Hi,yes it is a safety valve. You will have two on your tank/system . A temperature pressure one (t and p) and a pressure one.
These open when the temperature gets too high, or, when the pressure gets too high due to a faulty pressure reducing valve or a flat expansion tank.the temperature safety valve will open at around 90 _95 degrees c. The pressure safety valve will open at around 6 bar
Try turning the temperature down on the hot water to 62. It should be above 60 to kill bacteria and below 65 to prevent scale.
If the problem continues it may be that the temperature stat has failed or the pressure reducing valve or the expansion vessel.
To work on unvented cylinders a person must either be gas safe or have done a unvented course