Siân Willmott

2 April 2014 — ESD engineer Siân Willmot says there is much debate over the best solutions for hot water systems in the multi-unit residential sector. This article presents research conducted by GIW, which is provided to clients considering the best option for their development. She would like this piece to stimulate discussion in the industry about best-practice solutions, so please feel free to share your opinions.

There has been much confusion within the development industry between architects, services engineers, ESD consultants, other design consultants and planning authorities as to which hot water systems are most suitable for a development, particularly within the multi-unit residential sector.

With a vast portfolio within this sector, GIW has conducted research on the application of several hot water systems within medium-density multi-unit residential developments in Melbourne’s inner suburbs.

During the investigation, we have considered each major technology under a triple-bottom-line framework – economic, environmental and social impacts.

Technologies reflected within the research include the following:

  • Centralised gas storage
  • Centralised gas – continuous flow
  • Solar with gas backup
  • Centralised instantaneous condensing boiler
  • Commercial heat pumps
  • Instantaneous electric

Based on inputs from several manufacturers and suppliers on cost, maintenance, efficiencies, warranties, hot water delivery, redundancies and billing accuracy, we have graded each system under the appropriate triple-bottom-line analysis.

Due to reasonable capital cost, high efficiencies, low maintenance and gas fuel input, the condensing boiler met the best economic outcome. Condensing boilers for residential applications are currently available and have energy efficiency ratings of 6 Stars (recovery efficiency of 94 per cent).

Again, due to high efficiencies and gas fuel input, the condensing boiler maintained the lowest greenhouse gas emissions over a 10 year period, ahead of the gas boosted solar thermal system.

A social impact analysis considered hot water quality, warranty (for both minor and major components), billing convenience and accuracy, access and redundancy. As the electric instantaneous systems are provided per unit, they can be easily accessed, maintained and billed. In addition to this, the single units allow hot water to be shut down and maintained per individual apartment, without affecting the remainder of building occupants. This makes them the highest ranked performer under a social analysis.

Of course, other items need to be taken into account such as spatial considerations, access to fuel sources, administration and incorporation of renewable energy technologies. These and other site constraints will determine the final system selection.

Siân Willmott is an ESD engineer with GIW Environmental Solutions.

8 replies on “Choosing hot water systems for multi-unit residential”

  1. Hi Richard,

    The development in question is a 7 storey, 42 apartment development. The roof space is minimal due to the provision of other key building amenities for inner city multi-residential developments, such as a rooftop terrace. The annual contribution from the solar thermal system is 17%.

    Your points on the condensing boiler are valid and I have already agreed that some benefit will come from the coupling of these two systems. However, with the small amount of solar provided and the already highly energy efficient condensing boiler, the economics of coupling these two systems simply does not stack up. It is evident that the coupling of these systems would result in lower greenhouse gas emissions and as such, it was not considered an option as the condensing boiler was already the most environmentally friendly.

    Please see below for the efficiencies of the different systems used:

    • Centralised gas storage: 80%
    • Centralised gas – continuous flow: 81%
    • Condensing boiler: 93.5%
    • Heat pump: 320%
    • Instantaneous electrical: 97%

    Note that heat loss through the distributed insulated pipework was calculated based on length and size of pipe runs and attributed to all centralised systems as an additional loss.

    Hope this information helps.

    Sian Willmott

  2. Hi Sian,

    Thanks for responding, that makes more sense. So this is applicable for buildings where the solar plant space allocation is not going to enable more than a 5-10% reduction on normal water heating demand, so very limited roof space or poor climate for solar collection?

    Would you mind stating your assumptions regarding efficiency of the different systems, the % of demand met by the solar plant and also what size building you’re talking about here (number of floors and number of apartments).

    I would also argue against the idea for the condensing gas system that “little benefit will arise from coupling this system with solar preheat.” Whilst this is true if the solar collectors are delivering hot water to the heater (in which case the heater wouldn’t be needed), if you’re delivering cold water from the solar collectors (which is the case by the looks of things 90% of the time based on the assumed sizing of the solar plant) then the benefits of the condensing boiler would be realised. And then there’s a grey area of warm water from the solar collectors. All the same, I can’t imagine a scenario where a condensing boiler coupled to a solar plant wouldn’t perform better environmentally than a standard boiler coupled to a solar plant. If the solar plant was large enough to remove any benefit of the condensing heat exchange, then it would be supplying copious amounts of zero carbon, zero cost hot water and hence significantly reducing fossil fuel consumption.

