need unvented advice

[Megaflo-Experts]

ok but the incomings only 3 bar taken from a outside tap safety valves set at 6 bar,if i have 3 in hows this caused a 6 bar to go off?

The 6 bar pressure relief valve is there as a safety device. In your case it won't need to do anything if the PRV failed but in a property with 8 bar mains pressure, then it would discharge if the PRV did happen to fail.

It's all to prevent the pressure in the cylinder getting too high!