V8Druid
do it as well as you can,but learn to do it better
was fairly recent when i bought the place so say 12-ish yr old .... has a switch for heating and hot water or just hot water ... I've always left it on the dual settingIf your Grant Combi is working properly/wired properly and has the DHW control switch live then it should always maintain the primary store ready for instant hot water. Sounds like something wrong to me. Fault could depend on which model you have as they have evolved over the years.