Radiators on with hot water or not?

ThisIsWeird

purpleparrotuk said:

ThisIsWeird said:

Ok, a 1 to 2.5bar increase when getting hot suggests an Expansion Vessel that needs attention. It isn't 'critical' until it gets close to 3bar, as that's the point when a safety valve blows open and discharges excess system water outside. 
Altho' not ideal, as long as there's no other issues, 2.5bar can keep on trucking, but it just isn't 'ideal'. Do you have your boiler serviced? If so, bring it to the fellow's attention - to check and recharge the EV should be straight forward.
Anyhoo, this hiss. Could be some trapped air, so have you tried bleeding the rads when cold? Or, it could just be normal moving-water noise. Your boiler output temp isn't silly-high, is it?

We have only just had the boiler and tank serviced last month. I looked again and it was on 2 bar when hot tonight. I think it’s only a bit higher than 2 when quite a few rads needed heating up. I don’t think anything would is wrong with the vessel. Surely increasing from just over 1 cold to just over 2 when red hot is ok? The flow temperature is set to 68. I’ve bled the radiators and no air in them. This hissing has been there for a few years now. 

If it's working, then good.
Some pressure rise is quite normal. Above 2bar would have me wondering if all is fully well, and 2.5 or above is getting into 'summat amiss' territory. 
Keep an eye on it
A hiss is not unusual either - mine is doing so right now... The level will vary as the TRVs open and close.