EcoSolar Energy Conscious Design Limited
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drain the contents of the tank to solve the problem. If the glycol is replaced, it should be drained
from the system, contained and disposed of in a suitable manner.
Another commonly found problem is that of incorrectly sized expansion vessels. Any thermal
expansion of the antifreeze mixture is taken up within the expansion vessel, this vessel is often
undersized and the excess expansion results in discharge from the pressure relief valve. To ensure
effective long term system performance the expansion vessel should be suitably sized, most systems
will have an expansion vessel installed with a volume of 18 to 35 litres or more.
NOTE Do not use Ethylene Glycol, as used in vehicle coolant systems, in solar systems as it is toxic,
use only proprietary Propylene Glycol as recommended by the solar water heating supplier.
Drainback
Indirect and direct drainback systems can be designed to be the most reliable and appropriate solar
water heating system. The systems incorporate a solar controller which only turns the pump on when
there is heat available to be recovered from the collector. When the pump is off, i.e. there is no heat
to be recovered from the solar collector, the solar collectors are empty. When the controller detects a
greater temperature in the collector, it starts the pump, forcing water up in to the collector and
circulates it through to the cylinder. During times of cold weather the pump will not be operating,
therefore there will be no water in the collector and thus, providing the water level is suitable, no risk
of the collector freezing. This system effectively manages the risks of freezing and overheating.
Controller
The EcoSolar solar controllers have functions to start the pump and allow warm water to enter the
collector when low temperatures are recorded by its sensor in the collector, thereby reducing the risk
of the water in the collector freezing. The controller monitors the water temperature in the collector
using a sensor located in the solar collector. For effective control this sensor should be installed
within a dry pocket with heat transfer paste and adequately insulated from the ambient temperatures.
Under normal conditions the controller uses this temperature sensor to determine when the collector
is warmer than the bottom of the cylinder and turns the pump on and off to recover this heat.
If the collector temperature sensor detects that the water temperature within the collector has fallen
below a predetermined temperature, the controller will turn on the pump for a short period of time
to circulate some warm water from the cylinder to the collector to increase the collector temperature
and prevent it from freezing. This is not a fully reliable method of frost control as it relies upon
power being available to the controller and pump, so in the event of a power failure there will be no
frost protection. The accuracy of the temperature sensors also needs to be monitored during
maintenance to ensure they are recording an accurate water temperature.
Frost Valves
These mechanical valves open, when they are subjected to low temperatures, discharging water from
the collector which is replaced by water from the cylinder. This relies upon the correct flow patterns
through the system and collector and water being supplied to the collector being warm. These valves
should be installed at the top and bottom of collectors on direct heating systems. Please note this is
not a fully reliable method of freeze protection, are prone to going out of calibration and are slow to
