Vaillant Boiler pressure problem?

This combi boiler has been running trouble free for over 3 years, and
serviced annually. Got home from shopping, to a cold house and no hot
water. Pressure on boiler below 1, checked throughout house for leaks,
no found. Increase pressure to 1.5, boiler fired up fine. Checked
several minutes later pressure up to 3, so I had to bleed off a little
water. Any ideas as to the cause please? Is it indicative of a problem
lurking? I certainly do not want problems over the Christmas/ New Year
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If the pressure increases more than usual when it gets hot and doesn't continue to drop when it gets cold then I'd guess that the expansion vessel has sprung leak.
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It started with the pressure dropping, I increased it, then when it warmed up I had to drop it again. I have just had another look and had to drop it once more. My guess (and it is a pure guess)is that I have now taken as much water out as I put in when the pressure had dropped!
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In article , Broadback writes
You really need to reference this when it is cold, set it to 1bar cold and run the system. The pressure should not rise by much more than 0.5bar when hot, if it rises by more than 1 bar then as previously suggested the expansion vessel is suspect.
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On 14 Dec,
1 Did you isolate and disconnect the filling hose? A leak in the filling valve would cause the symptoms you described.
2 Does the pressure rise rapidly when the bioler heats and fall again rapidly as the boiler cools? If so has the pre-charge air leaked out of your pressure vessel. A top up with foot pump may be the cure. Before doing so read the sealed system FAQ (and your bioler manual).
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Thank you all for your help and suggestions. I will follow up on them tomorrow, when I can hopefully switch off and allow to cool down while her indoors is outdoors doing the weekly food shop. ;-)
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