03.Instantaneous water heaters
This type only heats as much water as you need, when you need it. If you turn on the tap, cold water flows through a heat exchanger, igniting a gas burner or switching on an electric element. So there are no heat losses, and as long as there's gas or electricity, you'll never run out of hot water.
The size you need (the flowrate in litres per minute) depends on the number of hot water outlets the heater has to serve, more than the number of people in the household. As a general rule, for a two-bathroom house you need a flowrate of about 22-24 L/min. But talk to your supplier to find the capacity most suitable for your situation.
Electric instantaneous water heaters have to be connected to the day-rate tariff, so the running costs will probably be higher than with an off-peak storage system. However, because there's no tank to lose heat, they're cheaper to run than day-rate storage heaters. Modern models have better temperature control than older ones you might have come across.
With older models, the water temperature varied depending on the water flow: the more cold water running through the heat exchanger, the lower the temperature. However, modern systems have electronic control that ensures a constant temperature up to the model's maximum flowrate. Only if you draw water at a higher rate than that will the temperature drop.
You can select different temperatures for different water outlets - 55 degrees celsius in the kitchen and laundry, say, and 40 degrees celsius in the bathroom (to avoid the risk of scalding).
Standard models have a pilot light, which wastes a certain amount of gas. Models with electric ignition are more economical. Gas instantaneous heaters can be installed externally, or internally with a flue. As there are no tank heat losses, they're likely to be cheaper to run than gas storage systems.