CESIO in 2026: the optimised solar water heater you can install without surprises

Understanding CESIO and CESI: what you're really selling on site

Principle of an individual solar water heater (CESI): collectors, tank, heat exchanger

A CESI captures heat from the sun via solar collectors. A heat-transfer fluid circulates to the tank, where a heat exchanger transmits the energy to the domestic water. The tank keeps hot water available, with a backup (electric, gas or heat pump) for days without sun.

Difference between CESI and CESIO: the "optimised" promise and points to check

The CESIO aims for "optimised" performance through a better-sized, better-controlled system as a whole. On site, what you're really selling is good tuning. Check: collector surface, tank volume, backup type, control system (anti-overheating, anti-freeze), and the quality of components and certifications. To go further on the variants, see the difference between an optimised CESI vs a CESI with separate backup.

Which homes and usage profiles: renovation, new-build, families, second homes

In new-build, integration is simple. In renovation, look at the roof (orientation, shading) and the space for the tank. Families gain the most, since needs are steady. In a second home, watch out for long periods without draw-off. Plan for absence management and overheating protection.

Optimised solar sizing: the simple rules that avoid callbacks

Choosing the collector surface and tank volume: rules of thumb and common pitfalls

For a residential CESI, often aim for 1 to 1.5 m² of collectors per occupant and a tank of about 50 to 80 L per m². Undersized, the backup runs too much. Oversized, you create summer stagnation, with overheating and fluid ageing. Also keep a thermostatic mixing valve and a consistent tank setpoint.

Orientation, tilt, shading: how to secure solar output

Ideally due south, but a drift towards east or west remains acceptable. A tilt around 30 to 60° secures output across the year. The real enemy is shade. Check for no obstacles between 9am and 3pm in winter. A photo survey or a sunshine tool avoids surprises and missing kWh.

Recirculation loop, distances and losses: optimising the DHW network from the design stage

Place the tank as close as possible to the draw-off points. Limit pipe lengths and insulate the tubing thoroughly, especially in the loft. If a DHW recirculation loop is essential, control it by clock or on demand. Otherwise it continuously drains the solar gain. Also take care with diameters and flow rates to avoid noise, losses and callbacks.

Installing an optimised CESI: on-site steps, safety and clean finishing

Preparing the layout: roof, support, pipe routing, waterproofing

Before lifting the collector, secure roof access. Lifeline, guardrail or harness depending on the context. Identify shaded zones and choose a sound area of the rafters. Hooks or rails are fixed to the structure, not to the tiles. For routing the CESI's pipework, favour a short, insulatable path. Every penetration is treated as a real waterproofing repair, with a suitable flashing and compatible sealant.

Hydraulics and control: expansion vessel, glycol, air venting, basic settings

Check the expansion vessel (pre-charge) and the safety valve. Fill with the suitable glycol mix, then pressurise per the instructions. Vent air at the high points until the flow rate stabilises. Check the pipe insulation and the absence of loops that trap air. On the control side, place the sensors correctly, then set a simple start-up differential and a maximum tank temperature. To go further on this topic, also see antifreeze and the heat-transfer fluid.

Commissioning and tests: checks to run before leaving the site

Run the pump manually to confirm circulation. Check collector and tank temperatures, leaks, hot pressure, and the safety valve's discharge in a test. Verify the fixings, the roof finishing, and take photos. Finish with a client explanation and a reminder of annual fluid and settings maintenance.

2026 grants and requirements around solar: what the client expects and what you must prove

MaPrimeRénov' and solar water heaters: key conditions and documents to provide

For a CESI, the client expects a clear grant and a clean file. You must prove installation by an RGE-qualified company, provide a detailed invoice and the technical data sheet (brand, reference, performance). Add the supporting documents requested by the platform, such as the tax notice and proof that the home is a main residence.

CEE and solar thermal: points of attention on the standard sheets and compliance

In CEE, everything rests on the applicable CEE standard sheet and its criteria. Check that the surfaces, volumes and product references recorded on the honour statement match the invoice. Keep the performance evidence (certificate, marking) and the works completion date.

RGE: scope for solar thermal, traceability and mentions on the invoice

Solar thermal falls under the Qualisol scope, or the equivalent depending on your certifying body. On the invoice, state the RGE mention, your qualification number, the job site address, then the brand, reference and serial numbers of the collectors and tank. This is your best protection in case of an audit.

Tradesperson's pitch: selling an optimised solar CESIO without overselling

Clear pricing: cost items, useful options, and what makes the price vary

A readable quote. Itemise materials (collectors, tank, circulator, control), installation, hydraulic connections, safety, and roof adaptations. Add the planned backup (electric or boiler) and commissioning.

• Useful options: anti-scald thermostatic mixing valve, network insulation, production monitoring.
• What makes the price vary: collector surface, tank volume, roof access, roof covering type, distance between tank and collectors, condition of the existing installation.

Real gains: DHW savings, comfort, and winter limits (without impossible promises)

A well-sized CESI often covers a large share of hot water needs over the year. In summer, comfort is excellent. In winter, the backup takes over, especially if the family consumes heavily in the morning. Talk in kWh and euros, based on the client's bill. No promises. To frame the hot water need and avoid vague estimates, rely on DHW draw-off profiles.

Maintenance contract and monitoring: preventing scaling, checking the glycol, reassuring the client

Simple monitoring. Offer a regular visit to check pressure, circulator, sensors, and valves. Watch for scaling if the water is hard, and check the glycol and its antifreeze protection.

Set the frequency. Annual check or every 2 years depending on use. Glycol analysis or replacement every 3 to 5 years. A clear report reassures the client and avoids unpleasant surprises.