Waste water heat recovery for showers

Reducing energy consumption in homes is high on the agenda for both the government and the general public battling with expensive utility costs. Heating water for showers can also be a huge waste of resources, with up to 90% of the energy used to create hot shower water simply going down the drain.

As space heating energy generation in a home becomes more efficient, the proportion of the home’s energy that is wasted through showering increases. In the average new-build home, around 15% of energy use is wasted through showering.¹ This is why companies like Recoup are championing the use of waste water heat recovery for showers (WWHRS).

A typical WWHRS installed into a home can reduce the energy required per shower use by up to 55%.² This technology uses a heat exchanger to transfer the residual heat energy from outflowing shower drain water, using it to preheat the incoming cold-water feed that supplies the shower mixer and/or water heater. Using this preheated water reduces the shower’s demand for hot water which saves energy. The water heater can be a combi-boiler, DHW cylinder, heat pump, heat interface unit, hydrogen boiler or practically any DHW generation source that will accept a preheated cold feed.

Not only do these systems reduce homes' hot water requirements, but they lower their CO2 emissions and energy bills. WWHRS is also a cost-effective compliance measure for housebuilders and developers looking to meet or exceed the new Part-L building regulations for energy efficiency. All of Recoup's WWHRS systems, for example, are SAP-listed energy efficiency measures, recognised for use in the UK to help achieve Part-L building regulation compliance.

Recoup is also calling for WWHRS to be represented in the National Calculation Methodology (NCM), so that it can be incorporated simply and efficiently into DSM modelling for non-residential projects. The company has finalised a number of consultation submissions regarding WWHRS and the use of showers in future homes and properties.

WWHRS explained

There are two primary types of WWHRS – Vertical systems and Horizontal systems:

Vertical systems, like the Recoup Pipe HEX, are multi-layered pipes up to 2.1m tall that are installed on the floor below the shower. These are popular in new homes because they offer the highest efficiency and are installed at first fit. The copper double-walled heat exchanger forms the inner pipe which the waste shower water runs through under gravity, transferring its heat energy. The mains cold water runs around the outside of the heat exchanger which separates the two water types. The mains cold water picks up the heat energy from the copper heat exchanger to create the preheated water. Ground floor showers cannot use these vertical systems unless combined with a low-power staged pump, as with the Recoup Pipe HEX Active.

Horizontal systems are typically less efficient than the longest vertical systems, but they do offer more flexibility for differing showering environments. The Recoup Easyfit+ is a horizontal system designed to install easily under a bath or larger shower tray, ideal as a retrofit solution. The Recoup Drain+ range are also a series of horizontal drain systems with varying efficiencies for wet room drain applications and particularly commercial environments like gyms and leisure centres with high levels of shower use. These horizontal systems work in the same way as the vertical systems except the mains cold water is contained within the heat exchanger’s copper pipe and the heat carrying waste shower water is directed to run over and around it to transfer the heat energy.

Installation

WWHRS systems can be installed in one of three different installation methods:

• System A (Figure 1, above) – Supplying preheated water to the shower and the water heater
• System B (Figure 2, above) – Supplying preheated water to the shower only
• System C (Figure 3, above) – Supplying preheated water to the water heater only

The performance for systems A, B and C are all fully tested, validated, and recognised through SAP. However, System A always produces the highest efficiency as both the cold and hot shower water are benefiting from the preheated water supplied from the WWHRS to the shower TMV. Therefore, it is recommended that System A is used wherever possible.

System B is the least efficient installation method, with System C efficiency between the two. Which method should be used within a property can be affected by design factors including distance from the shower and water heater, the number of showers into one WWHRS, or if more than one WWHRS system is installed into a property. Flow rates also make a difference and should be considered when installing WWHRS; generally lower flow produces greater heat efficiency.

Efficiency and energy saving calculations are specific to the product, installation method and property where the WWHRS is used. Recoup can calculate this for their customers’ specific project designs but Figure 4 (above) is a general performance and efficiency overview for the Recoup Pipe HEX.

If more than one WWHRS unit is installed in a property, only one of them can be installed as System A or System C, supplying preheated water to the water heater. The others need to be installed as System B.

The amount of pipework connected between the waste water heat recovery unit and the TMV of the shower is limited to 4.75m, due to the legionella risk assessment. The risk is deemed as low below this measurement, so a System A installation needs the water heater and shower TMV to be within 4.75m of the WWHRS unit, unless further legionella risk assessment for the specific design situation is undertaken. If the distance is greater than 4.75m then System C should be used.³

The 4.75m Legionella risk assessment restriction also applies to System B, but typically the WWHRS is already located close to the shower to ensure heat energy is not wasted from the shower drain pipework length. This is why System B is the primary installation method for retrofitting, especially when the water heater is not located close by and would require additional invasive work to configure the property’s pipework to reach it. System C is not affected by this distance as all water passes through the water heater before going to the shower. System C is the least common installation method, but is mainly used in properties where the 4.75m is exceeded. Townhouses are a common example, especially when more than one shower from differing floors is running into a single WWHRS unit.

The installation process for WWHRS can be completed by a competent plumber, without the need for product-specific qualifications, skills, or tools. There is also no planned maintenance required, no change of habit for users, and no impact on the showering experience the user receives. WWHRS systems provide a fit-and-forget technology that will save energy on every shower.

References

1. Average new house (2021 regs).
2. The Recoup Pipe HEX at a 9.2l/min flow rate has an efficiency of 65.2% using system A. 65.2% of 85% heat energy lost from drain water is 55.42%.
3. The amount of pipework connected between the waste water heat recovery unit and the TMV of the shower is limited to 4.75m due to legionella risk assessment. Based on Recoup’s 2013 risk assessment in conjunction with legionella control, created in line with HSG274 and ACOP L8, the risk is deemed as low below this measurement. So a System A installation needs the water heater and shower TMV to be within 4.75m of the WWHRS unit, unless further legionella risk assessment for the specific design situation is undertaken. If the distance is greater than 4.75m then System C should be used.
   

Find out more

Recoup runs monthly CPD Sessions, introducing WWHRS for different market sectors, including Residential New Build, Non-Residential, Retrofitting for Social Housing Decarbonisation, and an Installation and Technology Overview. Find out more at recoup.co.uk.