My boiler is a Worchester 28cdi pressure is increasing up 3 bar?

My boiler is a Worchester 28cdi pressure is increasing up 3 bar by itself ,but is working fine ,even though I have drained the water pressure with release valve ,and even though the boiler is not been used the pressure goes up by itself, I drained it to bring pressure under 1 bar ,but after few minutes it was back up to over 2bars.