A wall-hung gas combi with micro-storage is a combination heater within the meaning of Regulation (EU) No 813/2013: it declares a load profile and a water heating energy efficiency tied to that profile. It is that pair, not a flow rate in litres per minute, that lets you compare two appliances. Regulation (EU) No 814/2013 sets out in Annex III the reference energy Qref for each profile, namely 2.100 kWh for the XXS, XS and S profiles, 5.845 kWh for profile M and 11.655 kWh for profile L. On temperature, the French order of 30 November 2005 imposes a floor of 50 °C at every point of the distribution, which rules out lowering the setpoint to limit scaling.
What micro-storage changes, and how it is measured
The small internal store: what it brings and what it does not replace
Micro-storage adds a few litres held at temperature upstream of the hot water exchanger. That volume covers the start of the draw-off while the burner comes up to output, which removes the temperature dip in the first seconds and smooths short, repeated draw-offs. It does not change the instantaneous output available, so it changes nothing about the limit under simultaneous draw-off. Confusing the two is the leading cause of undersizing on this type of appliance.
Instantaneous, micro-storage, integrated cylinder: the technical boundary
With instantaneous production, all the energy comes from the burner at the moment of the draw-off, and the temperature follows variations in flow. With micro-storage, the reserve covers the start and the appliance then behaves as an instantaneous unit. With an integrated cylinder, storage covers a significant share of the demand, which shifts the constraint from output to volume and space. The boundary is not judged to the litre, it is judged on the load profile the appliance can actually hold.
The ErP load profile, the only comparable criterion between appliances
Regulation (EU) No 814/2013 defines in Annex III a standardised sequence of draw-offs per profile, and the reference energy Qref that follows from it. Every appliance producing hot water corresponds to at least one load profile, and it is that declared profile that appears on the product sheet.
| Load profile | Reference energy Qref |
|---|---|
| 3XS | 0.345 kWh |
| XXS | 2.100 kWh |
| XS | 2.100 kWh |
| S | 2.100 kWh |
| M | 5.845 kWh |
| L | 11.655 kWh |
| XL | 19.07 kWh |
| XXL | 24.53 kWh |
The XXS, XS and S profiles share the same reference energy: what separates them is the sequence of draw-offs, not the total. Comparing two appliances declared on the same profile is meaningful; comparing an S profile against an M profile on a quoted flow rate alone is not.
Sizing hot water on figures, not on an impression
Reading the reference energy Qref and setting it against real demand
Start from the number of outlets, how they are used and how likely they are to run together, then place the demand on the scale of profiles. A dwelling with one bathroom and short, repeated uses typically sits at the lower to middle profiles; as soon as a bath, a rain shower or a second simultaneous outlet enters the picture, the demand moves up a step. Write the profile you selected on the quote: it is a sizing assumption and it has to be defensible.
Simultaneous draw-off: where instantaneous output becomes the limit
When two outlets run at once the available output is shared and the temperature falls, all the more so because the incoming cold water is colder in winter. Micro-storage does not make up that shortfall, it only delays it by a few seconds. If simultaneous use is structural in the dwelling, the answer is a cylinder, not a small internal store.
Pipe runs and secondary circulation: what decides before the appliance does
The waiting time at the outlet depends first on the volume of water held in the pipe between appliance and tap, not on the internal reserve. Shortening runs, grouping wet rooms and insulating the pipework counts for more than the choice of model. A secondary circulation loop improves service but has to be controlled, otherwise it permanently holds a loop at temperature along with its losses.
The hot water temperatures you are not free to choose
The public health floor and the anti-scald ceiling
The French order of 30 November 2005 sets both ends. In distribution, the water temperature must be 50 °C or above at every point of the system, except for the final supply pipes to the outlets, whose volume must be as small as possible and in every case at or below 3 litres. At the outlets, the maximum temperature is set at 50 °C in rooms used for washing and limited to 60 °C in other rooms.
What that imposes on the appliance settings
The production setpoint sits above the distribution floor, with whatever margin the loop losses require if a loop exists. An economy mode that reduces temperature maintenance has to stay compatible with that floor. Log the value you programmed at commissioning and have it countersigned: a setpoint changed afterwards by the occupant falls outside your liability, provided the original value is written down.
Water hardness and scaling: treat upstream, not through the setpoint
Since the setpoint cannot come down, scaling is dealt with upstream. Measure the water hardness on site, size the treatment on that recorded value and log it. Add a filter on the incoming main and plan access to the components for servicing. On a hot water plate exchanger, the narrow channels concentrate the deposit, and accessibility drives the cost of maintenance far more than the purchase price does.
Installing and setting up: the points that come back as defects
Flue arrangement, ventilation and location
Choose the flue arrangement from what exists and from the applicable flue rules, and check the flue's compatibility with condensation, the falls and the terminals. Where an existing flue is reused, lining is the question to settle before ordering the appliance. Check the ventilation of the room, the safety clearances and the access for servicing.
Hot water connections and distribution losses
Take care over hot water pipe lengths and keep diameters consistent with the flow you are targeting: oversizing the pipe increases the volume to be run off before hot water arrives, and therefore the waiting time. Insulate the first metres and limit bends. If a secondary circulation loop is needed, control it on a timer or on demand, and check that the volume of the final supply pipes stays within the regulatory limit.
Commissioning: flow, ΔT, combustion, gas tightness
Record the actual hot water flow and the ΔT between inlet and outlet, check combustion with the analyser and carry out the pressure and tightness check on the circuit. Validate the safety functions: flue discharge, ventilation, circuit pressure, relief valve. Put all those values on the commissioning report with their measuring conditions, or they will be useless at the next service.
Positioning the offer in 2026
What a gas appliance no longer qualifies for
The gas appliance item no longer qualifies under the two French national schemes: the corresponding line was removed from the MaPrimeRénov' scale for applications filed from 1 January 2023, and the BAR-TH-106 energy savings certificate measure was withdrawn from 1 January 2024. Associated measures (controls, pipe insulation, insulation) follow their own specifications, to be checked before quoting. The detail of the texts is set out in our article on the condensing gas boiler.
What you document to stand up in a dispute
The load profile you selected and why, the hot water setpoint you programmed, the water hardness recorded, the volume of the final supply pipes, the commissioning report with the combustion values, and the gas compliance certificate where the installation was altered. It is that set, and not the satisfaction expressed at handover, that protects you if comfort is challenged two winters later.
The alternatives to price alongside
Where simultaneous draw-off is structural, or where the room allows it, a separate cylinder or a heat pump water heater changes the class of service entirely. In a house, an air-to-water heat pump with a cylinder combines heating and hot water and takes the hot water item out of the instantaneous output constraint, with sizing carried from the site survey through to the EN 12831-1 calculation. Price those work plans alongside with their assumptions stated, rather than as a footnote variant.
