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28 June 2021

Efficient Water Heating in Medium Density Housing

medium density housing nz

While district plans have been amended with local growth plans that cater to denser housing models, are we also taking time to consider the affordability of the housing we are building? While we increase the ratio of people to land area, we still want to maintain the quality of the urban lifestyle and connectivity between residents, while improving the outcomes for people living in a denser population.

Centralised hot water systems for efficiency

It is important that our construction industry is utilising technology to build efficiently to realise the economic benefits both for the developer and the end user. It is critical architects are designing in modern, well thought out ways to help increase desirability for this new style of living as an attractive alternative to New Zealanders who are accustomed to stand-alone housing.

When we start talking about medium density housing a good place to start making changes are with hot water heating — the largest single energy demand in modern residential buildings. This provides the greatest opportunity to reduce living costs by introducing better technology.

With higher density housing the opportunity for centralised hot water systems exists, allowing for more efficient and environmentally friendly systems, while reducing overheads of the tenants. While ultimately these systems would require some kind of housing management model for billing and maintenance, the reduced environmental impact and running costs will be a significant benefit to both the developer and the end user.

Heating our water with minimal environmental impact

Previously medium density housing and apartment buildings have generally completely relied upon gas to heat their hot water, but the Climate Change Commission is recommending the phasing out of fossil fuel gas consumption over coming years. This is not a challenge we should be afraid of as residential buildings have predictable and high hot water use so minimising and designing out fossil fuels in this long-lasting infrastructure is relatively easy and a high priority.

There is a range of sustainable technologies for hot water heating in medium density housing and apartments suitable for a mix of budgets. The keys to success are reliable performance modelling, complete system design and guaranteed product operation. The most appropriate option will vary depending on the building design and management. Ranked in terms of sustainability, the options include:

1. Wood pellet boilers

Wood pellet boilers are ideally suited to centralised hot water systems for residential buildings of any scale. They use 100% renewable fuel, manufactured in NZ from waste sawdust, with the lowest carbon emissions of any energy source in NZ (0.003kgCO2e/kWh). Modern pellet boilers are fully automatic with no manual handling required for refuelling or operation. Pellet fuel can be delivered and moved around buildings with vacuum systems for flexible placement of fuel store and boilers. The domestic hot water system can be combined with a centralised space heating system if required. Centralised hot water does not require cylinders in each dwelling but does require ring main pipework and generally per apartment metering for billing.

2. Hot water heat pumps

A high quality, appropriately specified heat pump system will reduce energy use and costs for hot water by 60% to 75% compared to electric elements. The highest performance systems, such as CO2 refrigerant models, do not require additional electrical boosting. Individual split systems provide a solution for apartments and townhouses with an indoor cylinder and an outdoor compressor. CO2 models are extremely quiet at 37dB and therefore suit higher density living environments. An alternative option is the use of centralised systems with the heat pump/s and cylinder/s in a single plant room plus ring main for hot water distribution and individual metering for billing to all dwellings.

3. Solar hot water

A commercial scale solar thermal system can reduce the energy needs for an apartment building's hot water by up to 75%. Additional energy will always be required for boosting on days with lower solar energy, this can be provided by electric elements, a hot water heat pump, pellet boiler or gas boiler. The building will require a centralised system with a ring main for hot water distribution and billing to all dwellings.

4. Electric instantaneous water heaters 


These water heaters don't store water so you don't get the heat losses that occur in traditional storage hot water systems (~15% per day). Installed per apartment they eliminate heat losses from a centralised system with a ring main and they take up less room than a cylinder. Some models can be provided with preheated water so a combined system with solar thermal is possible.

But a single instantaneous system may not be suitable for more than one shower running at same time in one apartment. Systems require a high instantaneous electrical input (>40A, three phases) so there may be building transformer size and cost implications. Also increasing peak loads on the electricity grid puts more pressure on the sustainability of our electricity generation and lines infrastructure, increasing the need for peaking electricity generation plants that usually have the highest carbon emissions per kWh.

5. Electric hot water cylinders

These standard water heaters are widely understood and used. They have relatively low install cost and simple, with hot water available for multiple outlets and on demand. Electricity use is included in apartment electricity meter so no additional metering or billing required.

But electric elements do not have any energy efficiency gains compared to above options and take up valuable space within the apartment.

6. Gas boilers for centralised hot water

There is a potential opportunity to continue to use gas boilers for domestic hot water if combined with a more renewable technology that does the majority of the water heating, such as solar thermal or hot water heat pump. In this case the system should be designed so the majority of the energy is provided by the renewable system and the gas boiler is used to boost the temperature during periods of low renewable contribution. This approach takes the maximum benefits and mitigates the negatives of both technologies, resulting in a cost and energy efficient complete system.

7. Gas instantaneous water heaters

Instantaneous water heaters are very low efficiency (average is 80%) and are using a non-renewable fossil fuel, contributing to the climate crisis. They are relatively cheap for output and easy to specify so have been a preferred technology for these reasons. With so many viable alternatives with lower environmental impacts and running costs there is very little justification for gas instantaneous water heaters in new or retrofit NZ residential buildings.

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.
Email: marcus@ecohotwater.co.nz
Phone: 07 312 3382

View more information on Apricus NZ Eco Energy, including contact details.
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