Vaillant boiler losing pressure

Nose put out again?
Ehh? What are you on about? I’m trying to deter people from taking incorrect and expensive advice.
I suggest you read it properly, what the OP actually wrote.

He clearly stated the pressure increased on cooling / on the heating going off and without him needing to top it up. I even asked the OP the clarify that point and he confirmed it. The OP also said he renovated his flat, which suggests he owns and is responsible for it.
I did read it properly, you didn’t. You also said it wouldn’t be at the OP’s expense o_O
 
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It’s nothing an annual service won’t fix. Vaillant are renowned for this and I dare say the expansion vessel runs on its limit.
Small temperature changes can help bring the boiler back on. As well as making it go off again.

It doesn't explain how the boiler can fail on an F22 due to low pressure, then mysteriously top up its own pressure to be able to function again. Heat input causes pressure to rise, when it cools pressure reduces. The complaint seems to be that it is doing the precise opposite of that - despite the protests to the contrary.
 
It doesn't explain how the boiler can fail on an F22 due to low pressure
What I believe has happened, the boiler pressure has dropped to 0.5 bar, on that model of pro once the pump kicks in (the 2nd component to be energised), the pressure goes up by 0.5 bar, so could have brought the heating on that way.
 
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I renovated my flat earlier this year and installed a new CH system as below:
* Vaillant ECO Tec Pro 24 combi boiler
Then it is under Vaillant warranty so get them out, I agree probably an expansion problem, which they would say should be checked at every service, but as it isnt 12 months old yet, that is not a get out they can use, as @jeff the gasman says the expansion vessels on these are very poor
 

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