boiler problem with british gas

[newbie1]

Called to look at a job that is under bg homecare
boiler main 24he in good working order
symptoms heating gets hot when hot tap runs
my theory/diagnosis was divertor valve

British gas young lad been twice and says its the bathroom mixer tap mains back pressure ,now i ran tap in both cold and then off then hot then off then hot and cold then off and heating stayed off
now ime allways willing to learn something new but ime tending to stick with my diagnosis but please feel free to chip in as they are calling bg back a third time and thy paid me for my time so its not a problem

 

[ecowarm]

British gas young lad been twice and says its the bathroom mixer tap mains back pressure ,now i ran tap in both cold and then off then hot then off then hot and cold then off and heating stayed off
now ime allways willing to learn something new but ime tending to stick with my diagnosis but please feel free to chip in as they are calling bg back a third time and thy paid me for my time so its not a problem

I dont see the how that could happen as hot / cold same pressure so I would say your right Newbie1

 

[Billy Bob Bob]

british gas numpty,its a knackered diverter classic symptoms