Hot water on a Vailllant ecoMAX 828/2E?

The hot water from our Vaillant ecoMAX 828/2E takes a long time to get hot even when you have the water flow low. The bolier fires up fine and there are no error codes. The diagnostic codes are all OK. It seems that when the hot water is called for it partially goes into the heating system. Is there a valve that diverts the hot water to one or the other, or both as required? Any help would be much appreciated.