solarWater heating systems, once installed, have a significant impact on the total energy consumption of the house for many years.

It is important therefore to think carefully about the various options available when building a new house, or undertaking major renovations.

Water heating typically makes up about 30 percent of the household energy bill however, for many households, water heating will be a considerably higher percentage. In recent years many households have replaced most of their lights with energy efficient bulbs and doing this increases the proportion of the energy bill that hot water heating makes up. Also, households with young families tend to use a lot more hot water, and it is this group that is usually particularly sensitive to increasing energy costs, as they earn less and they often have a parent working reduced hours or not working at all. At the other end of the spectrum, many houses with older occupants have quite low hot water demand.

Here we discuss the options commonly available for water heating in New Zealand, and will help guide you to the best option for your situation.

So, what are the options?

Electric

By far the most common water heating method in New Zealand is the electric storage water heater. This is a tank of typically either 135 litre or 180 litre capacity, and is heated with a 2-3 kW electric element. These systems have steadily improved in terms of their storage losses (about 20 percent of the energy used is lost) and are mostly installed on a controlled tariff, with a running cost of around 24c/kWh.

Gas systems

The next most common type is the gas storage system, which operates similarly to an electric storage system, except there is a gas heater at the base of it. These systems are relatively inefficient, with both a low gas conversion to heat efficiency and high storage losses. They are best suited to reticulated gas networks and so are not common in Christchurch. While the cost of the gas is cheaper, the monthly supply charge for low to moderate users results in this being a more expensive option than electricity.

Solid fuel / wetback

Another common system in New Zealand is the wetback system. They consist of a water jacket at the back of a solid fuel burner fire-box, use the natural thermosiphon effect to transfer the heated water to the tank and to draw the cold water from the tank for heating. These systems are usually installed in conjunction with an electric storage system.

Solar water heating

Solar water heating systems are steadily increasing in popularity. These systems use collector panels mounted on a north facing roof to heat the water. They come in different configurations, but for Christchurch the main options to consider are either glycol or drain-back flat panel systems, or an open circuit evacuated tube system.

In general, systems should be pumped systems, as the tank on roof types are unsuited for the colder climate in Christchurch, unless they are preheat only systems.

Heat pump water heating

An emerging option is the heat pump water heater system. These systems are an electric system, but use the electricity much more effectively than a conventional electric system, in the same way that a heat-pump space heater is much more efficient than an oil column heater.

Heat pump water heaters have a very similar performance during winter as during summer. For Christchurch it is important that only Class A Low Temperature rated systems are used. Systems using an electric element during colder conditions will perform poorly during winter.

So which option to choose?

Smaller households

This all depends on how much hot water you use and what you want to achieve. For households with low hot water usage, then the cheapest option is to heat the tank with an electric storage system, using a cheaper night time only tariff. Up to 30 percent of Christchurch households choose this option. This also has the lowest carbon emissions of any system, but does not suit larger uses of hot water. You will need about a 180 L, well insulated (add a thermal blanket) tank, and this will suit a two person household.

Larger households

If you are a moderate or larger hot water user (have four or more in the household), then seriously consider either a solar water heater or a heat pump water heater. The solar system will usually cost more to install and both systems will save about the same in energy. The carbon emission savings is usually greater with the heat pump, but the solar water heater gives you more flexibility in how you use the system if you want to maximise savings.

Solar retrofits

Retrofitting solar water heating to an existing tank is an option, although in most cases the tank will be too small  to get the best savings. It is cheaper though, and can work well as a two stage process. Add the solar initially, then upgrade the tank in a few years time when it needs replacement.

Adrian Kerr is CEO of the Solar Association of New Zealand and director of Project Solar Ltd, and can be contacted at Adrian.kerr@projectsolar.co.nz



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