Stardanny
Just read your question form yesterday the diffrence between the T&P and the PRV is simply this the T&P Operates on excess temperature or Pressure the PRV operates on excess pressure only it is usually sightlyt lower rated than the T&P and it is installed in the cold feed after the pressure reducing valve (Quite often on a Hydroblock unit) if the PRV lifts, only COLD water is discharged, this is both safer and does not waste energy (which would happen if you dumped hot water) the T&P will dump scalding hot water which can cause a hazard and of course wastes energy) However the T&P valve is the last line of defence and should only operate in the case of imminent danger (explosion)