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I generally don't notice a performance problem with the boiler, but after I run the heat to the radiators and turn the boiler off, after cooling down the the pressure gauge shows zero. I can easily top it up to the "green zone", and all is well. I don't always remember to do this, and it doesn't seem to be a problem. Next time turn the heat on, the gauge is back up to a normal level while heating. In other words, I've never seen the gauge read zero when the boiler has been running.

I don't see any signs of a leak anywhere near the radiators, and it doesn't seem to happen when using hot water -- though this may because the boiler is not running long enough.

Any ideas of what to check?

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When the boiler is running (especially in heating rather than hot water mode) watch the pressure gauge: does it rise almost all the way to maximum? And do you then get a sudden discharge of hot water through the pressure relief discharge (PRD) valve and pipe to the outside of the building? (If the boiler is mounted on an external wall the PRD pipe usually goes directly through the wall behind the boiler.)

If so this is where you're losing water (and hence pressure). It indicates that the expansion vessel (probably built-into the boiler) is faulty. It may just have lost pressure (there's a Schraeder(sp?) car-tyre-style valve you can use to top it up) but could have failed entirely (e.g. if water comes out of the air valve!). For expansion vessels built into boilers it's often more trouble and expense than it's worth to replace the fitted one and quite acceptable to fit a vessel externally.

The uk.d-i-y usenet newsgroup is generally a good place to discuss this sort of problem. There is also a wiki/faq at http://wiki.diyfaq.org.uk/

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We had this sort of problem with our boiler, firstly you have to find where the water is coming out, I:

  • Put paper under / round all pipe connectors and under the outlet from the over pressure relief discharge (PRD) valve.

It turn out that our pressure relief discharge (PRD) valve was operating, it should never be giving out water.

As a temp fix, I reduced the temperature of the radiator water and opened all radiator values all the way – this made the pressure relief discharge operate less often.

Then the first time the “gas man” came, he pumped up the expansion vessel, as its air pocket was smaller than normal. This did not fix the problem, but would have in 90% of the cases.

On his second visit after we both spent a long time tracing possible faults, he found that the small rubber hose that connects the expansion vessel to the rest of the combi was blocked, so there was nowhere for the pressure to go when the system heated up.

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Sounds like your expansion vessel is knackered.

You can either change the one in your boiler which will be expensive, or you can fit a new one somewhere else in your system, like in your loft space if possible, which would be cheap.

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