According to the manual of my AWB ThermoMaster 2HR 28.02WT Condensing boiler, it is compatible with a pre-heating solar system for tap-water only. It even has extra facilities to ensure that the coolest water goes through the condenser part (HR part) of the kettle before being preheated by the solar system to get a higher efficiency.
So this is the supported scenario:
They claim that it is not possible to use the solar system for pre-heating the central heating circuit. What is the reason for this? It should be a lot easier since I'd only have to insert the solar system in the circuit on the cold side just before it goes back into the CV?
In my situation it makes a lot more sense to use it for central heating instead of tap-water. The heating is on all day (my wife and children are at home most of the day). The kitchen has its own boiler, so the hot tap-water would only be useful for the shower (normally only used after the sun has set).
My reasoning was first that the CV system might be checking the temperature of the returning water to determine how much extra power it should add for heating, however the system looks simple and my guess is that it only takes the room-thermometer as input for determining the needed heating power.
Could anyone explain why it is not supported and what other systems do differently to support it? There seem to be many systems on the market that do support it, but what is the big difference between what I already have and those?