Pressure after boiler isolation

[JDL160]

My boiler pressure has been dropping for about 8 weeks. Topping up daily but more recently a couple of times a day. No problem could be found, so plumber has isolated the boiler. The system pressure has decreased and now the boiler pressure is going up. Can this be a thing? The plumber is more interested in the system pressure dropping and says it has to be a leak in the system but I’m wondering why the boiler pressure is going up? Any ideas would be very much appreciated.

 

[terryplumb]

Pressure dropping = leakage.
Pressure increasing whilst boiler is not in use and totally inactive = mains water is entering the system.

 

[JDL160]

After the boiler was isolated, the system pressure dropped 0.2 over the first couple of hours, but hasn’t dropped now for over 24 hours. The boiler pressure went up from 1.5 to 2.2 over the first 24 hours and has now settled to about 2.0. Has anybody got any ideas?

 

[spile]

I once had a filler valve that had a leaky tap. I disconnected it and blanked it off using an old lock shield valve.

 

[dilalio]

Are you running the boiler for hot water during these isolation tests?

 

[JDL160]

I did at first - the plumber said I could still access hot water - I’d thought this would be shut off too. When I noticed the pressure change on the system and the boiler, I stopped using the hot water too, hoping this would give a truer reading. Can running hot water affect the readings? Including the system valve reading?

 

[dilalio]

I did at first - the plumber said I could still access hot water - I’d thought this would be shut off too. When I noticed the pressure change on the system and the boiler, I stopped using the hot water too, hoping this would give a truer reading. Can running hot water affect the readings? Including the system valve reading?

Water, when heated, expands.
This increases pressure in whatever system the heated water is in.
So, boiler isolated and at 1.5bar cold; will increase in pressure when heated, within the boiler.