Energy

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

Water Heating: Key Principles

According to the BRANZ HEEP report, water heating accounts for 29% of energy use in an average New Zealand house. Average water use is in the order of 40-60 litres per person per day. Any reduction in energy used to heat water, or the amount of hot water used, can have a significant impact in terms of both national sustainability and cost to the user.

Achieving energy efficient water heating

Achieving energy efficiency and sustainability in water heating involves:

  • specifying efficient hot water outlets that minimise the amount of hot water used
  • specifying water heating energy sources that are efficient and do least harm to the environment
  • planning spaces and reticulation to minimise heat losses from the system.

Heat losses can be minimised by specifying a system that:

  • only heats/stores as much water as is needed to meet peak demand – by specifying the appropriate storage cylinder size and/or using a continuous flow water heater for higher demand fittings
  • only heats water to the temperature needed to meet regulatory requirements (see hot water supply and pipework)
  • uses water-efficiency measures such as fitting low-flow taps, shower heads and water-efficient appliances
  • for storage systems, incorporates a Grade A insulated cylinder located in a warm space (specifying an additional cylinder wrap will further reduce heat losses)
  • has all hot water pipes insulated
  • minimises the length of hot water pipework
  • sets the hot water supply at the right flow rate and temperature
  • incorporates appliances such as dishwashers that heat their own water.

Key decisions when specifying water heating systems

Key decisions that have to be made as part of the design process include:

  • locations of water heaters and outlets
  • heating energy source
  • whether to store hot water or use a continuous flow system
  • the capacity of the system.

Reducing the amount of water heating required

The amount of energy needed to meet a household’s water heating requirements depends on:

  • the amount of hot water used – the age and number of occupants
  • the increase in temperature from the cold water supply to heated water
  • the amount of energy lost while the hot water is stored or delivered to the taps (typically, 30% to 45% of the energy consumed in water heating).

Efficiency of water heating energy sources

Solar energy is the most cost efficient way of converting energy into heat because the sun's energy is free, but solar systems require backup from another energy source to guarantee hot water year-round. Heat pumps are the most efficient at converting electricity into heat. Electricity, solid fuel and gas also produce greenhouse emissions. In some circumstances, continuous flow gas water heating systems may be the most efficient option. Also see water heating energy sources.

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.