Gledhill Boilermate 2000 Thermal Store Query

Can anyone tell me how the above system prevents hot water from entering the heating circuit when either the thermostat is set to very low or the heating is switched off?
My system has constant hot water present in the highest radiators in the system.
I'm not a heating expert but I believe that with a standard system there is a motorised valve which prevents hot water from entering the heating circuit. I'm also lead to believe that the above system does not use these. If that is the case how should it do it?
Any info would be most appreciated.
Thanks,
Howard.
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