimum air permeability for detached (single family) English houses ventilated by a mechanical positive input or an extract ventilation strategy. Firstly a theoretical model of air infiltration and exfiltration is used to investigate the underlying relationship between the fabric airflow rate, the mechanical ventilation rate (supplied or extracted), and the air permeability in a single detached dwelling. A meta-model of the total ventilation rate and heating system energy demand is developed. Secondly, the infiltration model then utilises a sample of over 16,000 English houses located in 10 different regions to investigate the relationship between the fan power and air permeability in a randomly chosen sub-sample of detached English houses. Thirdly, it is shown that there is indeed an optimum permeability for detached English houses with such systems and the effects of heating system and fan efficiencies on this value are identified. Finally, these predictions are contextualised by exploring their relevance to the current English housing stock.