What has happened to my hot water system?

Brown water

If you have brown or rusty coloured water coming from your hot water taps, you need to have your hot water system inspected by a plumber. Rusty water indicates that your tank has become rusty on the inside, and will need to be replaced. The rust is caused by the inside of your hot water system corroding from holding water, inside your tank is a ‘sacrificial anode’ which will allow your tank to remain rust free for a period of up to 5 years, as it sacrifices itself to rust, rather than your tank. The sacrificial anode needs to be replaced every 5 years in order to prolong the life of your tank.

Water is leaking from the top (or bottom) of my tank

If you have a significant amount of water leaking from the torusty hotwater tankp or bottom of your tank, this indicates that your tank has ‘burst’ or ruptured. This is common in tanks around 10 years old (or older). The tank becomes less reliable to hold the pressure of containing hot water as it ages (which is why regular servicing of your tank is important), and will eventually ‘burst’ under the pressure. If this happens a new tank will be required.

Different types of tanks

Storage tanks

The water is heated to a relatively high set temperature 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.

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.

Instantaneous hot water

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 litres a minute.

But have your plumber investigate 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 also 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 ofscalding).

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.