Energy

Approximately 95% of New Zealand homes have their hot water stored in cylinders heated using an electric element or gas. Most of the remainder have a continuous flow system, predominantly using gas. There are, however, more energy efficient and sustainable options, namely solar, heat pump or recovered heat sources. In determining the appropriate water heating source and system type for a home, you need to consider its size and likely water use, and the upfront and ongoing costs, as well the energy efficiency and environmental impact of the energy used.

Storage or continuous flow system?

Hot water storage systems can be used with energy efficient heating sources such as solar, or they can use gas or electricity as the primary energy source. A disadvantage is that they can run out of hot water. Continuous flow systems have an endless supply of hot water, so long as the electricity or gas is available.

The choice between a storage or continuous flow system may be influenced by:

• the energy source readily available
• space available for installation
• use patterns and number of occupants within the house
• distance between the water heater and the point of use within the house.

Solar water heaters

Solar water heating is a highly efficient method of water heating. Solar systems can cut a typical household’s water heating costs substantially: up to 75% in summer and 25–45% in winter. Solar systems have low greenhouse gas emissions.

However, solar energy is variable and solar systems will not be as effective on cloudy days. In nearly all situations, a booster system is required to provide water heating in periods of high demand or low solar gain. This can greatly reduce the amount of energy that the solar water heating system replaces.

A recent study has shown that the single-most important factor in the efficiency of solar water heating systems is how individual systems are configured and particularly how well they are installed.

In colder climates, the solar water heater can be linked in with a wetback (see below) system to improve the balance between summer and winter water heating.

Heat pumps

Heat pump water heating transfers heat from the air outside a building to water stored for use in the building. Heat pumps can also use outdoor water and ground heat sources, although these are less common for New Zealand residential use.

Heat pumps are a highly efficient method of water heating. They offer the most efficient electricity-based option, as the electricity involved is used only to move the heat, not to create it. They have, on average, a Coefficient of Performance (COP) of between 2.0 and 3.0, meaning they create two to three times the energy for water heating than they use in electricity to run the pump and fan. Consequently, they also have lower levels of greenhouse gas emissions than other electricity-based water heating.

Heat pump energy efficiency is affected by low outdoor temperatures, especially air-to-water heat pumps. Ground-to-water heat pumps have a less variable heat source, but given their high set-up costs and ground area required, they are more suitable to multi-residential developments or very cold climates.

Year round, the energy efficiency of air-to-water heat pumps is comparable to solar water heating. This is because they have a heat source that is available 24 hours a day, 12 months of the year, and do not require supplementary water heating for high demand or low-solar periods. Heat pump water heating may particularly suit sites with poor solar options.

Electricity

Electricity can be used to heat either storage or continuous flow water heaters. However, in terms of sustainability and energy efficiency, conventional electric hot water heating is not a preferred option except as a backup to more efficient systems such as solar water heating or heat pumps.

The advantages of electric hot water heating include:

• electricity supply is available throughout the country
• electricity is typically always available in the building
• they can generally be set to use off-peak power supplies (though small cylinders may use peak electricity).

Disadvantages are that:

• they are less cost efficient in energy production than other options such as solar water heating and heat pumps
• generating electricity from fossil fuels produces greenhouse gas emissions
• electricity prices are rising
• they are expensive to run unless cheaper off-peak rates are available.

Gas

Gas can be used to heat either storage or continuous flow water heaters.

Continuous flow gas systems or small, high efficiency gas storage systems can be more efficient than electricity for small households. Gas may have a slightly lower cost (based on actual energy use) than electrical supply with the same heating capacity. For storage cylinder systems, gas has quicker heat recovery times than electric heating and can generally use a smaller cylinder.

Burning gas on site to heat water produces fewer greenhouse gas emissions than electricity generation using fossil fuels, so is generally seen as more sustainable.

However, disadvantages of gas include:

• gas systems have higher heat losses, with heat being lost in the products of combustion exhaust
• gas may not be available through a reticulated system, which means the occupier will have to rely on gas bottles; if it is available through a reticulated system, a gas line might have to be installed and the occupier will have to pay fixed charges for connection and line supply.

Wetbacks on solid fuel heaters

In a wetback in a solid fuel burner used for space heating, heat from the combustion process is used to heat water jackets installed within the firebox. Options are available for 2 or 3 kW rated water jackets (which may be combined to give up to 7 kW). Water is circulated through the water jackets and then through pipes to the storage heater. Older wetbacks may simply consist of pipes which pass through the firebox of a wood burner, wood stove or, in some cases, through older style open fires.

Most wetbacks circulate the water through a thermo-siphon effect (i.e. warmer water rises and cooler water falls, creating a natural flow through the water jacket within the solid fuel burner). As a consequence, the hot water cylinder should be close to the heat source. This may require longer pipe runs to hot water outlets than would otherwise be needed. Large diameter wetback water pipes (25 mm) are recommended to improve flow.

Some systems may incorporate an electric pump to increase pressure and allow transfer to storage cylinders located further away, however open venting is required.

Wetbacks are generally specified to provide winter boost to a storage cylinder system.

Key factors when incorporating a wetback system are:

• most wetbacks only make a substantial contribution to water heating in cold climates where a lot of space heating is required
• burners are available that produce a higher proportion of their output as hot water. These are suited to houses where space heating demand is lower
• supplementing the input from the wetback when the solid fuel burner is not being used, through solar or electric boost
• determining the emissions rating for the solid fuel burner incorporating the water jacket – as the water jacket cools the combustion process, the level of particulates emitted generally rises
• recognising that the installation of water jackets within the firebox reduces the heat output to the adjacent space
• ensuring the circuit between the water jacket and the storage cylinder is open vented for safety– this means that either a low pressure open vented cylinder is required or that an isolated circuit separate from the water in the cylinder is open vented and heat is supplied to the water in the cylinder by a heat exchanger coil (this option allows the cylinder to be run at mains pressure)
• specifying that isolated open vented supply lines incorporate a valve that allows the water to be replenished should overheating occur – overheating gives a drop in pressure in the line, which activates the valve to replace the water lost out the top of the vent pipe
• specifying copper piping only.

For your clients

To help your clients make decisions about water heating options, and to help them find ways of reducing their hot water bills, refer them to www.smarterhomes.org.nz/energy/water-heating and www.smarterhomes.org.nz/energy/solar-water-heating.