Current difficulties surrounding air conditioning systems involve an increase in input air aimed at improving indoor air quality (IAQ), the financial costs arising from energy consumption and external environmental impact, linked to the greenhouse effect (GWP) and the destruction of the ozone layer (ODP). One alternative technique which offers an adequate combination of IAQ and acceptable energy saving is the introduction of energy recovery systems using air extracted from air-conditioned premises. We have designed a mixed air-energy recovery system, consisting of two heat pipes and indirect evaporative recuperators. The experimental set-up used is described and the proposed energy recovery system is characterised. A staggered set-up has been chosen for the input air pipe and a parallel one for the extracted air for variable recirculation. By means of the trace gas method and photoacoustic spectroscopy (PAS) the amount of exfiltrated flow in the installation is measured. The energy characterisation of the mixed energy recovery system was performed by means of the experimental design technique. An analysis was carried out of the influence of factors such as temperature, flow, relative humidity, water flow, etc. on the basic characteristics defined by the mixed system: heat flow, heat efficiency and COP. (C) 2003 Elsevier B.V All rights reserved.