Unvented hot water question

[spanner]

Quick question about unvented systems! ??
The temperature and pressure relief valve should never operate.. Is this correct. As it implies that the water in the system is likely to be running at very dangerous thus the pressure.. The normal everyday relief is more likely to be of the expansion vessel's andand valves? The only problem. Or have I blown meself up already...

 

[lame plumber]

theres a series of safety devices and the tpr is the last in line being that it works as water hits 95 or the pressure hits 8 bar or so dependant on the manufacturer. so the pressure recing/limiting valve should keep it at low pressure initially and if that fails the pressure relief valve will go at 3.5 bar normally and finally the tpr valve if the initial ones fail to work. Same for temp where theres a thermostat, overheat stat cut off and finally the tpr valve