Solar thermal DHW with gas backup: a tradesperson's guide (2026)

Sizing a solar thermal DHW system with gas backup, without overspending or underperforming

Assessing actual DHW needs: usage profiles, volumes, peaks and seasonality

Start by counting occupants, usage patterns (showers, baths, kitchen) and peak hours. A well-sized DHW system targets the actual need across the day, not the theoretical maximum. Note the seasonality, especially in houses, since demand can drop in summer just as solar output rises.

Choosing the solar tank and collectors: area, orientation, tilt, losses

Aim for a tank matched to the draw-off pattern, with proper insulation to limit losses. On the collector side, south-facing orientation remains ideal, but east or west works if the area is increased accordingly. Tilt is chosen based on annual usage. Pay attention to the length of the connecting pipework and its insulation — that's kWh saved.

Setting the gas backup share: capacity, restart time, comfort and continuity of service

The gas backup must cover sunless days and restart quickly after a peak. Size the capacity to the flow rate needed at peak times and check the solar priority. A simple setting helps maintain comfort without overheating, with continuity of service even in winter.

Understanding the hydraulic diagram and key components of a solar + gas DHW system

Solar loop: circulator, control, sensors and glycol (points to watch on site)

The solar loop is a closed circuit. The circulator pushes a water-glycol mix between the collectors and the tank's heat exchanger. The controller compares the collector and tank sensors to start or stop the pump. On site, keep an eye on air (bleed valves), the expansion vessel, cold-fill pressure and glycol concentration, especially after a water top-up.

Heat exchangers, mixing valve and anti-legionella: sanitary safety and DHW temperature stability

The tank includes one or two heat exchangers. A thermostatic mixing valve at the outlet stabilizes the hot water and limits scalding when the solar loop runs hot. Anti-legionella mode aims for a periodic temperature rise in storage and good circulation, with no "dead" sections.

Connecting to the gas backup: boiler, micro-storage, DHW preparation and priorities

Solar preheats, gas tops up. Depending on the equipment, the setup is either in series (solar tank then boiler) or via a tank with backup heating. With a micro-storage boiler, check the minimum flow rate and the DHW priority logic to avoid short cycles. The goal is simple: let solar do the work, then boost with gas only when necessary.

Getting installation and commissioning right in 2026: the details that prevent callbacks

Roof placement and pipe runs: waterproofing, network insulation and UV protection

On the roof, every penetration is a sensitive point. Aim for durable waterproofing, with a slope that directs water outward, and fixings that don't distort the roofing membrane. On the pipework side, insulate all the way to the fittings, then protect the insulation outdoors (conduit, shell, coating) against sun, rain and birds.

Bleeding, filling, settings: flow rate, ΔT, setpoints and summer/winter modes

When filling the system, purge air at the high points, then check pressure both cold and hot. Set a stable flow rate to hold the target ΔT, and lock in simple setpoints. Activate summer mode if needed to limit unnecessary cycling when DHW demand drops.

Final checks: efficiency, temperatures, check valves, valves, and DHW distribution balancing

Validate production and draw-off temperatures, the direction of check valves, the actual opening of valves, and the absence of thermosiphon effects. On distribution, carry out DHW balancing on the recirculation loop to prevent wait times, overheating and lukewarm returns.

Making the installation profitable: 2026 incentives, compliance and client arguments around DHW

2026 incentive eligibility and structuring: MaPrimeRénov' and CEE for solar DHW production

In 2026, a solar water heater can qualify for MaPrimeRénov' and a CEE bonus. The principle is simple: you file the MaPrimeRénov' application before work starts, and get the CEE offer validated before the quote is signed. Combining them is possible under the rules in force, with the amount mainly varying by income, zone and equipment.

RGE and documents to prepare: quote, technical data sheets, certificates and traceability

To secure the file, the company must hold RGE certification for solar thermal. Keep the detailed quote, the technical data sheets (collectors, tank, controls), the invoice, and the CEE sworn statement. Add simple traceability: product references, serial numbers, installation photos, and proof of commissioning. An up-to-date RGE certificate must be included.

Simple talking points: gas savings, DHW comfort, property value and a "solar" image

For the client, aim for words that make things clear. Less gas for DHW, so a more stable bill. More comfort, with hot water available with no consumption "spike." The property gains appeal, especially if the DPE rating improves. And the solar image reassures — visible energy that gives the feeling of living in a well-thought-out project. Clear arguments, no promises.

Troubleshooting and optimizing a solar thermal DHW system with gas backup

Common faults: stagnation, overheating, lack of hot water, backup running too often

In solar thermal systems, stagnation and overheating often come from a circulation fault: a dead pump, air in the circuit, a closed valve, a badly placed sensor. A lack of hot water more often points to a scaled-up exchanger, a stuck mixing valve or a poorly stratified tank. If the gas backup starts up too much, the setpoint is too high or the solar priority is poorly managed.

Useful on-site measurements: temperatures, flow rates, pressure, heat-transfer fluid quality

Take readings of collector, flow and return temperatures, and top and bottom of the tank. Check actual flow rate with a flow meter and cold-fill pressure. Check the expansion vessel and the relief valve. On the fluid side, measure glycol pH and antifreeze protection, and note any discoloration or burnt smell.

Quick optimizations: control settings, backup setpoints, insulation, anti-legionella scheduling

Adjust the start thresholds based on the temperature differential, and cap the maximum tank temperature to avoid overheating. Lower the backup setpoint and schedule it for useful time windows. Improve the insulation of the connecting pipework. Keep a regular anti-legionella cycle while keeping gas spending under control.