Boiler Loosing Heating Water Pressure

2 Oct 2007
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United Kingdom
Over the summer when the heating wasn't on, I noticed the boiler was loosing water pressure (quite slowly).

This weekend we had the heating back on, so I filled the water to 2 bar, and within a few days, it had dropped to nearly zero. Last night, I filled it back up to 1.5 bar, and this morning it is at zero again.

Except for a leak in the pipework, what's probably going to be the problem? And predicted cost?

The boiler is a Worcester Bosch.

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Firstly I'd do a quick check of your radiators, make sure theres no signs of leaks from the valves, bleed points etc. also you will probably have a pipe which runs outside from the boiler check no water is coming from there, this will be from the pressure relief valve which may not be seated fully.
Post the model and we may be able to guide you as to how to check your expansion vessel charge.

Sorry for the late reply, I've had internet troubles.

I've checked the radiators, and there is definitely no leaks :)

The model is a 24i junior.

One thing I have noticed, is that if I fill the water pressure to just over 1 Bar, then put the heating on, after about 15-20mins the pressure rises to over 3 bar, then drops to 3 bar again which is well in to the red. Once it cools down, the pressure is back at 0 again.

Does that sort of point to a certain problem?

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Yes! expansion vessel.
When cold expansion vessel needs to have a 'air' charge of approx 0.8bar.
Also it needs to be empty of water.
The water pressure when cold needs to be approx 1bar.
When heating is operating, the air will get compressed resulting in an increase in pressure on both 'air' and 'water'
The water pressure should rise maybe around 1bar with heating on so it may reach 2bar.
The PRV is set to open at 3bar and if it does open, it indicates the volume of air was low which results in pressure well over 3bar.
PRVs once opened may not close again properly due to dirt lodging on the valve seating face leaving you with a small leak. The easy option would be to replace the PRV.
The PRV is a safety device, but it sounds as yours did not open.
When the PRV operates the pressure would drop from 3bar to about 0.5 bar instantly
Of course there is always the possability a small leak exists elsewhere in the system which would allow the pressure to drop, but over a longer time period

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