Hot water fades away!!

I have a sealed system central heating/hot water with a Potterton boiler.
It has recently developed a symmptom that when I turn on a hot water tap, it gushes away quite happily for a few seconds, but then the water flow fades away to nothing. Leave it a little while and a bit more comes out. Also recently the pressure vessel pressure keeps dropping to below 1 bar so it needs water adding to restore the pressure .... Any thoughts would be very much appreciated

the fading hot water problem is quite common, though I've never really worked out exactly what's happening. I presume it's mainly to do with the varying pressure of water from a combi (initially at mains pressure but reducing as flow is restricted through combi).

As for falling pressure in the CH system, if the fall is constant it's a leak, but if it occurs only when the heating cools down it may be caused by a faulty expansion vessel.

Sounds as if the heat exchanger may be scaled, did the pressure of th hot water gradually slow down?

could be a blocked filter if the installer fitted one, maybe, if you have one fitted, a faulty pressure reducing valve.

or as kevinfrombaxipotterton ( all hail potterton and their robust diverter valves) a blocked up DHW heat exchanger

what boiler is it?