Temperature/pressure relief valves - how do they work?

[cl]

Our boat calorifier (silly boat name for a hot water cylinder) has a temperature and pressure relief valve.

I can understand the pressure relief valve, it's simply a spring loaded thing that opens when the pressure exceeds 3 Bars and thus prevents the tank from exploding.

However I can't really understand what the temperature relief bit does. There is a probe from the valve that pokes down into the water and, presumably, lowers the valve seat as/when the temperature rises and opens the valve. How does opening the valve help anything? It will just let hot water out but won't reduce the temperature really, except that cold water will replace the hot. Is this all that it does? Why bother, what harm will too hot water do?

[Tim Lamb]

In message , [e-mail] writes

Also found in industrial sites where high pressure hot water or steam is used to produce hot water for hand washing etc.

I suppose over pressure could be from simple heat expansion. Do you have a non return valve to stop hot water expanding back to the cold water tank?

Over temperature might occur long before 3 bar is reached and might have exciting effects when a tap is opened:-)

Our boat person is still in bed and I suspect totally unaware of either issue! Her current problem is begging 5 gallons of red diesel from father after a *water in tank* drain down!

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[Robin]

(under "Valves").

[cl]

Ah, thanks, that does explain it quite well.

The boat calorifier is essentially an unvented cylinder but in practice runs at very low pressure as the pump providing the pressure is just an 'on demand' boat type pump with an accumulator. I'd guess the pressure is less than 1 bar.

[harryagain]

Calorifier is the correct term for all heat transfer devices where water is being heated. Large and small. The over temperature valve lets out hot water, cold water comes in and prevents overheating so you don't get scalded. Operates in the event the water temperature thermostat fails.

Also lets you know the thermostat is US.

[Roger Mills]

Or, in my case, that the valve was US!