vokera excell dhw problem

[niceday]

i have a vokera excell 96e

the boiler does not fire up for dhw when the boiler is hot and the heating is off.

if the heating is on then you can get dhw
if the boiler is cold it will fire up for dhw
but
if the ch has just switched off then it will not fire
if you have just drawn hot water and the boiler is hot then it will not fire.

when the temperature drops to 20deg it will then fire.
it has just been serviced and the diverter has been checked and the heat exchanger replaced (not sure if that was necessary)

any iadeas please

 

[corgiman]

get the bloke who change the he back and ask him why this didnt work, you paid your money to rectify this problem