Waste Water Heat Recovery for Showers
Residential Decarbonisation with ASHPs: Is there a DHW cylinder-shaped elephant in the room?
With 2025 and the possibility of homes heated without the use of traditional gas boilers looming ever closer, a new era of low energy, low carbon homes is on the horizon. Heat pump technology, and in particular Air Source Heat Pumps (ASHP) combined with a greener National electricity grid and increased levels of insulation and airtightness are gaining favour as the default method for decarbonising Britain’s homes and reducing occupants energy usage.However, many residential heat pumps tend to struggle to maintain high efficiency and capacity when it comes to domestic hot water (DHW) production, often resorting to grid-based electricity to meet demand.
Direct electric heating (fan heaters, electric radiators, infrared heating for space heating, as well as immersion heating for water) and electric storage heaters operate with 100% efficiency (commonly termed COP or Coefficient of Performance, where 100% = COP of 1.0). Heat pumps, on the other hand, can in practice operate with upwards of 200% efficiency (COP = 2.0), and often even as high as 400% (COP = 4.0) . In other words, they can provide two to four times as much space heating energy to the home as the electricity that they consume from the grid. However, these efficiencies generally only reference the space heating performance, and not the DHW production performance, which can be significantly lower [1].
Low-temperature emitter systems (e.g. underfloor heating) can make space heating with heat pumps very efficient, but with hot water, there is little opportunity to reduce the working temperatures to below 50-55oC. Regulations relating to legionella risk have lead to increased temperature storage requirement for DHW systems. It can therefore be quite a challenge to achieve high energy-efficiencies during hot water production when using a heat pump, as higher flow temperatures lead to lower operating efficiencies. UKGBC recently calculated an average typical residential ASHP efficiency of COP of 3.22 for space heating and COP of 2.06 for DHW* [2].
It is also significant that at present where the cost of electricity vs gas on average [3] (and even on the most cost-effective Green energy Tariffs [4]), is around 4- 6 times the price per kWh unit. By directly replacing traditional heating systems with ASHP there is the potential to cost the end-user much more for the same basic outcome: a comfortably warm home with readily available hot water. While this excessive cost outcome is less likely to be a feature of well-insulated, new-build homes. It is a potentially huge problem for the 24 million UK homes that need to be retrofitted to EPC A, B or C standard by 2030 if the UK is to have reasonable chance to meet its’ 2050 climate goals [5].
Fabric-First energy efficiency (FEES) is the mantra of current UK building regulations: The idea being to use as little energy as is practically possible, prioritising low-energy design before low emission implementation [2]. However, more often than not FEES and other energy efficiency measures in new-builds focus solely on space heating reduction. While this approach is effective in reducing the energy required for space-heating, DHW demand is largely unaffected by this higher fabric specification new build homes. As space heating demand falls relative to hot water demand, DHW generation in modern homes can be the most energy-intensive process. Showering has been shown to consume around 80% of generated DHW [6] in European homes, meaning showering can be the most energy-intensive process in a modern home.
Waste Water Heat Recovery for Showers (WWHRS) is an established, cost-effective heat exchanger technology that can recover heat energy from waste shower water and recycle this heat energy back to the shower, significantly reducing the DHW required for each shower use by up to 55% [7]. Fortunately, proposed changes to Part-L of UK building regulations (ADL2020: in Consultation [8]) look set to include WWHRS as a prominent technology, facilitating the reduction of DHW demand from showering in new homes, reducing CO2 emissions, energy use and householder bills.
The use of WWHRS on new-build homes also has the potential to reduce developer costs and also to ‘future-proof’ homes, where gas boilers are still to be fitted, but may have to be upgraded to a low-carbon source in the future, such as a hydrogen boiler or ASHP.
