Meteorological enthalpy analysis of temperate and maritime climates above latitude 45 degrees N suggests that the water-side evaporative cooling technique has considerable unrealised potential with contemporary "high temperature" building cooling systems-such as chilled ceilings and displacement ventilation. As low approach conditions are the key to exploiting the cooling potential of the ambient air, thermal performance at such conditions needs to be investigated. To address the research issues, an industrial scale test rig, based on a low approach open cooling tower and plate heat exchanger and designed to maximise evaporative cooling potential, has been constructed. The thermal effectiveness of such systems (as a measure of the degree to which the system has succeeded in exploiting the cooling potential of the ambient air) is a key parameter. This paper presents the results of experimental research into the thermal effectiveness of a water-side, open, indirect evaporative cooling test rig, designed to achieve low (1-4 K) approach conditions in the temperate maritime climate of northern Europe. The sensitivity of the thermal effectiveness to a series of key operating variables is investigated. High thermal effectiveness of up to 0.76 was found with both cooling tower air-flow rate and secondary water-flow rate having a strong impact. Primary water-flow rate however, has a weak impact on thermal effectiveness but a major impact on energy performance-indicating scope for a considerable improvement in energy performance at the expense of a minor reduction in thermal effectiveness. A proposed energy efficient control strategy for this form of cooling water generation is proposed and supported by an analysis of the measured results. (C) 2007 Elsevier B.V.. All rights reserved.