Water heating

To get the most heat from the least amount of non-renewable energy, go for solar water heating. The next most efficient water heating option is probably an electric heat pump, followed by instant gas.

Saving hot water

There are several easy ways to cut down on your use of hot water without sacrificing your lifestyle.

• Use cold water for washing clothes, rinsing, filling the jug etc.
• Turn off the water heater when you go on holiday.
• Have showers instead of baths and keep your showers relatively short.
• Fit low-flow shower heads which are water efficient (while still giving you a decent shower).
• Choose water-efficient household appliances.
• Fix leaks and drips.
• Don’t run the hot tap unless you need hot water.
• Reduce water flow from your taps.

Water heating options

Gas and electric

Most New Zealand homes use electric hot water cylinders. Others use gas or LPG hot water cylinders or instant gas hot water systems (which heat water as you need it). Both gas and electricity produce greenhouse gas emissions - but gas produces less than electricity when the latter is generated by burning fossil fuels.

Electric water heating

Instant electric water heaters are sometimes used for washbasins or showers and are energy efficient because they heat water where is is used, eliminating heat loss from storage cylinders and hot water pipes. Drawbacks with instant electric hot water systems are that they are typically used when electricity is charged at the peak rate and they also require separate wiring for large flow-rates.

Gas/LPG water heating

Instant gas hot water systems provide continuous hot water that never goes cold, as the water is heated as it flows to the tap. Gas is only used when your hot water tap is turned on.

There is no storage cylinder, which means there are no energy losses from keeping water in a tank hot. Systems can be up to 95% efficient and water temperature is set at a control panel reducing the risk of burns.

If you're looking at an instant gas hot water system, look for one with automatic ignition not one with a pilot light. Pilot lights use gas even when the water is not being heated.

Gas hot water storage systems are not affected by power cuts. However, standing losses from gas hot water storage cylinders are large (see 'Cylinders and pipes' below).

Costs - electricity and gas

The cost of electricity and gas to the consumer includes the line charges and connection fees. If you don't use much energy these can be a large part of your monthly bill.

Everybody needs electricity for lights and appliances. However the choice of having electricity or gas, or both, depends on your individual circumstances. The cost of switching from one form of energy to the other may outweigh any savings you make. Natural gas attracts a standing charge and this should be factored into your calculations if you are considering switching from electricity to gas. LPG does not have a standing charge, but a yearly rental has to be paid for the use of two 45kg LPG cylinders.

Electricity prices are expected to keep rising in future. Gas prices are also likely to increase as the Maui gas supply reduces and gas is sourced from other fields (including being imported).

See the Powerswitch section on the Consumer website for gas and electricity prices in your area.

Solar water heating

This is the most energy-efficient water heating option - the sun's energy is free, unlimited and non-polluting. For many households, it is also the most economical - converting to solar can pay for itself over time through lower energy bills.

Converting to solar is particularly worthwhile for larger households, households that use a lot of water and for homes in sunnier areas.

To maintain a hot water supply when the sun doesn't shine, solar hot water systems usually have backup heating - so you will still need to consider the pros and cons of other water heating systems too.

For more information, see solar water heating.

Electric heat pumps

Heat pumps use electricity far more efficiently than ordinary electric water heaters. They are usually used for space heating, but some are designed to heat water.

They work by extracting heat from the air outside, using a process that's like a refrigerator working in reverse.

One study of a heat pump hot water system in Dunedin showed it produced nearly 2.5 times more heat than direct electric heating (saving 60% of the electricity). Manufacturers often claim heat pumps give three times the heat.

Heat pumps lose efficiency as the temperature outside gets lower, so they are less efficient in winter. Ask suppliers for the heat output figures at an external air temperature of 2°C - the higher the figure the better.

In some heat pumps, the fluid used to extract heat is harmful to the ozone layer if it escapes. Old heat pumps should be disposed of carefully - contact your local landfill for advice on how to do this.

A heat pump water heater might cost around $5000 to install (not counting the cylinder). If you use 4500kWh per year to heat water now, then the heat pump should save you around $500 per year (at 18 cents per kWh).

Wetbacks

A wetback is a useful way to heat water in winter if you have a woodburner and a reliable source of dry, untreated wood. You can also use a wetback with an open fire, but open fires are very inefficient.

Wetbacks can complement solar water heaters since they are used in winter when the sun is weaker.

For any type of hot water storage, energy is lost through the walls of the cylinder even if no hot water is being used. This is called 'standing loss'.

Modern electric cylinders must have a high level of insulation so that the standing loss is relatively low.

The heat loss, even from a well-insulated 270-litre electrical cylinder, may be about 2.0 kWh/day, costing 36 cents at 18 cents/kWh.

If you have an older cylinder, which is not as well insulated, then adding a cylinder wrap could save you up to 1 kWh/day (saving about $66 per year). The wrap costs about $60-$90 (2006 prices) and could pay for itself in about a year.

Gas hot water cylinders can't be fully insulated as they have an exposed flue in the middle where the flame heats the water. They lose about three and a half times as much heat as a similar electrical storage cylinder.

Hot water cylinders are now sometimes installed outside the house to save space. The cold and rain outside mean that the cylinder insulation needs to be particularly good.

Heat loss from pipes

Pipes from hot water cylinders are often not insulated at all and lose a lot of energy. You can buy foam tube pipe insulation from plumbers' merchants very cheaply. The first metre of pipe from the cylinder is the most important to insulate, but installation can be difficult as pipes are sometimes hidden inside walls.

Pressure

Low-pressure cylinders are common in New Zealand and work well in most situations.

High-pressure cylinders are more expensive, give faster flow and allow you to use a wider range of taps. But this increases the water and energy used. High pressure systems can be made more efficient by using water efficient showers and taps. These have lower water flow than standard taps and fittings, but are designed to deliver the water effectively and comfortably. High flow showers do not always equate to better showers.

There are some low-pressure cylinders which give you high-pressure delivery. These are called indirect mains pressure cylinders. They are often operated at high temperatures, so standing losses can be high.

Size

A cylinder should hold enough water so that it rarely runs out under normal use. A family of four will probably need about 300 litres capacity.

Cylinders for solar and wetback systems

Some cylinders are specially designed to work with solar systems and wetbacks. Some older cylinders don't work well in these situations. Ask your installer if yours is suitable.

Wetbacks must be vented, so cannot be connected directly to a high-pressure supply. They can be connected directly to a vented, low pressure cylinder, or be connected to a vented heat exchanger that heats a high pressure supply.

For solar hot water, it's better to have a larger cylinder as you will be less likely to run out of water.

How long do cylinders last?

Life expectancy of modern copper or stainless steel cylinders is 20-40 years, but mains pressure glass-lined steel is shorter at 12-20 years.