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?
Best Answer
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/