A pumping and injection trial at the Royal Festival Hall simulated the operation of an open-loop ground sourced cooling system where all abstracted groundwater was returned to the originating aquifer. The trial used two existing boreholes (144 m apart) and existing heat input sources. The trial was conducted over a 4 week period in June–July 2007 and comprised step testing, a constant-rate abstraction–injection test and two tracer tests. The constant-rate test was undertaken over 29 days, at an average flow rate of 8.3 l s−1. The total heat input during the trial was approximately 194.1 MW h. The trial has provided an early indication that comparatively large (i.e. several hundred kilowatts and above) open-loop groundwater heating–cooling systems can be operated effectively in the Chalk aquifer without undue thermal interference, as defined by a measured thermal response in the abstraction borehole. Continued monitoring of water temperatures will confirm whether this remains the case over the longer term. The trial has shown that groundwater can be returned to the confined Chalk aquifer under gravity without undue complication. The trial method was developed in close consultation with the Environment Agency and may serve as a useful guide for subsequent trials.