Sewer heat recovery28 May 2021

There is a hidden and rarely used energy source in domestic buildings that has the potential to bring balance to the perceived uncertainty of the future energy grid mix. Now, wastewater heat recovery systems (WWHRS) are being included in new-builds to increase efficiency and reduce hot water use. By Tom Austin-Morgan

Utilising sewage as an energy source is not a new idea; it is already used in industrial buildings, but there has not been a technology developed to harness it economically in domestic buildings at large scale in homes, student accommodation, gyms and hotels, until now.

“There’s an awful lot of waste heat from showers that’s just missed,” says Ellis Maginn, business development manager at Recoup Energy Solutions. “Our calculations show from average shower use in a three-bed home that around 10% of the energy requirement for that home is going down the drain. With 27 million homes in the country, that’s a big number.”

Gains have been made in the energy efficiency of homes in the last few years in a bid to achieve the UK government’s net zero target for carbon emissions by 2050, especially in space heating. However, energy losses through wastewater haven’t really changed at all; and people are shown to shower more now than ever. If that waste heat isn’t being recycled, it’s just that: waste.

The latest solutions from companies like Recoup Energy Solutions and Baxi Heating UK and Ireland are simple and include no moving parts. They are heat exchangers that pipe used waste shower/bath water down to the soil pipe on one side, while simultaneously routing and ‘pre-heating’ the cold-water mains feed through the other. This is then fed to the cold side of the shower mixer or the boiler or, ideally, both. The incoming cold feed is consequently much warmer than it otherwise would be, reducing the amount of energy needed to heat it up to a usable temperature. WWHRS therefore reduces hot water use without compromising the user’s showering experience.

George Linder, new build category manager at Baxi Heating, explains: “On average the wastewater going down the shower plug hole is around 35°C. This raises the temperature of the cold mains water to between 16-21°C (depending on the flow rate of the shower), which means less work for the boiler. That will make a big difference over time.”

The Baxi Assure SHRU (Shower Heat Recovery Unit) range is available as a vertical system for showers and baths on floors above the ground floor, and a horizontal system that fits under the floor on ground floors. The product is more suited for new-builds, as the systems must be installed in walls or floors, making them difficult to retrofit.

Linder says: “At the moment the market is new-build; it’s quite a niche area. Is there a benefit to a homeowner to retrofit the system in their home? Ultimately yes, because of what the product does, but it’s a question of technical and financial viability. Unless you’re doing some kind of major refurbishment, the cost of installation outweighs the benefit for retrofit.”


According to Baxi Heating, the SHRU system offers up to 63% heat recovery efficiency and up to 8% SAP rating benefit on top of lower energy consumption. The wastewater in the Baxi Assure SHRU system travels through a stainless-steel heat exchanger, while the cold mains water travels up a PVC pipe around the coil. In the horizontal system, the heat exchange pipe is flattened out.

“It can be fitted behind the wall or in a floor, because there are no moving parts,” explains Linder. “It’s a truly fit-and-forget product. It will go on and on just doing its thing.”

Both the Baxi and Recoup vertical WWHRS units can be designed in so they can service two showers or showers over baths if they are within a few metres of each other.

As well as a range of vertical WWHRS pipes for showers on first floor or above, Recoup Energy Solutions’ product range includes WWHRS integrated into shower trays, wet-room drain channels and stand-alone, under-bath systems such as the Easyfit+. Like the vertical systems, these connect either straight into the shower mixer; the hot water source (for example, boiler/cylinder); or both for maximum efficiency. The most efficient WWHRS products can reduce the hot water required per shower by around 55%, ultimately recovering and recycling up to 67% of the waste heat energy that would otherwise just go down the drain.

The Recoup WWHRS system features a double-walled copper heat exchanger pipe that is encased within a PVC outer pipe. The waste shower water enters the WWHRS heat exchanger via a ‘turborotator’ which ensures the flow clings to the inside of a 50mm copper pipe via the Coandă effect to create a thin-film heat exchange, which maximises heat transfer to the exchanger. On the other side of the heat exchanger pipe, mains pressure cold water enters via the bottom of the vertical heat exchanger to create counter-flow heat transmission along the two-metre length of the exchanger (diagrammed in image). The use of a double-walled heat exchanger ensures direct compliance with UK and EU water regulations (NEN:1717).

“Industrial systems often try and save the grey water or store the energy for later use,” says Maginn. “But, while these have merit, they are big, complex and take up a lot of space. Our WWHRS are localised to individual showers and offer instantaneous energy saving.”

Where the vertical pipe systems must install on the floor below the shower they connect to, horizontal WWHRS systems may be installed in the void underneath the bath or shower tray rather than under the floor, making them easier to install as well as retrofit.

“We designed the horizontal Easyfit+ system specifically for new-build apartments and bungalows,” Maginn says. “But retrofit could be the biggest market for WWHRS. As the UK market opens out to more energy efficiency on retrofit, this is an ideal and a passive solution that will last 40-60 years. If you’ve got a plumber working in your bathroom or changing or servicing the boiler, WWHRS can be installed in two hours, and certainly for social housing it’s ideal.”

This technology could have fit into the government’s green homes grant scheme. However, on 31 March 2021 the £2.5 billion scheme was scrapped, just six months after it was announced by Chancellor Rishi Sunak as part of the prime minister’s ‘build back greener’ promise for after the COVID-19 pandemic. It has had a torrid time; builders complained of excessive red tape in registering for the scheme; heat pump installations in particular have been stymied by the rules;, while households found it difficult to access, and claim to have been given conflicting advice. The government has said it is allocating £300 million from the scheme to local authorities to improve homes for people on low incomes.

Both Linder and Maginn say that heat pumps, electric heating and other low-carbon heating technology will eventually replace traditional boilers as a way to heat homes. That means that cylinders will take over as the main source for hot water. Therefore, reducing hot water use will be the next big focus, making WWHRS more relevant as part of any low-carbon heating system.

Tom Austin-Morgan

Related Companies
Baxi Heating UK
Recoup Energy Solutions Ltd

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