The pros and cons of vented and unvented hot water supply are being discussed in depth on another forum.
This is a reasonably good explanantion of the differences that was quoted.
http://www.gasapplianceguide.co.uk/Mains_Pressure_Hot_Water_Systems.htm
When describing the safety requirements of the unvented system it doesn't give much information as to what can happen if the pressure limiting valve becomes defective and allows the water pressure to rise above the setting of the cylinder's pressure relief valve setting.
A pressure regulating valve did fail to control pressure and the results of that failure were the reason for the discussion starting on the other forum.
This is a reasonably good explanantion of the differences that was quoted.
http://www.gasapplianceguide.co.uk/Mains_Pressure_Hot_Water_Systems.htm
When describing the safety requirements of the unvented system it doesn't give much information as to what can happen if the pressure limiting valve becomes defective and allows the water pressure to rise above the setting of the cylinder's pressure relief valve setting.
A pressure regulating valve did fail to control pressure and the results of that failure were the reason for the discussion starting on the other forum.