Hot water fades away!!

[shenders]

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

 

[chrishutt]

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.

 

[KevinfromBaxiPotterton]

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

 

[corgiman]

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?