Replacing convectors with inertia radiators is fundable, contrary to what still circulates in many specifications. French standardised certificate sheet BAR-TH-158, in its A73.3 version published in the Official Journal of 22 August 2025, covers the installation of a fixed electric emitter, radiant panel or radiator, fitted with advanced electronic controls, for operations committed up to 1 November 2030. It allows 1,800 kWh cumac per emitter in a detached house in climate zone H1, 1,500 in H2 and 1,100 in H3, and imposes measurable characteristics: drift below 1 K, amplitude below 0.3 K, spatial variation in heating mode of 0.2 K or less. The point to hold in front of the client is this: at the emitter, any direct electric appliance returns as heat the whole of the energy it consumes whatever its mass, so that what is gained is gained on temperature stability and on sizing, never on efficiency.
What inertia covers, and what it does not
At the emitter there is no efficiency left to recover
A convector, a radiant panel and an inertia radiator all convert electricity into heat in the room with no loss at the generator. No consumption gap can therefore be quoted to a client on the basis of the emitter type alone: the promised percentage saving between a convector and an inertia appliance rests on no standardised figure and has no place on a quote. What can be demonstrated is control quality, because it is measured and required by the certificate sheet.
What inertia actually changes: the dynamics, not the efficiency
The heated mass shifts the emitter's time constant. It smooths the swings, cuts the cycling frequency and makes the perceived temperature steadier, at the price of a slower rise and less responsiveness on restart. That is an operating choice, not a performance choice. The differences between storage technologies are set out in our comparison of dry and fluid inertia, and the positioning against the other emitter families in our article on convectors, radiant panels and inertia radiators.
The only enforceable quantities
| Control criterion | Value required by sheet BAR-TH-158 | Where to read it |
|---|---|---|
| Control drift | below 1 K | technical data sheet or manufacturer's attestation |
| Control amplitude | below 0.3 K | technical data sheet or manufacturer's attestation |
| Spatial variation in heating mode | 0.2 K or less, TH-C-E ex method | technical data sheet or manufacturer's attestation |
| Presumption of conformity | NF Électricité performance three-star eye certification, with 100% active mode seasonal energy efficiency calculated under Regulation (EU) 2024/1103 | certificate from the certifying body |
Regulation (EU) 2024/1103, applicable since 1 July 2025, replaced Regulation (EU) 2015/1188: that is the text to cite in a specification, not the 2015 one.
Certificate sheet BAR-TH-158 in practice
Amounts, scope and conventional lifetime
The operation covers existing residential buildings and is counted per emitter installed. The evidence of completion states the quantity fitted and the characteristics of the equipment.
| Climate zone | Detached house | Flat |
|---|---|---|
| H1 | 1,800 kWh cumac per emitter | 1,500 kWh cumac per emitter |
| H2 | 1,500 kWh cumac per emitter | 1,200 kWh cumac per emitter |
| H3 | 1,100 kWh cumac per emitter | 900 kWh cumac per emitter |
The conventional lifetime is 16 years. The sheet cannot be combined with sheet BAR-TH-173, and applies to operations committed up to 1 November 2030.
The advanced functions required, to check in the catalogue before ordering
- Electronic room temperature control with a weekly timer.
- Room temperature control with open window detection.
- Remote control.
- Adaptive start control.
- Self-learning capability.
- Overconsumption indicator giving visual feedback at a minimum of three consumption levels based on the set point.
- Automatic presence detection, stepping the output down progressively into a reduced consumption mode.
An appliance sold as an inertia radiator without those functions falls outside the sheet. The marketing description never replaces the declared characteristic.
Evidence of completion and the qualification required
The contractor must hold a quality label matching point 7 of section I of article 1 of French decree no. 2014-812 of 16 July 2014. The evidence of completion states the installation of one or more emitters with advanced electronic controls, the quantity installed and the characteristics of the equipment. Failing that, it states brand, reference and quantity, supported by a document from the manufacturer or from a body accredited to EN ISO/IEC 17065 attesting to those characteristics.
Sizing without a watts-per-square-metre rule
Why a rule of thumb does not hold in retrofit
A watts-per-square-metre figure ignores ceiling height, the real heat loss area, orientation, the design outdoor temperature of the location and the state of the envelope. On the same floor plan, two corner rooms and a central room do not call for the same output, and an oversized emitter cycles constantly, which degrades precisely the stability inertia was meant to deliver. Real demand rests on heat losses calculated room by room against the weather file for the zone.
What the survey has to bring back, room by room
For each room: areas and build-ups of the walls facing outside or an unheated space, glazed areas and joinery type, ceiling height, any pronounced thermal bridges, and the target indoor temperature for the use. It is that data set, and not a survey coefficient, that produces a room output defensible if comfort is later disputed.
Distribution and set point
The output retained per room is then split between emitters according to the wall lengths available, keeping the appliance under the most heat-losing window where the layout allows. A bathroom calls for a higher output relative to its volume, not because its heat losses are greater but because the target temperature there is higher for a short period.
Fitting and connecting: what commits the contractor
NF C 15-100: circuits, protection, cross sections
Heating circuits are dedicated circuits. Check the rating of the protective device, the conductor cross section and the presence of a 30 mA residual current device on the circuits concerned, in line with NF C 15-100. A saturated board or an undersized cross section shows up as recurring trips that the client will blame on the appliances, not on the installation.
Pilot wire and load manager
The pilot wire carries operating commands from a programmer or load manager and allows zoning without multiplying thermostats. Its presence and continuity are checked before settling on a centralised control architecture. The commands and their use are set out in our article on the six pilot wire commands.
Installed load and contracted supply capacity
Adding heating load without revisiting the supply contract invites trips at peak, particularly in homes with an electric water heater and an electric hob. The subject is dealt with at quotation stage, with the client, as set out in our article on contracted capacity and electric heating.
Settings to hand over and recurring mistakes
Set points to document in the handover notes
- Living spaces, around 19 °C when occupied.
- Bedrooms, set point lowered at night.
- Bathroom, high set point during use and setback the rest of the time.
Weekly programming and absence scenarios
Adaptive start control, required by the sheet, anticipates the restart so the set point is reached at the hour of occupancy: that is precisely the function that makes a high inertia emitter usable without overheating. On a long absence, a programmed setback beats a full shutdown, which then imposes a long restart on a cold mass.
Recurring commissioning mistakes
- Room sensor placed near a cold source or in direct sunlight, giving a false reading.
- Oversized emitter that cycles and cancels the benefit of the inertia.
- Programming not matched to how the home is actually occupied.
- Advanced functions left inactive at handover, so unused and unjustifiable on file.
