kitchen tap problem

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
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DenisP wrote:

Airlock perhaps? try connecting a short length of hose from hot to cold and turning both on, this should force cold water through the hot pipe, but I don't know about the pump, never used one, nor know anything about them
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Phil L
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Phil L wrote:

should've said .. it's a monoblock mixer tap and there are no leaks either.
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DenisP wrote:

If the cylinder is upstairs why the pump? Also it's not clear what your 2) means. What does it mean to supply HW to the pump? How/where did you move the HW supplied to the pump?
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Existing kitchen gravity HW feed that came from top of 4 year old HW cylinder (in airing cupboard upstairs) is now fed from the outlet of the pump (also in airing cupboard) which is a 22mm feed from the side of the cylinder. The cold water tank (softened water) is on a raised platform in the loft and feeds the cylinder using 28mm pipework. Reason for pumping kitchen circuit was to increase flow (which it does) and try and get over the lag which is now worse
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DenisP wrote:

'lag' as you call it can only be air getting into the system. When yoiu turn the tap on, can you hear air being forced out prior to the hw appearing? - what happens if you put your finger over the tap before turning it on? try it and let us know
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Phil L
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Phil L wrote:

Tried that .. no air being expelled. It can only be insufficient water pressure to operate the ceramic valve although it does eventually come on.
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