Biasi Riva Plus M90D.28SR, ho****er doesnt always work when heating is on

[jezabel222000]

Hi, we moved house this year so this is the first time we have had the heating on on our Biasi Riva Plus M90D.28SR. Throughout the summer we had hot water instantly but since we've started putting the heating on, whether it be manually or with the timer, its extremely hard to get any hot water and it takes anywhere from 2-10 times of turning the hot tap on and off before we eventually get any hot water. The red light comes on on the operation lights. We only have this problem when the heating is on, it works perfectly fine when the heating isnt on. Any ideas what it can be?
Thanks

 

[Dotty]

Sorry Jez, you need a gsr in to look at it