Thermal output, surface temperatures, and supply and return water temperature were measured for a wall cooling system involving pipe attached to a wall section made of thermally insulating blocks. The experiment was performed for warm climatic conditions typical of, e.g., summer in Central and Northern Europe. The outdoor environment was simulated by a climatic chamber while the indoor climate was simulated by attaching a hotbox to the wall surface. The sensitivity of thermal output to several design parameters was investigated by 2D numerical simulations. The measurements showed a fast thermal response of the wall system. The cooling output was 38.3 W per m2 of the cooling area which equalled about 4.8 W/m2 per 1 K temperature difference between water and hotbox. The lowest surface temperature of 19.6 °C was measured at the pipe. Thus, the cooling output could be enhanced by reducing the surface temperature closer to the dew point temperature. The temperature of water in the pipe was very close to the surface temperature. It was illustrated how this characteristic of the wall cooling system tested positively affects the efficiency and cooling capacity of an air-to-water heat pump.