Mother has lived in her house for forty years and it has the original hot water system in the roof still supplying hot water. As far as I know, it has never been touched. The system, as most did, has a header tank with a cock and ball valve to keep the water at a set level. The hot water unit takes its water from the header tank as required and is gravity fed to the taps. That means the water pressure is not very high. It heats the water overnight at a lower electricity tariff and has its own meter and timer. The biggest problem with such a unit is that if you run out of hot water, there is no more until it goes through its overnight heating cycle. This often causes family disputes with blame apportioned to someone who spends a long time in a shower, or the person who does too much washing in one day. Some units do have an over-ride where you can switch it on manually during the day, but it will heat the extra water at the more expensive general household tariff.
I forget how big the units were, that is how much hot water they held. Maybe 200 litres, although 80 gallons seems to ring a bell, which is much more than 200 litres. Maybe it was 60 gallons.
When we first moved to the farm when I was four years old, hot water in the kitchen came from boiling water on the black cast iron stove, for the bath from a chip heater and for the laundry, an immersion heater. It wasn't long before my father installed an electric hot water service of the type described above and then later connected the new slow combustion stove that heated the water in jackets around the firebox. The electric system only came on if the stove was off.
Leaving a young me at home and to my own devices was always a mistake and mischief would ensue. There were always things to investigate and thought lines to pursue. I thought I might see how super hot I could get the hot water and loaded up the stove with many more briquettes of coal than was normally burnt. Extending from the roof and shaped like a shepherd's crook was the hot water system overflow. I had never known it to operate, but operate it did. Like a dragon it spat out steam and boiling water on to the roof. The fire in the stove became hotter and hotter and boiling water continued to flow in a steady stream. What to do? I can't take burning briquettes out of the fire box but I did close down the fire damper.
Eventually it settled down and by the time my parents returned, all they knew was that the water was nice and hot and I am sure they did not notice the higher than usual consumption of briquettes for the week.
Now, it is always good to add a photo to a tale but how boring is round metal cylinder. But wait, the oldest hot water service in the world was anything but boring and so I have taken the liberty of using one of River's photos of her failed behemoth of a hot water heater. It looks to be quite a complicated unit.