A combined solar system, or CSS, covers two uses with the same installation, domestic hot water and heating, where a solar water heater is limited to hot water. Orders of magnitude on a house are 10 to 20 m² of collectors for coverage of 50 to 70% of hot water demand and 25 to 50% of heating demand, the rest coming from a boiler, heat pump or immersion backup. A CSS only holds those levels on a home already insulated and fitted with low temperature emitters, underfloor heating or radiators sized for warm water. The pairing of collector area and cylinder volume is settled before ordering: too many collectors for the storage installed produces summer overheating and stagnation, which degrade the fluid and trigger callbacks.
Understanding the CSS: solar power serving heating and DHW
How it works: solar collectors, tank and backup heating
A CSS captures solar energy via thermal panels. A heat transfer fluid warms up, then passes its heat to a storage tank fitted with heat exchangers. The tank feeds DHW and, depending on the controller, a heating circuit. When the sun isn't enough, a backup (boiler, heat pump, electric element) takes over to guarantee the temperature.
Differences between CSS, solar water heater and "simple" solar thermal
A CSS covers two uses: hot water and heating. It generally requires more collector area and a larger tank. A solar water heater is limited to DHW. "Simple" solar thermal often targets a single, more occasional need (pre-heating, pool), with lighter hydraulics.
Which homes solar heating suits (area, insulation, emitters)
It's worthwhile if you have a well-exposed roof, room for the tank, and a home that is already insulated. The gain is better with low-temperature emitters, such as underfloor heating or radiators sized for lukewarm water. In renovation, an audit helps size accurately, without overpromising.
Sizing a combined solar system correctly: avoiding disappointment and callbacks
Assessing needs: heating, DHW, habits, climate zones
A combined solar system is first sized on real data: heating needs, DHW volume, number of occupants, hours of presence, comfort temperature. Add the climate zone and altitude. A well-insulated home completely changes the picture. Without this information, you oversell solar, the backup then runs constantly, and callbacks follow. On the heating side, Argile computes the home's heat losses to NF EN 12831-1 from the survey taken during the visit, which gives the solar sizing a solid base.
Choosing the right components: collector area, tank volume, controller, hydraulics
Aim for balance between collectors and storage. Too small, low gains. Too large, summer overheating and hydraulic constraints. Check tank insulation, stratification, expansion vessel, antifreeze protection and stagnation management. The controller must drive the circulators and prioritise the right exchanges, with well-placed sensors.
Managing the backup: boiler, heat pump, electric element... and DHW/heating priorities
The backup isn't a plan B, it's the co-pilot. Calibrate its power and setpoints to avoid conflicts. In practice, secure DHW first, then heating, with a clear sanitary priority logic. Clean integration limits short cycles, lukewarm returns and endless adjustments.
Installing a CSS on site: points of attention for lasting solar performance
Collector placement: orientation, tilt, shading, roof integration
Aim for well-oriented collectors, ideally due south. Keep a tilt consistent with the use, often around 30 to 60°. Watch for shading, trees, chimneys, dormer windows, especially in winter. On the roof, take care with waterproofing, ventilation under the collectors, and cable and pipe routing to limit losses.
Hydraulic schemes, safety and commissioning: bleeding, glycol, vessel, relief valves
A reliable CSS means a solar circuit that has been made safe. Plan suitable air vents, a correctly sized expansion vessel, relief valve, pressure gauge and check valves. At commissioning, bleed at sufficient flow rate, set the flow, then check the pressure. Check the glycol, antifreeze protection, pH, and the controller settings.
Maintenance and monitoring: checks to plan to preserve solar output
Solar power needs to be managed over time. Schedule an annual check of the pressure, relief valves, circulators and sensors. Monitor temperatures and energy produced to detect a drop in output. Check the glycol quality periodically and inspect the seals and the condition of the collectors (see also circuit pressure).
Grants and requirements in 2026: what to check for a compliant solar job
RGE and key documents: quote, technical datasheets, commissioning report, maintenance manual
For a solar job, check that the company holds a suitable RGE certification (photovoltaic or solar thermal). For the file, keep a dated and signed quote from before work starts, the equipment's technical datasheets, the commissioning report and the maintenance manual. Add the certificates required by the insurer and, if needed, electrical compliance.
Available grants depending on the case: MaPrimeRénov' and CEE (checks to do in 2026)
Careful, the ranking changed mid-year. Since 1 September 2026, the MaPrimeRénov' per-measure pathway only funds heat pumps, connection to a heat network, the energy audit and oil tank removal: solar thermal, both solar water heaters and combined solar systems, has been taken out. On a CSS sold on its own, MaPrimeRénov' therefore no longer enters the pricing, and the file only recovers it under the accompanied pathway, with criteria on the home, income and the number of energy classes gained. CEE can supplement this if the operation matches a standardised sheet and if the offer is accepted before work starts. Argile carries that funding into the funding plan and shows the remaining cost and the monthly instalments.
Compliance points: performance, compatibility, safety and best practice
Check the sizing, roof and structural compatibility, waterproofing, electrical protections and earthing. In solar thermal, monitor the hydraulic circuit and network insulation. In photovoltaic, secure the wiring and the connection. The goal is a durable and traceable installation.
Client pitch: selling a CSS without promising the impossible
Concrete benefits: comfort, lower consumption, complementarity with the backup
A combined solar system brings stable comfort. It preheats the hot water and supports heating when the sun is out. As a result, you consume less energy over the year, without changing habits. And above all, it works with the existing backup (boiler, heat pump, stove) which automatically takes over.
Simple figures and explanations: solar share, seasons, limits and realistic expectations
Explain the "solar share" as the portion of annual need covered by the collectors. It varies by region, insulation, usage and tank size. In winter, solar output drops and the backup does most of the work. In the quote, give ranges, base them on a study, and set realistic expectations about the return on investment.
Typical cases: high-performance renovation, family home, boiler replacement with solar DHW
It works well in high-performance renovation (reduced heat loss), in a family home with high DHW needs, or when replacing a boiler while keeping a backup, with solar DHW as the priority. You're selling a hybrid solution, not a promise of 100% free heating.
