EcoSolar Energy Conscious Design Limited
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The pumps are installed and set to the high speed (3 on Grundfos) setting. The pump will then
always start on the high speed thereby providing the maximum torque to start the pump. Many solar
system failures have been due to the pump being unable to start, they are often set to low speed to
minimise mixing in the cylinder but this low speed reduces the amount of torque that the pump has
to start, this is especially a problem in direct or open loop systems as there could be a build-up of
particulates which restrict the starting of the pump. Once the pump has started the controller reduces
the pump speed to maintain a constant temperature differential between the collector and the
bottom of the cylinder. The pump will rotate faster with an increase in solar gain and slow down with
less solar gain. As the pump slows to maintain the temperature differential the pump will also slow
down if there is mixing in the cylinder, thereby minimising the mixing and increasing system
performance. Currently the slowest pump speed should be set at 40% of the maximum speed.
Overheating
Over heating within solar water heating systems is a significant issue that needs to be carefully
managed in order to ensure the system’s long term reliability. Over heating puts excessive stresses
upon all of the components within the system. Lets look at some of these issues,
Cylinders
Cylinders generally have a maximum design temperature that should not be exceeded. Many of the
hot water cylinders sold in New Zealand are lined steel cylinders, these generally have a lower
maximum design temperature than the copper or stainless steel cylinders. Particular attention should
be paid to retrofitting solar water heating on to existing mains pressure lined steel cylinders, as this
often results in the cylinder experiencing excessive stresses that can void any warranty on the cylinder
and potentially lead to premature failure.
Collectors
One aspect to choosing a collector should be their resilience to the maximum temperatures that are
likely to be experienced in the system, some collectors are more at risk than others and this is a key
question to ask any solar supplier.
Valves
Many systems are installed reliant upon valves managing the risks associated with overheating and
excessive pressure. These valves discharge water from the system, resulting in excessive water
consumption and wear and tear on the valves. New Zealand and Australian Standards will require
another solution to managing the problem. Tempering valves will also have maximum hot water
temperatures and an uncontrolled heat source will heat water above these maximum temperatures.
All the valves should be correctly selected and maintained.
P
umps
Pumps also have maximum design temperatures that should not be exceeded, however another issue
that needs to be managed is the boiling of water at the inlet of a pump. Water boils at lower
temperatures as the pressure decreases, at the inlet of the pump, there is potential, depending upon
