I have a mixer tap in the kitchen and there is a problem in that when I
turn on the hot tap (quarter turn valve) there is often up to a 15
second delay before any water comes out. There is no problem with the
mains cold and the hot is fed from the cylinder which is upstairs and
about 12 feet away. Pipewaork downstairs is 22mm feed reducing to 15mm.
Hot water is softened. There is a hot feed to the washer from the
undersking cupboard but turning that off makes no difference

So far I have tried ...
1) A replacement tap (now a Franke mixer)same problem although it didn't
do it immediatley.
2) Moving the HW supplied to the 3.5bar Monsoon pump

So I am now at a loss .. I'd have thought the overrun on the pump would
keep the pressure in the pipe, but it seems not to be the case. the tap
can be turned off for just a couple of minutes and when it is turned on
again we get the same lag. I can only assume it's a sticky valve but
then why has it happened to 2 different taps. Plumber is also confused
as to what it might be.

Any advice/observations appreciated

Denis