The number of splits does not come from a watts-per-square-metre ratio but from a room-by-room heat-loss calculation, corrected for how the rooms are used: a bedroom closed off at night and an open-plan living space do not call for the same unit. In the field, a three or four-room home is usually handled with two indoor units, one per day and night zone, and a two-storey house with at least one unit per floor. Two ceilings then frame the project, both set by the manufacturer rather than by any regulation: the maximum number of indoor units per outdoor group, and the acceptable connection ratio. The SCOP thresholds, on the other hand, are regulatory: 3.9 for the French BAR-TH-129 certificate sheet, 4.00 for the A+ class that unlocks the 5.5% VAT rate in multi-split.
What determines the number of splits in a home
Floor area, volumes and insulation level: the basis of heat pump sizing
The number of splits starts from a heat-loss assessment. Even with an efficient heat pump, a high ceiling or average insulation increases the power to cover. You look at floor area, heated volumes, walls, windows and ventilation. The better the envelope is treated, the more you can reduce the number of units or choose smaller power ratings.
Living areas, bedrooms, closed-off rooms: sizing the need without over-equipping
Next, you break the need down by usage. An open living room can often be handled by a single well-placed split, while bedrooms with closed doors need dedicated diffusion or a small grouping. The goal is to avoid unnecessary over-equipping, which is costly and performs poorly in mid-season. When a zone is only heated occasionally, a different solution may be enough.
Quick answer
How many indoor units for this home?
The rules of thumb from the section above, applied to your case. The number moves with the partitioning and the number of floors, not with floor area alone.
Indoor units advised
2 units
Why that number
The baseline for the layout: one unit for the living space, and one on the sleeping side from two bedrooms up.
Refine it room by room with the configurator further down the article.
A first estimate, to be confirmed by a room-by-room heat-loss calculation.
An Argile tool
Installation constraints: line lengths, routing, condensate drainage
Finally, installation constraints settle the matter. Manufacturers set limits on the length and height difference of refrigerant lines. Attic runs, penetrations, the outdoor unit's location and condensate drainage determine how many splits are possible. Simple rule: the shorter and more accessible the path, the more robust the installation.
Sizing rules for a multi-split air-to-air heat pump (and mistakes to avoid)
Power per room: gains, heat loss and occupancy patterns
For an air-to-air heat pump, start room by room: that breakdown is what lets you place the indoor units on the plan and work out a coverage rate for each room. The right power depends on heat loss (insulation, air leaks, ventilation), but also on free gains (sun, cooking, occupants). Avoid the "so many W per m²" rule without checking actual occupancy. Doors closed at night, remote work during the day, a bedroom that's rarely heated. These patterns change the need.
Configurator
Sizing the multi-split, room by room
Describe the home room by room: the model derives the power of each indoor unit, the power of the outdoor unit, and checks the connection ratio between the two.
Design temperature -7 °C
2.50 m
The rooms
32 m²
2,246 W of heat loss
2.5 kW unit
12 m²
749 W of heat loss
2 kW unit
11 m²
824 W of heat loss
Heated by transfer
Indoor units
2
Connected power
4.5 kW
Design heat load
3.4 kW
Outdoor unit
4 kW
Connection ratio
113 %
Target 100 to 150 %
Oversized indoor units
A bedroom often asks less than the smallest unit in the catalogue. That is not a fault in itself, but do not count those extra kW as useful power: all they do is make the unit cycle.
A room without a unit loses more than a room with one
Transfer through the door does not close that gap. Either you equip it, or you accept a supplementary heater for that room.
Rounded teaching model, non-contractual. The final choice of outdoor unit and indoor units goes through the manufacturer’s technical data and selection software.
An Argile tool
Outdoor unit: limiting oversizing and undersizing in a multi-split system
In a multi-split system, the outdoor unit must cover the need at the coldest point, without falling into short cycling when demand is low. Too big, and the unit starts and stops, consumes more and wears out. Too small, and the setpoint isn't held and defrost cycles are noticeable. Respect the minimum and maximum ranges, and the connection ratio recommended by the manufacturer. For more, see the most common sizing mistakes.
