The water is heated to a relatively high set temperature (usually between 60 degrees celsius and 70 degrees celsius) and kept ready for use in a tank. When you use hot water, it's drawn from the top of the tank and replaced by a layer of cold water at the bottom. The temperature drop is sensed by a thermostat, which turns on the heater at the bottom of the tank.
Although the tank is insulated, it's constantly losing energy. So the water temperature drops over time unless it's reheated. If you draw off hot water faster than the cold water can be heated up, the cold layer can eventually move to the top of the tank - and you'll run out of hot water.
Storage tanks must withstand high water temperatures and pressure, and have to be protected against corrosion.
Mild-steel tanks are protected from water corrosion by a lining of one or two layers of spun glass, often called vitreous enamel. The mild steel can also be corroded by forming an electrolytic cell with copper water pipes, which are common. So these tanks have a magnesium rod (called a sacrificial anode) inside them that corrodes first, and which you need to check (and usually replace) at least every five years in order to prevent damage to the tank. Mild-steel tanks usually have five- to 10-year warranties.
Stainless steel is resistant to water and electrolytic corrosion. Tanks made from it are more expensive to buy, but generally last longer and don't require as much maintenance as mild-steel tanks. They usually carry a 10-year warranty.
- Regardless of the tank material, hot water systems will require ongoing maintenance such as occasional replacement of valves and seals.
Note: Tank warranties usually only apply if the water quality is within certain limits. For example, depending on the concentration of suspended solids in your water, mild-steel tanks may require different types of anodes, and stainless steel may not be compatible with bore water. Check with the installer whether a particular type of tank is suitable for the water in your area before you buy.
Electric systems have to meet minimum energy performance standards regarding the tank's heat loss.
Most electricity providers offer a choice of tariffs for water heaters.
If you connect your heater to a continuous or day-rate supply, electricity is available 24 hours a day and the heater can replace tank heat losses and hot water that's been drawn off relatively quickly. You're unlikely to run out of hot water - at least not for very long. So the tank can be smaller than with an off-peak system and you'll save on the purchase cost. A four-person household needs a tank of about 125-160L. However, the high day-rate tariff makes this type the most expensive to run.
Off-peak or night-rate systems only receive power for a period during the night. Tank heat losses and hot water that's drawn off can't be replaced until the following night. So you need a large tank that can cope with your day's demand, otherwise you may end up without hot water (about 250–315L for a four-person household). You can get a twin-element tank (see diagram). The bottom element is connected to off-peak supply and does most of the heating. If you run low on hot water, a second element connected to continuous supply kicks in to heat a small portion of water at the top of the tank.
Gas water heaters have a gas energy rating label: the more stars on the label, the more efficient the heater. There are standard and high-efficiency gas storage heaters. High-efficiency models are more expensive to buy, but cheaper and more environmentally friendly to run.
Gas storage heaters only need relatively small tanks, as gas is available 24 hours a day and heat losses can be replaced quickly. A four-person household needs a tank of about 135–170L. Gas heaters can be installed externally, or internally with a flue, which may require more installation work (and cost more) than an electric system. Some gas suppliers may have a specific tariff for gas storage water heaters.
Solar and heat pump
Solar water heaters use the sun's rays to heat your water. They basically consist of solar collector panels and a storage tank. A heat pump works on the same principle as a fridge or air conditioner. It doesn't create heat, but transfers it from the air to your hot water tank.
As a general rule, one person uses about 50L of hot water a day (more if you use your dishwasher often, take very long hot showers or often wash clothes in warm or hot water). Based on this, a four-person household needs about four square metres of solar collector area (two panels) and a 300-360L tank, to allow for days with lower radiation or a higher demand. If your panels can't be installed in an ideal location, their efficiency may drop and you'll need a larger collector area.
Heat pumps don't rely on the sun, so a slightly smaller system is sufficient (270-315L tank for four people).
Check our article on solar and heat pump water heaters for more details.
Slightly unusual these days, gravity-fed hot water systems are non-pressurised tanks located in the roof space. They were common before mains-pressure systems became popular some decades ago. They are still available. Some solar hot water systems are also available with gravity-fed storage tanks.
You're probably aware of the main disadvantage: as the hot-water pressure depends on the height difference between the tank in the roof space and the water outlet, it's usually not enough to allow several hot-water outlets to draw from the tank at the same time. However, these systems don't require much maintenance, and can last for a very long time. And you can connect solar collector panels.