combi boiler broken

halstead sime trio, combi boiler.
has been working fine, if a little noisy - partly the fan, partly the pump. The pump also vibrates a bit, but I'm not sure if this is normal or not.
i've tried it set on heating and water, as well as water only, but the following applies either way.
it runs for about a minute, with the burners firing normally, and then the whole thing gets progressively noisier within the space of about 20 seconds, then the black screw-down type pressure valve on the expansion vessel opens.
At that point, I get concerned, and turn the electrical supply off.
My thinking is that its either a blockage or an airlock in the heat exchanger, causing the pressure to increase, and the valve to open.
I've run the pump with the bleed screw off, and I can see the thing spinning, so I am inclined to suggest that the pump is ok, if a little noisy, but I am happy to be proved wrong.
The system had been working fine, with no change in displayed pressure, and no changes have been made to settings, etc, so I think anything involving air would be unlikely??
Which leaves a blockage.
Any thoughts anyone?
Also, anyone know a plumber / heating engineer in the Waterlooville or Havant area of Hampshire, with Combi experience?
Thanks
Rob
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It could be the pressure vessel. It might have become deflated (in which case it can be pumped up again) or the diaphragm blown (in which case it should be replaced).
Christian.
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I've been reading up on it this morning, and one thing I forgot to mention is that when the expansion vessel valve opens, water comes out, so it does seem very likely that the diaphragm has blown.
I guess its a job for the Halstead parts department.
Rob
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