How does cold water pressure get used by a hot water heater to produce hot water pressure?

Question

Recently I had to change a washer in the shower for my hot water tap. I couldn't find the pressure tap for the hot water on my hot water boiler.

I asked a friend how to switch off the hot water pressure and he said:

It's easy! You just turn off the cold water pressure at the tap out the front, then drain the hot and cold water pressure off at the basin, then you can change your hot water washer.

He was correct, but it didn't seem right. I couldn't visualise how the valves in the heater could make that possible.

As you can see in this diagram, there is an overflow pipe, as well as a valve to stop pressure going back up the cold water pipe. But if the pressure is being supplied by the cold water pipe, surely it just overflows through the overflow pipe and doesn't apply any pressure to the hot water outlet?

My question is: How does cold water pressure get used by a hot water heater to produce hot water pressure?

EDIT: This hot water system is in Australia.

Answer 1

The "overflow pipe" in the diagram is not an overflow pipe.

The water heater is fitted with a relief valve which remains closed against the usual water pressure in your plumbing, but which will pop open if either the pressure or the temperature in the heater tank gets dangerously high. Overpressure will occur if the cold water inlet is somehow blocked, all faucets are closed, and the tank heats up. Overtemperature will occur if the thermostat fails and the heater (element or flame) remains constantly on.

The opening of this safety valve is such a rare event that ordinances and good practice don't even require that it be connected to a drain.

The "overflow pipe" in the diagram is merely a guide to prevent hot water from spraying everywhere if the safety valve ever does open.

Answer 2

The hot water is at the top of the tank and is pushed out by introducing the cold water to the bottom of the tank through the dip tube, as shown in your diagram. If you run the tank out of hot water or shut the heating elements off you will just have the same temperature water coming out of your hot water faucets as you do coming in your cold line.