'Mains Pressure' Cylinder (without a discharge pipe?)

Looking at the diagram, I dont' see how that can be a "thermal relief valve" because its on the inlet side to the cylinder. So its unlikely it will see 100 celsius
Sanj, you're confusing the location of the PRV with that of the T&PRV, which is in the top of the cylinder (at least in the UK). You should be very careful what you post about unvented cylinders. There may be some readers who will think that they can ignore the requirement for the T&PRV with potentially serious consequences.

From the diagram it looks as though its a PSV. Spring loaded to relieve at a pressure. Nothing to do with temperature.

My mum and dad's santon has the T&P valve in the side of the cylinder so i don't think it *has* to be in the top
 
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From the diagram it looks as though its a PSV. Spring loaded to relieve at a pressure. Nothing to do with temperature.
The diagram neglects to show the T&PRV in the top of the cylinder. The PRV shown on the cold supply is also required in the UK but is not in any way a substitute for the T&PRV. The PRV is to release excess pressure but the T&PRV is primarily to release excess temperature, though it will also release excess pressure but at a higher setting than the PRV.
 
The diagram neglects to show the T&PRV in the top of the cylinder. The PRV shown on the cold supply is also required in the UK but is not in any way a substitute for the T&PRV. The PRV is to release excess pressure but the T&PRV is primarily to release excess temperature, though it will also release excess pressure but at a higher setting than the PRV.

The diagram hasn't neglected anything. If you install to DIN 1988 you don't need the T&P valve. That diagram shows DIN 1988 hence no T&P

I don't know why the UK building regs are so strict. the hydrotest pressure of most cylinders is 16 bar with max working pressure of 10 bar. The PSV will lift at 4.5 bar, i would have thought that coupled with the 2 port energizing valve and the boiler controls would have been plenty.
 
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its not the pressure thats the main problem,the USA found this out as theyve been installing these for years. at first they only used prvs but found at cost that its the temp thats the problem. at atmospheric pressure water boils at 100c . at 3 bar pressure about 143c ,if say for instance, theres no temp release valve, and water was to boil at 143c, then someone turned a tap on and brought the water to atmospheric pressure, it would expand 1600 times to steam causing a flash bang.1 litre of water x 1600 say 100-200 litre. work it out, and its a recipie for carnage. even more dangerous than a gas explosion.
 

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