Optimised CESI vs separate backup: which one should you choose in 2026?

Understanding the two CESI architectures: optimised and separate backup

Optimised CESI: principle, basic diagram and role of the solar tank

In an optimised CESI, the solar tank does everything. It stores water heated by the collectors via a heat exchanger, then handles the backup in the upper section (immersion element or boiler coil). The result: a single DHW production point and a stratification that needs preserving. In practice, you aim for a "cold" bottom of the tank to keep capturing solar energy for longer, and a hot enough top of the tank to cover usage.

CESI with separate backup: how it works with a solar tank + independent DHW production

Here, the solar tank mainly acts as a pre-heater. The water then passes through an independent DHW production unit (boiler, heat-pump water heater, instantaneous heater). The backup doesn't reheat the solar tank. This limits unnecessary reheating and keeps the logic simple. The key point is getting the switchover threshold right.

Solar thermal and DHW: what changes on the hydraulic and controls side

In both cases, you find a primary solar loop (often glycol-filled), a circulator, sensors and differential controls. The hydraulics hinge on non-return valves, fill valves, and a mixing valve on the DHW outlet to keep the temperature safe. With separate backup, the controls mainly manage solar priority and avoid recirculation loops that "break" stratification.

Job-site comparison: performance, DHW comfort and ease of maintenance

Solar output and DHW coverage by use case (family home, holiday cottage, small collective)

On a well-sized CESI, the solar share mainly varies with occupancy. In a family home, correct sizing (collectors, tank) delivers steady coverage across the year. In a holiday cottage, intermittent use demands responsive controls and a backup able to quickly catch up on reheating. In small collective housing, pooling usage smooths things out, but requires real follow-up on settings and the recirculation loop.

Hot water comfort: availability, recovery time and peak-demand management

Comfort depends first on storage volume and backup power. With a well-insulated tank, you keep stable water for longer, even when sunshine drops. For peaks (back-to-back showers, households), recovery depends on the setpoint, the backup schedule and DHW priority. A controlled backup avoids "burning" kWh just as the sun comes out.

Maintenance and repairs: sensitive components, access, upkeep cost

On the maintenance side, the sensitive points stay simple. Checking the heat-transfer fluid, venting, the circulation pump, sensors and safety devices. On the roof, access determines how long the intervention takes. A light service contract, with a periodic visit, keeps maintenance simple and limits avoidable breakdowns (overpressure, priming loss, control faults).

How to choose based on the building and existing equipment

Existing home: space constraints, network rework and an existing tank

In renovation, the starting point is often available space. Plant room, ceiling height, access for handling and duct runs matter as much as performance. If a hot water tank is already there, check its volume, condition and above all whether a heat exchanger can be added. Otherwise, go for a dedicated solar tank. The goal: a clean job, without unnecessary rework.

Pairing with a heat pump, a boiler or an electric water heater

A CESI works very well as a top-up system. With a heat pump or a boiler, you keep an automatic backup for when sunshine drops. With an electric water heater, the backup runs via the immersion element, ideal if the panel is close by and correctly sized. Think about controls and solar priority to avoid heating "for nothing". A simple top-up, and comfort stays consistent.

Orientation, solar thermal collector area and storage volume: the right ballpark figures

Aim for a clear roof, ideally south-facing, with a tilt of around 30 to 60°. In a house, you're often looking at 2 to 5 m² of collectors for 2 to 5 people. On the storage side, count roughly 50 to 80 litres per m² of collectors. To fine-tune, also see how to size DHW storage volume. This gives a balanced system, neither overheating in summer nor calling on backup too often in winter. Just right.

Incentives and 2026 requirements: points to watch to offer a profitable CESI

MaPrimeRénov' 2026 and CEE: checking CESI eligibility and required documents

Before pricing a CESI, confirm 2026 eligibility for MaPrimeRénov' and CEE. Request a thoroughly detailed quote and invoice, the collector and tank technical datasheet, and the RGE certificate. For CEE, check the "individual solar water heater" fact sheet and its performance requirements.

RGE and compliance: standard practice, instructions, commissioning and proof of performance

The file holds up if the installation is clean. Follow standard practice, the manufacturer's instructions, the hydraulics and pipe insulation. Carry out a documented commissioning with settings, safety tests, and a signed report. Keep photos, the diagram, product references and certification (such as Solar Keymark) as evidence.

Client pitch: costing out DHW savings and setting expectations

To sell accurately, start from DHW usage. Estimate the solar coverage rate based on region and tank size, then translate it into kWh and euros. Remind the client that backup is still needed and that winter will be less productive. With incentives, work out a realistic payback and set a clear framework. To secure the administrative side, also see how to avoid MaPrimeRénov' application rejections.

Tradesperson checklist: decide fast on the quote and secure the installation

Questions to ask the client before deciding (DHW profile, habits, priorities)

Before pricing, clarify the DHW profile. How many occupants, showers or baths, morning or evening peaks, and tolerance for variation. Ask about the client's priority: comfort, savings, sobriety, or simplicity. Also check the constraints: available surface area and orientation, shading, roof access, tank location, distance to draw-off points. For a CESI, note the existing backup energy source and expectations around control.

Installation checkpoints: safety, frost protection, mixer, recirculation loop and insulation

• Safety and durability. Fixings, roof sealing, electrical protection, valve labelling.
• Solar circuit. Careful venting, stable pressure, correctly sized expansion vessel, frost-resistant heat-transfer fluid compliant with the instructions.
• DHW. Safety valve, non-return valves, and a thermostatic mixer to limit scalding risk.
• If there's a recirculation loop. Controls, non-return valve, and continuous pipe insulation to avoid losses.
• Insulation. Tank, hydraulic connections and sensitive points, especially in unheated spaces.

Commissioning settings: setpoints, backup time windows and monitoring the first season

At commissioning, set an appropriate storage setpoint and a periodic sanitary cycle per the manufacturer's instructions. Set the backup time windows. Off-peak hours where possible, without overriding solar input. Explain the simple indicators to the client. Temperature, backup hours, alerts. Schedule a check-in after a few weeks, then at the end of the first season. Pressure, fluid condition, leaks, insulation, and feedback from use.