Solar start
In solar thermal systems, the heated heat transfer medium sometimes takes too long to reach the
collector sensor, causing the system to start too late. This insufficient gravity rise occurs mainly with
flat-mounted collector arrays, absorber strips in meander configurations, and vacuum tube
collectors.
The start function puts the solar circuit pump into operation briefly at intervals, transporting the
content of the collector to the sensor. To prevent energy losses, this interval operation is started
only within a time window and only when the insolation at the GBS01 radiation sensor (special
accessory) reaches a certain level, or subject to monitoring of the collector temperature. Without a
radiation sensor, the controller first attempts to determine the current weather conditions by means
of the actual [captured] collector temperature. That allows it to identify the right time for the solar
start function's flushing process.
A separate start function is required for each collector array with an assigned collector sensor.
Drainback
In drainback solar thermal systems, the collector area is drained outside the circulation time. In the
simplest scenario, an open expansion vessel is installed near the solar circuit pump, which collects
all the heat transfer medium higher than the vessel when the pump is stopped.
The system is started either by means of a radiation sensor or when the Diff. on temperature
differential between the collector sensor and cylinder sensor is exceeded.
During the filling time, the pump must raise the heat transfer medium over the highest point of the
system. If there is variable speed control, it must be programmed so that the pump runs at full
speed (e.g. digital command on analogue output). Optionally, a second pump (booster pump) can
also be connected to a free output in order to increase the filling pressure.
At the end of the filling time, the stabilisation time starts (setting in the Solar control function). The
collector sensor should reach the start differential within the stabilisation time, after having been
cooled down by the filling process. Speed control is recommended as the pump will then run at
least at its minimum speed during this time, allowing the collector to be heated more quickly. If the
collector sensor does not reach the start differential at the end of the stabilisation time, the system
will be drained and a restart cannot occur until the blocking time has elapsed.
If the pump is switched off during standard operation (e.g. due to the temperature differential falling
below Diff. off or shutdown due to excess temperature in the collector), the system will be drained.
A restart is only possible once the blocking time has elapsed and the start condition is met.
A separate Drainback function is required for each collector array.
The Solar priority function and the Solar start function must not be used with drainback
systems.
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