and talking of TPRV being set off we were called to an emergency job where the gas boiler unit thermostat had failed to fail to safe and had pumped out about 500litres + of boiling water and steam out the poor little 20mm TPRVon a 300litre cylinder before anyone stopped it...when we got there the tundish line had almost melted away such was the ferosity of the discharge it was clipped every 600mm and in between each clip the pvc had sagged at least 200mm i wish i had taken photos now but at the time that was the last thing i was thinking about...subsquently talking to one of the body corp residents no checks had been done on the unit in almost 5 years...of course we told them to get servicing done each year of course we got no complaints as they could see what the result was of lack of servicing...also such was the pressure build up it blew a 40mm copper croxed fitting off but i must say the installer had brazed too close to the crox and had annealed the pipe somewhat...but still blowing a big crox off like that takes some doing!.