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26 August 2019

Sustainable Water Heating Options for Stand-Alone Houses

laura smetsers wanaka tree

This article explores all the domestic hot water (DHW) options currently available in NZ for individual homes. Given the range of choices, it focuses on the key considerations of environmental sustainability, capital cost, running cost, and ease of installation and maintenance. Not every system will be appropriate for every situation so it's important to involve an experienced, sustainability-focused designer or supplier early in the process to get the best outcomes for your project.

The domestic hot water systems are ranked to indicate how well they perform overall with long term environmental sustainability as a critical priority for buildings in 2019 and beyond. With domestic hot water being the biggest single energy demand in individual homes (making up on average 46% of new homes’ energy use), sustainability and affordability of this service are critical.

All systems considered are assumed to be high-performance examples of their technology, appropriately designed and specified, well installed and (if required) maintained.

Water-efficient tapware

Water Efficient tapware benefits

Specifying water-efficient tapware is the first and most important choice in delivering sustainable, affordable domestic hot water systems. Choose high WELS rated tapware and showers. Low flow rates do not necessarily mean a reduction in performance and comfort.

Solar hot water (SHW)

Solar hot water benefits

Joint first place for the best long-term environmental performance, life cycle sustainability and low running cost.


  • Will reduce the energy used, and the associated costs and environmental impacts by around 75%
  • Clear evidence of energy reduction with controller display and savings log
  • Can be combined with overnight (hydro) electric boost for very close to 100% renewable energy for DHW
  • Long product lifespan and good recyclability at end of life


  • Product and installation quality can have a big impact on performance and longevity — high quality, proven reliable systems must be specified
  • Appropriate design for climate and DHW use is important
  • Requires roof-mounted collector/s
  • Circulates water and uses mechanical components

Hot water heat pumps (HWHP)

Hot water heat pumps benefits

Joint first place because of practicality and ease of installation, although longevity and savings may be less than SHW. Also some concerns about end of life.


  • No roof-mounted collector or pipework to run between roof and cylinder
  • Generally a 60% reduction in energy costs, and the highest performance units can get up to 75% reduction
  • Can have combined function with underfloor heating and other heat loads
  • Some units are available without electric element boost or very clearly controlled electric boost
  • Ideally provide three to five times more heat energy than electric energy input — Coefficient of Performance (COP). COP figures should be given as a range over different temperature conditions to be accurate
  • Latest generation hot water heat pumps use CO2 as a refrigerant with minimal Global Warming Potential


  • Performance can be substantially reduced in colder conditions (>5°C)
  • Units incorporating electric elements often not transparent on contribution of electricity vs. heat pump operation
  • All in one units have high replacement cost which may undermine cost savings
  • Global Warming Potential of conventional refrigerants is a major concern globally. Appropriate end of life and thoughtful maintenance is absolutely essential
  • End of life recyclability is complex and not adequately resourced

Electric elements in cylinder (especially for night heating)

electric elements in cylinder benefits

Best choice after SHW and HWHP because of the high proportion of renewable energy in NZ electricity supply. Especially good environmental credentials and reduced running cost if used with off-peak, night rate with close to 100% hydro generation.


  • Long-lasting, low tech, low-cost technology
  • Renewable energy infrastructure already all in place (hydro dams and wind turbines) so switch to close to 100% renewable relatively easy with overnight heating
  • Highly recyclable and end of life processing already established (if not sent to landfill)
  • Space can be freed indoors with outdoor cylinders


  • High running cost compared to most other systems
  • Not very exciting or inspiring, explaining benefits can be challenging and unintuitive
  • Night tariff heating requires an adequately sized cylinder to store adequate DHW for the following day

Solar photovoltaics (PV)

solar photovoltaic benefits2

The high cost of installation relative to the guaranteed delivery of heat energy to the cylinder means PV scores poorly for install and running cost.  PV systems are better suited to purely electric loads such as charging EVs and refrigeration.  In many installations excess PV electricity is “dumped” to the cylinder but this should not be the main reason for installing PV, and cylinder heating performance will be poor for the roof area required and capital invested.  PV can be installed alongside SHW or HWHP for the best of both worlds.  Size the PV array for electrical not DHW loads.


  • Panels are a relatively simple technology to install
  • Provides electricity which can be put to multiple uses
  • No water circulating and no refrigerants used


  • Highest capital cost for energy delivered to heat water
  • Complex and generally not transparent total reduction in water heating energy use. Additional control equipment and cost required for water heating
  • Large area of roof required for output (2kWp = 13m² of PV vs 4.4m² of SHW)
  • End of life disposal and recyclability very unclear and challenging

Wood and biomass: wetbacks, pellet or wood boilers

wetback benefits

Wood burners with wetbacks stir many nostalgic thoughts; modern, mainly urbanised reality means an update is required. Clean Air requirements have limited their use, while combined with central heating some technologies have a very important future.


  • 100% renewable resource, using “current” carbon sequestered by trees
  • Single system can provide DHW and central/space heating
  • Advancing technology of pellets making systems easier and more efficient
  • Rural properties can be completely energy independent for hot water with free wood


  • Particulate emissions and release of carbon, despite being “current” still a concern
  • Modern, Clean Air wood burners generally have low output
  • Hassle factor can be off-putting to many, especially for wood burners requiring lighting every couple of days, year-round for 100% wetback heated water
  • More automated and higher performing systems have high capital

Fossil fuels: LPG, natural gas, coal, diesel

fossil fuel water heating2

The future of DHW for stand-alone homes does not have a place for burning fossil fuels. Homes designed now need to be built for the coming decades. Every other DHW option is more environmentally sustainable and often has lower operating costs. There are more important roles for fossil fuels than DHW during the transition to zero-carbon (transport fuels, plastic components, medicines, peaking and emergency electricity generation, etc).


  • Natural gas has low running cost, while the cost of carbon emissions are minimised
  • High recovery rate and energy density for space required
  • Can be combined with space heating systems
  • Can have medium capital cost
  • Instantaneous water heaters don’t require space inside the house


  • Burning fossilised carbon and contributing to the climate crisis
  • LPG has an equal or higher cost of delivered energy compared to mains electricity
  • Combining DHW and space heating with a fossil fuel increase total carbon emissions

If you are considering sustainable water heating for your next project, contact Marcus to discuss requirements, and discover the most effective hot water heating solution for your project.

View the Apricus Apartment Solar Hot Water System
View the Apricus Townhouse Solar Hot Water System
View the Reclaim Energy CO2 Hot Water Heat Pump

View more information on Apricus Eco Hotwater, including contact details.
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