EcoSolar Energy Conscious Design Limited
16
System Types
There are two overall types of thermal hot water systems;
•
Direct System
•
Indirect System
In a direct system the hot water that is heated as it passes through the solar collector is supplied to
the taps.
An indirect system has a heat exchanger, generally within the hot water cylinder which transfers heat
between the liquid being heated in the solar collector and the hot water that is drawn off at the taps.
This system has the advantage that a suitable antifreeze mixture can be used in place of water in the
solar collector thus limiting the potential of the collector freezing.
All direct and indirect heating solar systems shall incorporate additional suitable relief valves on the
solar circuit such to ensure adequate relief of heat or pressure. These will be in addition to other relief
valves on the cylinder or in the system. Direct systems will usually have pressure relief valves
installed, no isolation valves shall be installed between the cylinder and collectors or relief valve.
Indirect systems will usually have pressure relief valves installed which will be suitably rated for the
installed expansion vessel, if installed. If the system installed is a drainback system there will be an
overflow from the system.
If the hot water cylinder is valve vented, the existing relief valves should be checked for correct
operation and adequate sizing. This needs to be completed by a competent and trained plumber. If
the relief valves sizing or function is in doubt then it should be replaced with a correctly sized relief
valve. The open vent should be checked to ensure that it is not blocked and not becoming blocked.
Mains Pressure Systems
If a high pressure system is installed, it is important to determine the material the hot water cylinder
is made from and to obtain guidance from the manufacturer as to the cylinder’s maximum allowable
temperature. Many high pressure cylinders have a vitreous or glass internal surface that is damaged at
the high temperatures, as experienced in solar hot water heating systems. It is important to limit the
storage temperature within these recommended limits, this can be done by programming the solar
controller and installing three sensors, the third sensor being at the top of the cylinder. At EcoSolar
we tend to specify duplex grade stainless steel cylinders, due to their resilience to temperatures and
pressures and have found them to be a reliable option.
The solar system may have a pressure reducing valve on the inlet to the cylinder and will be vented in
one of two ways either;