Energy

Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Energy sources

Approximately 95% of New Zealand homes have stored hot water using electricity or gas for heating. The remaining 5% use a continuous flow system, generally gas heated, although electric, continuous flow systems are also available.

More energy-efficient hot water heating options include solar, heat pump, wetback systems as well as improved technologies such as condensing gas burners.

To determine a suitable water heating system, you need to consider size, likely water use, initial and on-going costs, and the energy efficiency and environmental impact of the energy used.

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 back-up 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.

Disadvantages are that:

  • they are less cost-efficient in energy production than other options such as solar water heating and heat pump water heaters
  • generating electricity from fossil fuels produces greenhouse gas emissions
  • electricity prices are rising.

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:

  • may be more efficient than electricity for small households
  • may have a slightly lower cost (based on actual energy use) than electrical supply with the same heating capacity
  • have a quicker heat recovery time than electric for storage cylinder systems
  • 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.

Disadvantages of gas include:

  • gas systems have higher heat losses, with heat being lost in the products of combustion exhaust (although condensing gas burners reduce this lost heat)
  • gas may not be available through a reticulated system, which means gas bottles will have to be used
  • if gas is available through a reticulated system, the gas line may have connection and line supply charges.

Wetbacks on solid fuel heaters

Wetbacks generally provide a boost to the water heating system, particularly in the winter when most required.

The combustion process of the solid fuel burner heats a water jacket installed in the firebox. Options are available for 2 or 3 kW rated water jackets (which may be combined to give up to 7 kW). Water circulates through the water jacket, then back to the storage heater.

Older wetbacks may simply consist of pipes that pass through the firebox, wood stove or, in some cases, through open fires.

Most wetbacks circulate the water by a thermo-siphon effect (i.e. warmer water rises and cooler water falls, creating a natural flow through the water jacket). The hot water cylinder should be close to the heat source to minimise heat loss. The compromise may be that longer pipe runs are required to hot water outlets than would otherwise be needed. Large diameter wetback water pipes (25 mm) are recommended to improve flow.

Some systems incorporate an electric pump to increase pressure and allow the storage cylinder to be located further away. In this situation, open venting is required.

Key factors when incorporating a wetback system:

  • Wetbacks generally only make a substantial contribution to water heating in cold climates where a lot of space heating is used.
  • Some burners produce a higher proportion of their output as hot water – these are suited to houses where space heating demand is lower.
  • The input from the wetback must be supplemented by other means of heating when the solid fuel burner is not being used.
  • The emissions rating for a solid fuel burner incorporating the water jacket must be determined – as the water jacket cools the combustion process, the particulates emissions level will increase.
  • The installation of water jackets within the firebox reduces the heat output to the adjacent space.
  • The circuit between the water jacket and the storage cylinder must be 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.)
  • The isolated open-vented supply line must incorporate a valve to allow the water to be replenished if overheating occurs. Overheating will cause a drop in pressure in the line, which will activate the valve to replace the water lost from the top of the vent pipe.
  • Only copper piping must be used.

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