Just like their gas-driven counter-parts, whether for new-build or retrofit applications, heat pumps have to be correctly sized to accommodate for space heating demand and DHW demand across a range of potential future usage scenarios. It would seem inconceivable that a competent heating engineer or specifier would consider an ASHP for use in a poorly insulated home. Yet, with DHW demand from showers inevitably being the largest user of hot water in the home, it would seem logical that measures such as WWHRS should be considered to help reduce DHW demand, in the same way that FEES is considered to reduce space heating demand. Heat pump systems, unless dramatically oversized or designed to incorporate a suitably sized DHW storage vessel (eg. a DHW cylinder or thermal store) will commonly require direct electric heating to maintain hot water capacity, particularly during the winter months when the heat pump must provide both space heating and DHW simultaneously.
This reliance on Grid-based electricity for DHW production can be a major, often unanticipated cost to the operation of a residential heat pump system: A cost that could become a barrier to the mass uptake of ASHPs, if not properly addressed. By considering WWHRS as part of the ASHP system design, DHW storage capacity can be significantly reduced, while peak DHW demand is also reduced meaning hot water should only ever be produced at the lowest possible cost by the heat pump, rather than at high-cost by the immersion heater element. Furthermore, and possibly most importantly, the addition of WWHRS as a holistic part of residential heat pump system can increase the effective system COP significantly.
As an example, a home with a single shower, with 3 uses per day (based on SAP defaults, and an ASHP with COP of 2.06 for DHW production), could be increased to an effective System COP of 3.83. That’s an increase of 86% for DHW generation though the inclusion of a WWHRS (see table below).
Where heat pumps are being considered as an option for existing housing stock, insulation and airtightness are equally important; and a ‘whole-house’ strategy approach including energy efficiency, ventilation and thermal management is essential to avoid unintentional negative results such as condensation, overheating or increased energy bills. Retrofittable WWHRS should then also be considered as part of a whole-house approach to help balance space heating and DHW heating demand load, and reduce primary energy demand. While retrofittable WWHRS are generally slightly less efficient than the WWHRS products favoured for new build, our calculations show that an effective System COP increase of between 30-50% could still be achievable if used as part of a retrofit ASHP system.
In their recent publication: The Future for Home Heating – life without fossil fuels, September 2020, NHBC acknowledged numerous possible issues associated with heat pumps and adequate DHW production. In conclusion, they identified that heat pumps will “become the predominant method for heating homes” yet in terms of understanding how heat pumps could provide adequate provision of DHW, could only suggest “specialist advice [be] sought from a suitably qualified and experienced heating designer”. With upcoming building regulation changes due in 2020/21; Future Homes Standard due to take effect in 2025; and the Government’s flagship retrofit programme, the Green Homes Grants now live, shouldn’t we be addressing the possibility of ASHP meeting residential hot water requirements as easily and efficiently as they can meet space heating requirements? Is efficiently delivered DHW really the ‘elephant in the room’ for the mass residential adoption of heat pumps in the UK?
[1] NHBC / Cutland Consulting Ltd, NF87: The Future for Home Heating – life without fossil fuels. September 2020: https://www.nhbcfoundation.org/wp-content/uploads/2020/09/NF87_The-Future-for-Home-Heating-%E2%80%93-life-without-fossil-fuels-1.pdf
[2] UK Green Building Council, Advancing Net Zero, September 2020: https://www.ukgbc.org/wp-content/uploads/2020/09/Building-the-Case-for-Net-Zero_UKGBC.pdf
[3] https://energysavingtrust.org.uk/about-us/our-calculations: Average price, Electricity (standard) Unit rate = 16.364p per kWh; Gas Unit rate 4.13p per kWh
[4] Bulb Energy, Vari-fair tariff, Oct 2020: Electricity Unit rate = 16.0114p per kWh; Gas Unit rate 2.7195p per kWh
[5] UK Labour Party / ARUP, Thirty Recommendations by 2030. October 2019: https://labour.org.uk/wp-content/uploads/2019/10/ThirtyBy2030report.pdf
[6] European Commission, Joint Research Center, MEErP Preparatory Study on Taps and Showers, 2014
[7] Based on 1no Recoup Pipe+ HE attached to a thermostatic mixer shower as System A: SAP default shower data assumptions.
