System:

System boiler, pressurised DHW tank in loft..

Symptom: water stains around tundish area and occasional drips in
pressure blow off exit pipe outside house.

Someone enlighten me as to whether the safety valve(s) operate on the
DHW circuit, the CH/DHW primary pressure, or both...and what pressure
they operate at?

Mains pressure is a stonking 2 bar plus...and I normally run the
primaries at 1.5-1.8 bar...

I assume the boiler has an expansion vessel in it, but should there be
one in the DHW circuit? I ocasionally get water hammer on one hot tap...