Impact of air leakages and short circuits in ventilation units with heat recovery on ventilation efficiency and energy requirements for heating

The impact of unintentional air flows on the performance of ventilation uni ts with heat recovery is discussed on the basis of single room ventilation units. Assuming an external short circuit (outdoor) and internal (inside th e ventilation unit) air leakages, which lead to internal short circuits, a model is developed and characteristic numbers for ventilation efficiency, e fficiency of heating load reduction and effectiveness of electrical energy use are derived. Differences between supply and extract air flow rates, res ulting in increased air flows through cracks in the building envelope, are taken into consideration too. The use of tracer gas techniques to measure a ir leakage rates from ventilation units is described briefly. It is shown b y numerical examples that unintentional air flows can considerably reduce t he performance of ventilation units in terms of ventilation efficiency and, in combination with unintentional heat flows through the casing, energy sa vings. Therefore, these flows should be avoided or at least reduced to an a cceptable level by an appropriate construction, manufacturing process and i nstallation of the units. (C) 2001 Elsevier Science B.V. All rights reserve d.