Fault in combi boiler's pressure guage, and/or pressure-relief valve?

I ancountered a new problem with my Brittony 80 combi boiler.
The other day, I topped up the pressure in the central heating cirquit (to about the maker's recommended 2-bar) and then switched th boiler back on. I happenned to notice that the pressure guage then rose to about 3 bar, so I relieved the pressure via the pressure relief valve.
Then sometheing odd started happening. The relief valve, which is supposed to hold pressure up to 3-bar, started blowing out water at anything above 2 bar. I took the relief valve apart and looked inside. All seemed in order, but when I reinstalled it, the problem continued.
Suspecting a fault in the relief valve (tired spring perhaps), I bought a replacement today. I installed the replacement, and found that the new valve behaves exactly the same as the old one, blowing at 2-bar instead of 3-bar.
Logic tells me (but tell me if you disagree) that the most likely diagnosis is that the pressure guage is no longer giving a correct reading. But if that's the case, it seems strange that the fault in the pressure guage would happen immediately after I released the system pressure via the relief valve yesterday, don't you think?
I guess the fault could be in the actual pressure guage dial or on the pressure sensor from which it takes its unput. What do you think?
Thanks...
Dave L
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Do they really recommend 2 bar? 1 bar should be plenty for any 2-storey house.
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wrote:

Built in espansion vessel pre-charge pressure 0.7 bar Maximum heating pressure 3.0 bar
That the relevant bit?
Britony System II 80 Specs
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wrote:

To be more exact, the pressure guage on the boiler has part part of the pressure scale highlighted, suggesting that the advisable system pressure as 1 to 3 bar. The manual, on the other hand, says "Check that the system is above 0.7 bar and below 2.5 bar, as seen on the pressure guage".
The pressure relief valve is designed to relieve pressure at 3 bar.
What you said about 1 bar being sufficient was helpful - thanks (Yes it is a 3-bed, 2-floor house).
Dave L
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Check that the diaphragm in the expansion vessel is not holed. If it is not holed then check the charge pressure. If low pump it back up to the makers charge pressure.
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Thanks for that. How important would you say it is to check out the integrity of the expansion vessel, bearing in mind that the boiler is not leaking and the system seems to be working fine?
I've never delved into the expansion vessel before. I have a vague idea where it's located, though it's not even illustrated in the manual. Is it a big and/or skilled job, or dead simple? Any special tools likely to be required?
Thanks again. Dave L
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PS.. If it was holed, would there not be some water leaking out? I can't see any signs of leakage.
The pressure relief valve however, has expelled more water and now the system is down to 0.8 bar according to the guage. (It was 2.0 bar when I toppd up the pressure just 5 hours ago).
Dave L
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How much does the pressure rise when between cold and hot? If the pressure rise is > 1 bar, then the expansion vessel is either shot or undersized. It is better to calculate a system that only gives <= 0.5 bar rise and pressurise to 1 bar when cold. With > 3 storeys, the design should be more specific and will depend on the relative vertical locations of the components.
I suspect that if you have a system that rises 1 bar and starts at 2 bar cold, then it will be activating the pressure relief, which may depressurise the system considerably. The fix is to repair/add an additional vessel (if required) and set to 1 bar cold.
Christian.
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On Tue, 19 Oct 2004 10:47:24 +0100, Christian McArdle wrote:

Full info in the FAQ below.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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On Tue, 19 Oct 2004 10:47:24 +0100, "Christian McArdle"

Funnily enough, the system seems to be working fine since I finished the work on it yeaterday, depite the odd symptoms I mentioned: (pressure valve blowing at 2-bar instead or 3-bar, according to the guage). When cold, the system, reads around 1 bar on the guage and rises to about 2 bar when hot. Does that sound about right? (it's a smallish 3-bed house) The technical help guy at the boiler's main office, told me the system can typically rise as much as 1.5 bar as it gets hotter. No more water has been expelled from the pressure guage since yesterday.
The only symptom that really puzzles me is that the 3-bar pressure-relief valve blows at 2 bar instead of 3-bar. So did the old one I replaced. So I suspect that the pressure guage is giving an incorrect reading. I'm not sure how long it's ben doing that for.
I guess there is no real danger, since the pressure relief valve blows at somwhere between 2 and 3 bar. Would you agree?
Cheers,
Dave L
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On Tue, 19 Oct 2004 18:34:45 GMT, snipped-for-privacy@nowhere99.com (Dave) wrote:

PS Slight correction: I looked at it at night, after the rads had stoped heating and the pressure was down to almost 0-bar on the guage. However it still fired up when I turned a hot tap on. I have now topped up the pressure to 1 bar while it was relatively cool.
Dave L
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wrote:

not
makers
Have you checked the link pipe to the expansion vessel is not blocked?
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On Tue, 19 Oct 2004 16:31:01 +0000 (UTC), "John"

No - not yet. Please see my latest reply to Christian. System seems to be working fine, except that I suspect the pressure guage is reading incorectly, e.g., showing 2 bar instead of 3 bar.
Dave L
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