On a solar domestic hot water system the backup fuel does not only change the hydraulic layout, it changes the performance bar the installation has to clear. Water heating efficiency, as defined by Regulation (EU) 814/2013, is declared for a given tapping profile, and support schemes set their requirement against that single figure. The French CEE scheme illustrates the effect plainly: at least 36 % for profile M with a Joule effect electric backup, but 95 % for the same profile as soon as the backup is anything else, which includes gas. The ratio is close to one to three, and it is decided by the declared figure for the package, not by the boiler on its own.
What a support scheme actually asks of a solar plus gas system
A narrow scope: forced circulation, thermosiphon excluded
The eligible definition is tighter than most people assume. Forced circulation is mandatory, which rules out thermosiphon systems and integral collector storage units. Collectors have to be glazed, non-hybrid, running water or a water and glycol mixture, and carry third-party certification such as Solar Keymark or an equivalent from an accredited European body. Total gross collector area has to be at least 2 m². A kit that misses any of those boxes opens no entitlement, whatever its intrinsic quality.
The declared tapping profile, the entry point of any file
The paperwork does not ask for a number of occupants, it asks for a declared tapping profile among M, L, XL or XXL. That is a product datum, set out in Regulation (EU) 814/2013, describing a standardised day of draw-offs. It is read on the appliance documentation and it commits the file. An installer who declares a profile without being able to evidence it on the product sheet exposes the client to rejection at audit. Matching real demand to a profile is a sizing question, handled with the reference energy of each profile.
The efficiency threshold that changes with the backup fuel
| Backup energy | Profile M | Profile L | Profile XL | Profile XXL |
|---|---|---|---|---|
| Joule effect electricity | 36 % | 37 % | 38 % | 60 % |
| Anything else, including gas | 95 % | 100 % | 110 % | 120 % |
Minimum water heating efficiency, as defined by Regulation (EU) 814/2013, required by the French CEE scheme fiche BAR-TH-101 v. A78.3 for the declared tapping profile.
Why a gas backup moves the performance bar
Where the gap between 36 % and 95 % comes from
The gap is not arbitrary, it comes from the conversion coefficient in Regulation (EU) 814/2013. Electricity is counted in primary energy through a coefficient CC of 2.5, corresponding to an average energy efficiency estimated at 40 % across the Union. Multiply the 36 to 38 % required with an electric backup by that coefficient and you land on the order of the 95 to 110 % required with a gas backup. The 95 to 120 % band is therefore not a harder requirement in absolute terms: it is the same requirement read on the final energy scale rather than the primary one.
What that imposes on the boiler and cylinder pairing
In practice, reaching 100 % on profile L with a gas backup calls for a properly sized condensing boiler, a solar cylinder with low standing losses, and pipework that lets the solar do its work. An oversized or poorly insulated cylinder drags the declared package figure down whatever the quality of the boiler. That is why the choice of cylinder and backup is treated as a line item in its own right, not as an accessory.
The combi boiler case
A boiler with a small internal store complicates the series layout. Its minimum firing flow rate and hot water priority logic can make it fire while the solar cylinder is already delivering water at a good temperature, which destroys the point of preheating and drags the measured figure down. Check the minimum flow, the delay and the ability to raise the firing threshold. If the appliance does not allow it, the twin coil cylinder layout is safer.
Choosing the layout: series or twin coil
Solar preheat then boiler in series
The solar cylinder preheats, the boiler tops up instantaneously. The layout is simple, keeps the footprint of a single cylinder and avoids reheating the solar volume. It does require a boiler able to modulate low and to accept an already hot inlet, and a correctly sized thermostatic blending valve on the outlet. It is the layout that performs best where the appliance can follow.
Solar cylinder with an integrated boiler coil
The solar coil sits in the lower part, the boiler coil in the upper part. The advantage is robustness: one appliance, one volume, readable controls. The drawback is the risk of the backup reheating too low down the cylinder and depriving the collectors of a cold base. Backup setpoint and coil height are the two parameters that make the difference, and they are set at commissioning, not at design stage.
Solar priority and the switchover threshold
In both cases the controls have to let solar take priority and only allow the backup on useful time bands. Enough hysteresis avoids rapid cycling. Programming the backup late in the day rather than early in the morning gives the day's sunshine a chance, which improves real coverage without touching the hardware.
Hygiene safety and commissioning settings
Thermostatic blending valve on the outlet
A solar cylinder can rise well above the working setpoint when the sun pushes. The thermostatic blending valve on the outlet is what makes that rise acceptable, by stabilising temperature at the draw-off points. Its absence is a classic non-conformity, and it shows up immediately on inspection.
A legionella cycle that does not cancel the solar gain
The periodic temperature raise is programmed on the backup, preferably late in the day, so that it does not short-circuit the day's solar production. Check that the controls allow that cycle without cancelling solar priority, and that the distribution has no poorly circulated legs. Hot water hygiene safety devices are set up with the client, not against them.
What you measure before leaving site
A peak draw-off validates flow rate, stability at the blending valve and recovery time. Record top and bottom cylinder temperatures, the direction of the non-return valves, the actual opening of the valves and the absence of parasitic thermosiphon. Note the cold pressure of the solar circuit and the insulation of the pipework, particularly on external runs.
Building the file and defending it
The values the paperwork asks for
Declarations typically call for the gross collector area, the declared tapping profile, the water heating efficiency for that profile, the number and capacity of solar cylinders, and the energy efficiency class of the cylinder where its capacity is 500 litres or less. They also call for confirmation of collector certification and of their non-hybrid nature. Each of those values is evidenced on the product sheet, never from memory.
Watch the exclusions between schemes
Support schemes routinely make measures mutually exclusive. The French fiche, for instance, is not combinable with the air to water and water to water heat pump measures. On a retrofit that touches both the heat generator and the hot water, that arbitration is made before the quotation goes out, not at submission. The installer also has to hold the qualification the scheme recognises.
Arguing for a gas backup without overselling
The case rests on two verifiable points. First, a gas backup has to clear efficiency thresholds that schemes deliberately set high, which means equipment chosen rather than inherited, so picked from a catalogue whose technical characteristics are verified. Second, solar covers most of the warm season and the backup only works on sunless days. What is not promised is a percentage saving: coverage depends on region, orientation and habits, and it is observed over the first season of monitoring.
