In message , AJH
writes
On 17/11/2020 23:33, John Rumm wrote:
that causes the emergency pressure reduction valve to dump some water
out of the blow off pipe outside

IME once one of these cracks open at over pressure they do not reseal
well and continually leak slightly thereafter.

Oh! That's a bit worrying. I suspect mine has been opening due to water
hammer when the changeover valve operated. I have put in a relief valve
set at around 0.8 bar shunting the boiler output through the DHW coil
and then back to the boiler.

--
Tim Lamb