After heating, domestic water is the second largest item in the energy balance of homes. A significant proportion of this energy escapes into the sewer system together with the shower. Heat recovery from so-called grey water makes it possible to partially reverse this loss and to reduce the costs of hot water preparation in real terms.
Where do we get heat from and how much of it?
In domestic wastewater, especially from showers and washbasins, the temperature is usually in the range 18-35 °C. Tap water is sometimes several dozen degrees cooler. The temperature difference between the waste water stream and cold water is precisely the resource to be recovered.
Technology in practice
Three mature solutions are used in residential buildings: vertical shower-to-shower (equal-flow) exchangers, easier-to-install horizontal/tray exchangers and small grey water heat pumps that heat the hot water cylinder. Each option has different efficiencies, installation requirements and scale of effect.
| Solution | Parameter/scope | Conditions for correct operation | Best applications |
|---|---|---|---|
| Vertical WWHR (equal-flow) | Declared efficiency according to CSA B55.1 tests: approximately 40-70% (depending on length and diameter of exchanger) | Simultaneous drainage from shower and cold water intake; short plumbing path; connection to shower mixer and heater input simultaneously | New buildings and renovations with riser access; “shower” farms |
| Horizontal/tray WWHR | Typically 25-40% | Stable drainage in a thin layer, access under the shower tray or at the bathtub | Retrofitting without forging risers, walk-in showers |
| Heat pump on grey water | Seasonal COP typically above 3.5; ability to cover a large proportion of hot water needs | Dirt filtration, convenient access to the exchanger, operation control | Multi-family buildings, student residences, hotels, PRS resources |
How does it work?
In passive WWHR (Waste Water Heat Recovery), wastewater flows around the walls of the exchanger, with cold water from the water supply flowing on the other side. When both flows occur simultaneously, the cold water is pre-heated and enters the shower mixer and the heater feed. The higher the declared efficiency of the appliance and the better the simultaneity, the greater the energy reduction.
The greatest effect is achieved on showers, where the simultaneity of flows is natural. In baths, with long sections without flow or in complex plumbing with circulation, the benefits diminish. In multi-family buildings, the diurnal profile and scale of flows favour central heat pump systems.
In practice, the most advantageous system is one in which the preheated water enters the shower mixer and the storage tank or heater inlet simultaneously. Certified solutions have a double media separation barrier and meet drinking water protection requirements, eliminating the risk of backflow.
How much does this give?
Let’s assume an 8-minute shower with a flow rate of 8 l/min (64 l), a mixture temperature of 38 °C, cold water of 10 °C and a storage tank of 55 °C. The energy per shower without recovery is approximately 2.08 kWh. A vertical exchanger with an efficiency of 50% reduces the energy required by about half. For three people every day, this is an order of magnitude saving of 1.1 MWh per year for the showers alone. With electric power, this translates into a noticeable reduction in costs and emissions.
| Mini-Calculation | Before the WWHR | After WWHR (η = 0.50) |
|---|---|---|
| Energy per 1 shower | approx. 2.08 kWh | approx. 1.04 kWh |
| Savings per shower | – | approx. 1.04 kWh |
| Three people, 365 days a year | – | approx. 1,140 kWh/year |
Heat pump on grey water in a block of flats
In multi-family buildings, a central exchanger at the grey water outlet combined with a small heat pump for the storage tank can cover a significant proportion of the hot water demand with a seasonal COP of around 3.5-4 and above. The prerequisite for a lasting effect is filtration and regular cleaning of the exchanger from deposits and biofilm.
Risks and the most common errors
Most often, the problem is a lack of simultaneity of flows, a riser that is too short or a sub-optimal connection to only the mixer or only the heater. In active systems, the drop in efficiency is usually due to neglected filtration and lack of service of the exchanger on the waste water side.
When does it make the most sense?
Showers are prevalent in houses and flats, so a vertical WWHR equal-flow is a simple and relatively inexpensive way to permanently improve the efficiency of the hot water system. In larger buildings, where the diurnal profile is more stable, a central heat pump on grey water gives a scalable effect without interference in the flats.
Grey water heat recovery in housing is not exotic. Passive exchangers in showers can reduce the energy required for hot water by tens of percent, and in multi-family buildings, heat pump systems are becoming a viable, permanent heat source for the storage tank.
The key remains proper hydraulics, certified equipment, simultaneity of flows and – with active systems – regular servicing.
Sources :
Eurostat, household energy (2023, hot water – share of consumption)
SAP 10.2 (UK), methodology for inclusion of WWHR in building energy audit
CSA B55.1/B55.2, test procedures for grey water heat recovery exchangers
IAPMO PS-92, requirements for intermediate heat exchangers
Title 24 (California), WWHR recognition and efficiency thresholds (≥42%)
Manufacturers’ technical data sheets (e.g. Power-Pipe – efficiencies according to CSA; Zypho – connection variants, separation according to EN 1717)
EN 1717, protection of drinking water against backflow contamination
Scientific reviews and case studies (NREL, EPRI/Incubatenergy) for wastewater heat pumps
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