Using a solar system with a condensing boiler for the central heating instead of tap-water?
You can solar heat the primary loop with high temp evac tubes in principle but what happens is you mess up the control of the return temperature which is critical for the operation of the condensing boiler. Preheating the tap water is the better approach because you are heating the water before the boiler loop tops it off. This doesn't affect the existing control logic at all.