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.