    Our findings, for buildings of 5-10 stories is that it’s quite easy to supply large percentages of the hot water demand via solar collectors (40%+). The roof space does need some planning but on these projects we’re also installing PV and other services on the roof so there is certainly room. I think if we ever came across a project where a full roof allocation of solar hot water collectors only yielded a 5-10% reduction in hot water heating demand we’d probably rethink this technology due to the added complexities and costs perhaps being better spend elsewhere. I would hazard to guess this would be for a scale of building akin to 20 storey apartments. At the very least I think there should be a caveat in your article that states these results are only applicable for small contributions of solar.


  3. Hi Richard,

    Thank you for your comments. The study was provided to compare the most common solutions we see in the industry today and therefore the reason behind coupling the solar hot water system with the central gas storage system (efficiency 80%). In addition to this, the condensing boiler has its own form of preheat by capturing and transferring the heat from the exhaust gas to the water via a heat exchanger. Therefore, little benefit will arise from couple this system with solar preheat.

    Unfortunately, like many multi-unit developments, our roof space was limited in comparison to the hot water demand, hence the small reduction in greenhouse gas emissions.

    This work by GIW was part funded under the AusIndustry R&D Tax Incentive Scheme. No other third party funding has been received.

  4. Hi Greg,

    Thank you for your comments. I read the report published by RMIT some time ago. Its suggestion that point of use electric systems are up to 3.1 more times efficient than centralised gas systems is viable. Instantaneous electric systems have a coefficient of performance of nearly 1 in comparison to their centralised gas counterparts which have efficiency generally in the range of 0.8-0.94, in addition to the losses associated with centralised reticulation (particularly for larger buildings with one plant) 3.1 times the efficiency for electricity vs gas does not seem unreasonable. However, due to the cost of electricity and the greenhouse gases associated with the harvesting of brown coal (currently 1.32 kg/CO2-e/kWh in comparison to 0.0512kg/CO2-e/MJ gas consumed as listed by the Australian National Greenhouse Accounts – National Greenhouse Accounts Factors, published by the Department of Industry, Innovation, Climate Change, Science, Research and Tertiary Education, July 2013), electricity without a renewable source is significantly more costly and greenhouse gas intensive that it’s natural gas counterpart.

  5. Hi Kathryn,

    Thank you for your comments. Certainly the longer term greenhouse gas emissions would make for an interesting additional study. However, unfortunately we are not in a position to determine when such greening of the grid would occur. Regrettably with a lot of multi-unit residential units, their roof space is limited relative to demand and therefore the yield from an onsite renewable source renders any contribution to the hot water demand minimal. It is also important to consider that in these cases, the peak use of hot water is in the morning before work; whereas the peak output of PV occurs in the afternoon. Hopefully with the progression of technology in storage systems, batteries or the like will become an efficient and economically viable addition to developments. It may also be worth considering future replacements for gas systems such as biofuels.

    Dead legs and heat losses have both been accounted for with the centralised systems in this case. As have the capital costs associated with materials, although a full LCA of materials has not been undertaken to determine the environmental impact in this case.

  6. Hi Sian,

    I had some questions about your report. Why would the solar plant be coupled to an inefficient boiler then be compared to an efficient boiler? I’m also not sure I understand how a solar plant only achieves a 5% reduction in GHG against a straight gas plant. I would perhaps expect that with an electric boosted solar plant, but not gas, unless the solar plant was drastically undersized. Could you please enlighten me in case I’m missing something? We work in this area and our findings differ significantly to yours so would be keen to compare notes. Also, who commissioned this study?



  7. Hi Sian,

    I think it’s worth considering the longer term greenhouse gas implications of each option. The gas options will always involve the burning of fossil fuels, whereas (as noted in the recent BZE Buildings Plan) the electric options can either be offset by on-site renewables or can benefit from the greening of the grid.

    Another point is much lower heat loss (through piping and because the temp setting can safely be sub-60C) and clearing dead legs with the instant electric option. I understand there is a point where instant elec performs better on a greenhouse emissions basis – this occurs when hot water demand is at the lower end of the scale. Also reduced materials in piping and lagging etc.

    As you note, there is going be a different best option for every case. Also I really like that you include the social impact in your analysis.

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