There are a number of reasons why the water in a solar geyser can overheat. One reason could be that the settings are not optimised to match the usage pattern of the household.

In one case, around 2017-2018, the Western Cape was struck by a severe drought and they had to seriously reduce their water consumption. This, together with the number of family members being reduced, resulted so that in the hot summer months, the solar water geyser started overheating. This was simply because the Sun was producing more hot water than they were consuming, driving the geyser temperature to near boiling point.

Shadow net can be used to reduce the solar radiation received by the vacuum tubes, effectively reducing the sun's heat during the height of summer. It was found that 40% shade net worked best in their case. Since it comes 3 metres wide, they bought 2 metre length, and doubled up the excess, meaning the effective shadow percentage is probably around 50-55%. This results in the geyser temperature ending up around 60-65 degrees Celsius on a hot day, which is ideal. Without the net on an equivalent day, the geyser would have ended up around 85 degrees, plus.

When the geyser gets so hot, the element never switches on anyway. All the energy in the system is thus from the Sun. The only way to prevent overheating is to reduce the Sun's heat - hence the shadow net. Shadow net is ideal for withstanding wind - it is strong but also not a solid barrier. To increase its wind survival, the ends of the net were sandwiched between two strips of wood - the net was first stapled to the one, before screwing them together. Using a hot soldering iron, holes were then made in the net at strategic points which help to prevent the net from sliding down the solar geyser frame. Cables ties, 5 on every side, work very effectively to strap the net to the frame.

An eye is kept on the geyser temperature and when it starts to exceed 70 degrees at the end of a sunny day. The net is installed and kept in place during summer, until it drops below 55 degrees again. If we leave the net on for too long, the system uses electricity unnecessarily to maintain the geyser temperature - the geyserwise is set to 55 degrees Celsius.

Second system, using 85% shadow net, for when they go away on holiday. With nobody home and no hot water being drawn, the geyser quickly overheats, even when the Geyserwise is switched to "vacation mode". This was made the same way as the 40% shade net system and was simply installed over it, if it was already in place. Installed by itself and with the geyser element switched off (geyserwise in "vacation mode"), it was found to keep the geyser to 53 degrees after a week-long hot spell in mid summer. It gets removed immediately after returning from holiday by simply cutting the cable ties.

Makeshift anti-bird spikes were installed to stop them from messing up the vacuum tubes. The proper PVC, Polycarbonate or even wire anti-bird spikes are actually more expensive than galvanised sheet-steel burglar spikes. In fact, the PVC ones don't last in the Sun and are also known sometimes not to work.