Pressure fluctuation on combi boiler central heating

12 Jan 2010
Reaction score
United Kingdom
Hi all.

This is my first post, so bear with me. I have a Valliant combi boiler anal experiencing some issues with the pressure of the central heating loop. When cold, the pressure reads as 0.5 bar (analogue dial and digital read out), which is min. pressure required for the system to start up. Once at operating temperature, the pressure rapidly rises to almost 3 bar, and no doubt exceeds the release valve. The problem is that this causes the pressure when cold to drop below the min. threshold resulting in the boiler not firing up when it's next required. This can be sorted by opening the mains feed, but of course the cycle continues, constantly requiring top ups when cold, and overshooting when hot.

What causes are there for this sensitivity to temperature? All radiators have been bled.

Any help on this would be greatly apreciated!

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Thanks for the reply. Can you explain what you mean by this? Is this an easy fix?

I have just had or should i say have this problem ,first i would say look in the FAQ'S and you will see what the pressure vessel is and go from there ,unfortunatley mine seems to be no good so it needs changing but it could be fixed with a bicycle pump if your lucky
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the expansion vessel cushions they increase in pressure when system gets hot, the pressure shud rise around 2 bar. if there not air charge in it the pressure will shoot up to 3 bar which will cause the pressure relief valve to open and discharge water pressure out side, which wud leave the 0.5 bar

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