Understanding wastewater heat recovery: where the potential hides
Showers, hand basins, laundry room: the outlets that discharge the most heat
In a home, the biggest potential comes from showers. Water often leaves around 38 to 40°C and reaches the drain still lukewarm. Hand basins add small but frequent flows. The laundry room (washing at 30 to 60°C) can also matter, especially in multi-unit buildings. Toilets, on the other hand, mostly discharge cold volume.
Basic principle: the exchanger that transfers heat without mixing the water
The principle is simple. Hot greywater flows on one side, cold incoming water on the other. A heat exchanger lets heat pass through, not water. This recovery preheats the water heading to the tank or mixer, which reduces the power needed downstream.
What you can recover depending on temperatures and flow rates
The hotter the wastewater and the more regular the flow, the higher the efficiency. On a shower, the target is often preheating by a few degrees, sometimes more, depending on the length of flow and the exchange surface area. In practice, recovery is mainly worthwhile when hot water is used every day. The right approach is to think in terms of kWh avoided, not a miracle promise.
Choosing the right solution on site: equipment types and use cases
Shower recovery: vertical or horizontal exchanger, depending on the configuration
For a shower, the vertical exchanger is placed on the wastewater stack. It makes sense when you have available height in a duct or under the floor. The horizontal type suits a short or offset stack, for example on a single-storey layout. Either way, recovery preheats the cold water before the mixer or the tank.
Recovery on a greywater riser: multi-unit housing, hotels, changing rooms
On a shared greywater riser, you're looking for regular flow rates. This is often the right terrain in multi-unit buildings, hotels or changing rooms. The exchanger is fitted on the main riser, and preheated water is then redistributed to the outlets. Plan for a non-return valve, insulation and easy access for inspection.
Recovery in retrofit work: access constraints, slope, traps and maintenance
In a retrofit, the site dictates the solution. Check access to the networks, the slope, and the space needed for a suitable trap. An inspection hatch and a bypass make maintenance easier. A simple maintenance plan, rinsing and checking for deposits, secures recovery performance over time.
Sizing and points to watch: securing recovery performance
Flow rate, simultaneity and losses: the parameters that make the difference
Correct sizing starts from actual usage. Look at draw-off rates, peaks and simultaneity (showers, kitchen, laundry). A recovery unit that's too small quickly hits a ceiling; one that's too big costs more and can lose efficiency at low flow. Also take care with losses. Network length, insulation, pressure losses and setpoint temperature all affect the final gain.
Water quality and fouling: filters, access and check frequency
Performance often depends on the condition of the exchanger. Plan for a pre-filter and easy access for dismantling. In hard-water areas, scaling can significantly reduce efficiency and increase pressure losses. Set up light monitoring: visual inspection, temperature delta measurement, cleaning based on usage, at least at start-up and then on a regular basis.
Compatibility: hot water tank, heat pump, sanitary loop and controls
Check the hydraulic diagram. Recovery is often placed as preheating before the hot water tank or the heat pump inlet. With a sanitary loop, watch out for lukewarm returns that can "eat into" the gain. Simple controls, with consistent schedules and setpoints, avoid unnecessary cycles and maintain hygiene temperatures.
Pricing, gains and grants: making recovery profitable in 2026
Energy savings: estimating gains on hot water and preheating
To price a heat recovery installation, start from the hot water needs. Daily volume, inlet temperature, target temperature. You calculate the energy to be supplied, then apply the exchanger's stated efficiency to estimate the energy recovered. On site, a simple measurement of flow rates and temperatures secures your order of magnitude.
Supply/installation costs: items to plan for and labour time
The usual items: exchanger and hydraulic accessories, plumbing connections, possible controls, network insulation, wall repairs, drainage. Time mainly depends on access to the risers and coordination with the hot water system. Plan for installation in an occupied site with unexpected issues, and keep a time buffer.
Possible grants in 2026: CEE, evidence requirements and documents to prepare
In 2026, the most common grant route is through CEE if the operation is eligible. Anticipate the evidence needed. Signed quote before work starts, detailed invoice, equipment technical sheet (performance, reference), installation photos if requested, and a sworn statement. Without these documents, recovery can lose its real profitability. To go further on claiming and building applications, see how to claim energy savings certificates.
Site method: fitting in wastewater recovery without losing time
Survey and quick audit: where to connect, where to route, what to change
In 30 minutes, identify the collection points (shower, washing machine), the technical space, future access for maintenance, and the network layout. Check the separation between potable water and wastewater, ventilation, and freezing risks if you're routing through an unheated space. Make a simple plan with dimensions and photos.
Installation steps and checks: sealing, slope, network insulation
Install the bypass, the valves and the filter, then the recovery equipment (heat exchanger or tank depending on the project). Keep a consistent slope, avoid reverse falls and fix the pipes to limit noise. Check sealing by filling with water and insulate the networks in cold zones. Run short tests before closing everything up.
Handover and explaining to the client: usage, maintenance, savings tracking
At handover, explain what can feed into the recovery network and what must stay on the standard drainage. Hand over a maintenance sheet (filter cleaning, descaling, seal checks) and identify the valves. Offer a 3-month check-in to verify flow rates and gains: to frame this savings tracking, use a simple before/after reading. The goal: an independent client, a stress-free job.
Key figures
25 to 35°C
Greywater temperature
15 to 25%
Hot water savings
30 to 50%
Recovery rate
Frequently asked questions
There isn't a single systematic national grant for this measure: depending on the programme and building type, recovery can be included in an eligible package of works (for example CEE, local grants, sometimes MaPrimeRénov' for a deep renovation). Check eligibility case by case and have the equipment validated (CE marking, manufacturer performance data) before quoting. In practice, allow 4 to 8 weeks for processing depending on the scheme, and require approval before signing if the grant makes that a condition.
Louis Airy
COO of Argile