Counter-intuitive
Why an oversized outdoor unit heats worse
The outdoor unit is sized for the coldest night of the year — a few hours. For the rest of the season it has to turn right down. Below its modulation floor it has one option left: start and stop.
4.0 kW
8 kW
30 % of nominal
19 °C
Season short-cycling
69 %
4,012 h a year below the floor
Season in deficit
0 %
0 h a year above the outdoor unit
Load
Modulation floor
Hours in the season
The outdoor unit spends most of the season below its floor
This is where the wasted energy and the wear come from: repeated starts for a load the machine cannot produce that small. Go down one size.
The heating season is modelled as a temperature distribution, not read from a real weather file. The order of magnitude holds; the individual hour does not.
An Argile tool
Balancing the splits: simultaneity, control and real comfort
The sum of the splits' power ratings isn't a simple "adder," and each indoor unit is set first against the cooling capacity worked out room by room. Think about simultaneity. The living room and bedrooms rarely demand 100% at the same time, but a peak is possible. Pay attention to controls, sensors, and per-zone setpoints to maintain real comfort without overheating one room while leaving another under-served.
Check
The connection ratio, live
The sum of the indoor units is not an adder: the outdoor unit is deliberately smaller, because the rooms never all call for 100 % at the same moment. The question is by how much.
Indoor units
3.5 kW
2 kW
2 kW
5.3 kW
Connected power
7.5 kW
Connection ratio
142 %
Between 100 and 150 %: the working range
Real-world simultaneity is what makes that overshoot possible. The outdoor unit covers the whole-home load without being sized on the sum of every room’s peak.
Indicative range. Every manufacturer publishes its own in the technical data, and varies it by product line.
An Argile tool
How many splits by home type: field benchmarks
Studio and 2-room apartments: prioritize the main areas with a well-placed split
In a studio, a single split often suffices if the space is open. Place it to blow toward the living area, without aiming directly at the sofa. In a 2-room apartment, if the bedroom is closed off, a second split can avoid overheating the living room to gain comfort. With an air-to-air heat pump, insulation and solar gains quickly shift the need.
3- and 4-room apartments/houses: aim for the right compromise between comfort and budget
The most common field benchmark is 2 splits: one in the living area, one on the night side. You move to 3 if the home is a through-layout, heavily partitioned, or if a west-facing bedroom struggles in summer. The goal is a good balance without multiplying units unnecessarily.
Multi-story houses: managing temperature differences and air circulation
Upstairs, heat accumulates. Plan for at least one split per level, and pay attention to placement. A split in a landing helps, but doesn't replace diffusion in closed bedrooms. Also check for air leaks and ventilation. This is often where consistency is decided.
Choosing the right configuration: multi-split, mono-split or ducted
When a multi-split makes sense (and when it doesn't)
A multi-split is practical if you want to heat or cool several rooms with a single outdoor unit. It's often a good compromise in renovation, when each room has a different need. On the other hand, for a small home or rarely used bedrooms, the extra cost and line lengths can reduce the benefit.
One costing point not to miss: at equal performance, multi-split is treated more favourably than mono-split by French tax rules. The reduced VAT rate asks for class A++ in mono-split and only A+ in multi-split, for the same refrigerant and connectivity requirements.
Ducted: to treat several rooms without multiplying indoor units
A ducted system distributes air via ducts and diffusers, with a single indoor unit. The result is a discreet and even outcome. It makes sense if you have dropped ceilings or accessible attics. Plan for a well-insulated network and easy access for maintenance: on this point, see also insulating ventilation ducts in unheated attics.
Combining with a backup: stove, radiators, or another solution depending on the home
An air-to-air heat pump can cover most needs, but a backup secures very cold days or fast reheating. Depending on the building, keep a stove, a few controlled electric radiators, or another existing generator. The right choice comes after sizing and looking at actual room usage.
