Water Heating

The main issues you should consider are:

• Where the heater can be installed

• The price of the heater (as well as installation, maintenance and running costs)

• The rebates and other purchase cost assistance applicable to the heater type and to your area

• The energy source (electricity – for electric storage, instantaneous, heat pump water heaters and solar water heaters with an electric boost,  gas water heaters – natural or LPG,) and its availability

• Specific issues that require consideration with different heater types – your climate, access to direct sun, shading, layout of your house, etc

As with any large purchase, it’s a good idea to do your own research first and also to discuss your options with a number of suppliers or installers so they can advise on the heaters that may be appropriate for you. 

The efficiency of any unit will depend on a large number of factors so it’s best to do plenty of research, including product reviews on websites, talking to several different retailers and suppliers before making any decisions.

While the majority of the water produced from a solar or heat pump system should be from the sun or warmth from the air respectively, you will also have to account for any boosting that may be required when the hot water supply is insufficient.

A solar hot water system can provide up to 90% of your required hot water, depending on where you live. Have a chat with your supplier to get an idea of the solar gain you can expect in your climate.

Heat pumps work best in warm climates as they can gain more warmth from the air to transfer in to the water. Once temperatures get lower, it becomes more difficult for the heat pump to extract warmth from the air, so they will need to work for longer to produce the same amount of hot water.

The energy.gov.au website has an abundance of practical tips and information for reducing your hot water usage around the home.

You can also search and compare appliances at www.energyrating.gov.au.

Storage units are usually sized according to the number of people in the house, and instantaneous units according to the maximum number of hot water outlets likely to be used at the same time. Gas storage systems can have a smaller tank size than electric off-peak systems as they reheat the water whenever it cools down.

The way you use hot water will also impact on the size you should choose. If everyone in the household tends to shower at the same time of day, such as first thing in the morning, you may want a bigger storage tank to ensure you don't run out of hot water. But remember, the bigger the tank the greater the standing heat loss, so aim for the smallest tank that will meet your needs.

The table below gives an indication of what size heater you might need, but you should always ask the supplier or manufacturer for specific size guidelines for the products you are considering.

Climate considerations need to be made when sizing and selecting a water heater. Storage heaters are an effective choice for any climate. However, in very cold climates, hot water usage tends to be higher, so it may be worth considering purchasing a larger storage tank than otherwise might be suggested for your household size. If you have the space available, then you could also consider installing the heater indoors, for example in a laundry, to help reduce heat loss from the tank.

In a climate with very cold winters, such as alpine areas, you should choose a system with a higher capacity than is normally recommended for your needs because instantaneous heaters are limited in how much they can increase the temperature of the cold water. They can raise the temperature of the cold water by 25 degrees at their nominal maximum rated flow. Reducing the flow rate will also allow the water to be heated to a higher temperature.

For electric hot water systems, the capacity (called the ’hot water delivery‘) is a measure of the storage and is defined as the volume of water that can be delivered by the tank while maintaining the temperature within 12°C of the initial delivery temperature. This volume is relevant to storage water heaters (gas and electric) where the element rating is very much lower than the rate of energy delivered in the form of hot water. For large electric tanks, which are usually run on off-peak tariffs, this is a reasonable measure of the total hot water that can be delivered in a day. Note however, many utilities offer a tariff which allows boosting through a top element at off-peak rates if certain sizing requirements for the off-peak tank are met (typical > X litres for a Y bedroom house).For smaller electric tanks, which are typically run on continuous electric tariffs, the capacity is the amount of water that can be drawn off in a single sitting. Clearly, the recharge period depends on the element rating and the tank size, but is typically around an hour for smaller tanks.

For gas storage water heaters, the tank storage capacity is a (very approximate) measure of the volume of hot water that can be drawn off in a single sitting. The recovery time depends on the rated burner capacity, but would typically be 1 to 3 hours.

Gas instantaneous (also called continuous flow) and electric instantaneous can supply hot water as long as there is gas/electricity available. Note that these units typically require very high power inputs. Electric units are fairly rare and almost always require three phase power.

A booster will provide additional hot water needs should there be insufficient energy from the sun to heat the water to the required temperature. If you, however, want to reduce your consumption of energy used you may have to be mindful of choosing the right Solar Water Heater or Heat Pump Water Heater with the right tank size for you and your family.

A booster is a back-up heater which uses an alternative source of energy for times when there is insufficient energy from the sun to heat the water to the required temperature. Generally this source of energy is either electricity of gas.

An in-tank boosting unit should ideally only activate when the water temperature is below the thermostat setting and should turn off when the desired temperature is reached. A manual booster switch can be used to maximise solar contribution.

Make sure you discuss your tariff options with your installer before the installation of your unit to ensure you have a good understanding of how different tariffs will affect both its running costs and performance.

Generally, be aware that some low-emission water heaters are less suitable for off-peak tariffs. The following water heaters are better suited to continuous/day rate electric supply:

• Instantaneous gas systems that require electricity to ignite the burner

• Solar systems with a circulation pump or some gas boosts that require electricity for burner ignition

• Some heat pumps – especially in colder climates where the pump will need to operate during the day

To gain a better understanding of your tariff options, you should talk to your electricity supplier.

Many models of solar are suitable to run on off-peak power. However, you should be aware that if you don’t use a lot of water in the mornings, operating your boost on off-peak can mean you have a full hot tank in the morning, meaning your solar collectors will be less able to make use of the sun’s contribution during the day.

Some heat pumps may also be suitable to run on off-peak power but you should talk to your supplier before you decide on a model. If you live in a climate with cold overnight temperatures be aware that the heat pump will have to work a lot harder to heat your water than during the day when temperatures are warmer.

In either case you should research your options and speak with suppliers before you make any final decisions.