This is really
Where I am (Nottinghamshire England) there are four hot water types that I know of:
- Gas "combi boilers" that produce hot water at need. No tank. Whole-house non-gas tankless heaters are extremely rare.
- "Power showers", an electric heater for water in a shower, are very common. Some houses don't have any central hot water, instead using these wherever hot water is required, such as the kitchen sink.
- Water mains pressure, where the tank has a secondary pressure control air tank. The secondary tank prevents the water tank from exploding from over pressure. It is a mystery to me why English tanks require this extra tank so they don't explode while French and US tanks don't (I've lived in all three countries).
- Gravity feed, where the tank is in the attic, open to the air.
It is worth noting that some houses here have cold water cisterns in the attic, filled by the water main and emptied by gravity. Reason: the water main can't supply enough water at pressure to be usable. These cisterns must be emptied and cleaned occasionally to avoid mold and critters taking up residence. There are frequent stories about finding floating rodents in the cisterns.
The hot water tanks are heated by one or both:
- Indirect heating, where the water in a tank is heated by a boiler (oil or gas), a heat pump, some solar system, or perhaps an alternate fuel burner such as wood or peat.
- Electric immersion heater(s). In my experience an immersion heater is 3kW (13 amps at 240v). In my flat, one tank has two of them and another tank has one of them.