Points of attention in 2026: regulation, grants and site requirements
SCOP decides everything: the class scale and the thresholds
On a multi-split, a single figure drives the sales pitch, the energy certificates and the VAT rate at once: the SCOP for the average heating season, declared under Commission Delegated Regulation (EU) No 626/2011. Here is the official scale, together with the SEER that mirrors it on the cooling side.
| Class | SCOP (average season) | SEER |
|---|---|---|
| A+++ | ≥ 5.10 | ≥ 8.50 |
| A++ | 4.60 to 5.10 | 6.10 to 8.50 |
| A+ | 4.00 to 4.60 | 5.60 to 6.10 |
| A | 3.40 to 4.00 | 5.10 to 5.60 |
| B | 3.10 to 3.40 | 4.60 to 5.10 |
| C | 2.80 to 3.10 | 4.10 to 4.60 |
| D | 2.50 to 2.80 | 3.60 to 4.10 |
Lower bounds included, upper bounds excluded. Classes E, F and G sit below a SCOP of 2.50.
Grants in 2026: energy certificates through BAR-TH-129 and 5.5% VAT
Air-to-air heat pumps are not on the list of equipment eligible for MaPrimeRénov'. They do have their own certificate sheet, BAR-TH-129, which requires no RGE quality mark: rated output of 12 kW or less, SCOP of at least 3.9, residential building more than two years old. And since 18 July 2026, the order of 13 July 2026 brings air-to-air heat pumps into the reduced rate set out in article 278-0 bis A of the French tax code, on supply-and-install. The criteria are detailed in our article on the 5.5% VAT rate applied to air-to-air heat pumps.
| Requirement | BAR-TH-129 certificates | 5.5% VAT (≤ 12 kW) |
|---|---|---|
| Mono-split | SCOP ≥ 3.9 | class A++ (SCOP ≥ 4.60 and SEER ≥ 6.10) |
| Multi-split | SCOP ≥ 3.9 | class A+ (SCOP ≥ 4.00 and SEER ≥ 5.60) |
| Refrigerant | not covered by the sheet | compliant with Regulation (EU) 2024/573 |
| Controls | not covered by the sheet | connection to a digital network required |
| Qualification | "a professional" | not covered by the order |
F-Gas and competency certificate: what commits you on site
Working on the refrigerant circuit requires the competency certificate of article R. 543-99 of the French environmental code, issued for five years at most and per establishment. On the operating side, Regulation (EU) 2024/573 triggers leak checks from 5 tonnes of CO2 equivalent for Annex I gases, or 1 kg for those in section 1 of Annex II. In residential buildings, hermetically sealed equipment holding less than 3 kg is exempt, which takes a good share of domestic multi-splits out of scope. Above that, the frequency depends on the charge.
| Equipment charge | Without leak detection | With a detection system |
|---|---|---|
| 5 to 50 t CO2 eq. | every 12 months | every 24 months |
| 50 to 500 t CO2 eq. | every 6 months | every 12 months |
| ≥ 500 t CO2 eq. | every 3 months | every 6 months |
Obligation
Leak checks: is this job covered?
The obligation is not triggered by the kilograms of refrigerant but by tonnes of CO₂ equivalent: the charge times the fluid’s global warming potential. Hence different answers for the same charge depending on the fluid.
3.0 kg
Equivalent charge
2.0 t CO₂e
GWP 675
Leak check
Not required
Trigger threshold
5 t CO₂e
that is 7.4 kg of this fluid
Below the threshold: no periodic check
This is the usual case for a residential R32 multi-split. What still applies: the company’s certification, the tightness test and evacuation at commissioning, the intervention record and the logbook.
Regulation (EU) 2024/573, article 5. Periodic checks are one thing; the company’s certification, the tightness test at commissioning and the record keeping apply whatever the charge.
An Argile tool
Commissioning and settings: limiting callbacks (noise, condensate, comfort)
Most callbacks come from settings. Pay attention to anti-vibration mounts and clearances for noise: the French order of 30 June 1999 caps the normalised sound pressure level of individual heating or air-conditioning equipment at 35 dB(A) in main rooms and 50 dB(A) in the kitchen, a useful benchmark when arbitrating a location even where it does not strictly apply in renovation. Check condensate drainage, charge and tightness, then configure setpoints and time schedules. A simple client handover reduces calls and improves comfort.
