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This weekend I decided it was about time that I twiddled all the various taps and valves associated with our water system – just to make sure none of them get seized in the ‘on’ position – wouldn’t be good if can’t get the water turned off when you need to most. Perhaps checking was a good move – because what I ended up with was hot water exiting through the pressure release valve at the top of the tank. The valve wouldn’t reseal, and the only way to stop the tank of water draining non-stop was to turn off the supply to it.  Time to call a plumber in.

This is the valve that sits on top of the tank that is the emergency pressure release. If, say, the thermostat in the tank fails, and the element just piles more and more heat into the tank, one would end up with very hot water under pressure. 

In normal conditions (e.g. a pot of water on the stove), as you shove heat into it the temperature increases, until the water hits boiling point. At this point any further energy you put in doesn’t raise the temperature of the water, instead it is used to turn the water into steam. If you keep the pot on the stove, eventually all the water will boil away. However, if you don’t allow the vapour to escape (pressure cooker), the pressure will build up inside and this acts to raise the boiling point of the water. Up comes the temperature – beyond 100 degrees, and up goes the pressure as well. At some point, if you keep putting more energy into the system, the pressure is going to be too great for the structure to cope with, and something’s going to fail.

The pressure cooker has a valve on the lid that will blow if it gets too hot and pressurized inside, releasing the pressure in a controlled manner, before the pressure cooker explodes. Likewise, the hot water cylinder has a similar device on it. You really don’t want it to get stuck in the closed position (or for the drain pipe to be blocked) hence the warning on the cylinder to release it every six months to check it works. Unfortunately, in my case, the valve is now stuck ‘open’ – which ain’t much use. So my tank is very safe, but it’s also not doing its job, since hot water promptly disappears down the drain. At least that’s better than having an exploding tank.

You can have a look at a water phase diagram here. You can see the boundary line between liquid (water) and gas (steam) phases (the boiling point) and how this increases as pressure increases. If you increase temperature and pressure enough you get to the ‘critical point’ – this is where there is no longer any distinction between water and steam – the two have become the same phase. This is really quite hot indeed!