Weird boiler pressure issue

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I wonder if anyone can help with a strange problem I've had in the last couple of days... I have a plumber booked to come out, but not till Friday and I would like some idea of what the problem might be in advance...

On Monday morning the boiler pressure was pretty much flat. There had been no issues whatsoever in the days before and I had noticed on Sunday that the pressure was normal (I often check it as it's in the garage next to my car). I immediately panicked, thinking there must have been a big leak somewhere in the house. I checked everywhere visible, no leaks. I then topped the water back up to see if I could hear any leaks under floorboards, or anything like that. Nothing obvious.

So, I left it like that to see what happened during the day. The pressure was still where I'd left it come bedtime. It was still OK yesterday morning and all day yesterday. There are still no water stains anywhere. This morning it was flat again. I am baffled. I thought it could be the heat exchanger but there's no sign of any leaks and the condy pipe isn't dripping any more than usual. There is also no water coming from the PRV pipe.

Any ideas? I don't see how it could be a leak in the system, as surely this would mean the pressure would be steadily dropping all the time? If it's the boiler what could be wrong?!

Oh, and it's a Baxi DuoTec combi boiler.
 
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Any water dripping from the PRV pipe outside ?
When c/heating is on ,does pressure increase to around 3 bar ?
 
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No water from the PRV pipe. The pressure only increases from around 1.5 bar to 2 bar, which is what it's always done.
 
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Then you have a leak. Either on the system side ( pipework / rads etc) or on the boilers heat ex ,if no obvious sign of water loss within the boiler.
 
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Thanks, that's what I assumed. I think it must be the heat exchanger, just weird that it only happens overnight?! The boiler is 13 years old so probably time for a new one...
 
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