Please can anyone help.
We have a 3 year old Protherm Combi Boiler and its been losing Bar Pressure for months... We have to keep filling it with water every 4/5 Hours.

We have diagnosed that Water is going straight out the Over Flow Pipe outside.
Can we buy a Part and fix this or will we need an Engineer and How much,Thank you.

Replace the safety valve and reset the maximum operating pressure switch/fill.

Good advice from hvac1000 , but after doing what was recommended keep an eye on your system, valves usually go because of an underlying reason, i.e. too much pressure venting off too often.