RCD tripping when using hot water

After a long process of elimination I have found out the RCD was tripping whenever the hot water was being used and the boiler started working.
Time to time this doesn't happen but 9 out of 10 times using hot water it does.
I have no idea what to do about this, if anyone has any advice on what to check I could give a go before contacting someone to repair the boiler.
Thank you so much