The Thermomax tubes used for heating the water. The temperature of the water in the hot water tank in the house was compared with the water running across the top of the tubes. If the water was 4°C hotter at the tubes, hot water was pumped up to the house. Notice the very thick, highly insulated tube in the bottom right of the photograph through which water travelled up to the house. The distance from the Thermomax tubes to the house was about as far as it could be before energy loss became too great.
The decision was made to place the tubes at this place in the garden and not on the roof of the house simply because this part of the garden got more sunlight. Trees near to the house would keep the roof in shade for more of the day than where the Thermomax tubes were situated.