Boiler Pressure Questions... too low then too high!

ma9gm

Right, all set to slightly just above 1. Thanks for the help.

ma9gm

Ok, turns out that only fixed it temporarily!! The problem is back. :0(
So the problem seems to be this, during the day when the boiler is off, the water pressure is just above one bar. It doesn't move all day. The boiler comes on in the afternoon/evening without any issues. When the boiler is on it then goes up to about 3.5 bars (which it shouldn't do!!!). During the day it doesn't seem to release the valve. But I can turn it on and off without any issue other than the 3.5bar pressure thing!
Then here's the problem, when it's meant to turn on in the morning it doesn't. When I go and have a look it gives me an E119 error message which means the system pressure is low. I fill it up, and it switches on again! From my simple reading around, it seems like the expansion vessel needs replacing, as does the PRV? Would that sound about right?
Also, I have direct line home insurance plus cover, which includes emergency cover and fixes up to £500. Would this be covered by that or would the engineer just come out top up the boiler and leave?