Utilization of heat recovery ventilation: steady-state two-zone heat loss analysis and field studies

Abstract

In new houses in Europe the share of mechanical ventilation with heat recovery is increasing as a result of more severe energy performance requirements and of energy labelling for residential ventilation units. The methods used to assess the influence of heat recovery ventilation on the heating energy use in energy labelling and certification are typically based on single zone energy balance equations, although heating behaviour and set-points differ in different rooms of a dwelling. As a result of this the energy savings of heat recovery ventilation as assessed with single zone methods may be larger than when the spatial variations in dwellings are taken into account. This is related to the fact that the recovered heat supplied to the dwelling through the ventilation system is not 'useful' to reduce space heating and cooling demand at all time and in every room. A two-zone steady-state heat loss analysis was conducted to investigate the relation between spatial variations in a dwelling and the utilization of heat recovery. One zone represents the rooms in a house which are regularly heated and are typically equipped with heat emitters and local controls. The other zone represents the rooms which are rarely heated or have no individual heat emitters or controls. The results show the differences between a single zone and two-zone approach in terms of the effects of heat recovery ventilation on building heat loss, and define the main influencing parameters for the utilization of heat recovery in residential ventilation systems. The analysis is supported by results of a field study where energy use in 114 low-energy houses was monitored. Half of the houses had mechanical ventilation systems with heat recovery, while the other half had demand-controlled mechanical extract ventilation. Apart from the differences in ventilation systems, the houses were largely identical.