Sizing a heat pump means calculating the design heat loss of the dwelling at the design outdoor temperature, then choosing a machine whose output at that temperature and at the planned flow temperature matches that figure rather than exceeding it. The safety margin added out of habit belongs to the back-up, not to the heat pump: it is the heat pump plus its back-up that covers the peak, and the back-up has to be an integral part of the system rather than an appliance the occupier happens to own. The design outdoor temperature is not read off a broad climate zone, it comes from the national annex for the specific location and its altitude.
Understanding a home's heat loss before sizing a heat pump
Heat loss: transmission, ventilation, thermal bridges and air infiltration
Before choosing a heat pump, you start from the home's actual heat loss. It comes from the envelope (walls, roof, floor), ventilation, thermal bridges (wall/floor junctions, window reveals) and air infiltration. The higher these figures, the more output is needed, with the risk of oversizing if you go by floor area alone.
Data to collect on site: areas, insulation, joinery, volumes and orientation
On site, record the area of each surface, the thickness and type of insulation, the type of joinery and glazing, the heated volumes, and the orientation of the façades. Also note the existing emitters. A reliable survey beats an estimate, especially in renovation where cladding sometimes hides the reality underneath.
Base outdoor temperature and flow temperature: what changes depending on your zone
The calculation uses a design outdoor temperature specific to the location, taken from the national annex and then corrected for altitude. Broad climate zones do not enter this calculation. That temperature determines the output needed on peak-cold days. The water flow temperature then depends on the emitters. Underfloor heating runs around 35 to 40°C, radiators higher. A well-tuned weather compensation curve limits consumption.
On-site calculation method: from heat loss to useful heat pump output
Quick calculation of needs (W/K then kW): simple steps and points to watch
On site, start from a heat-loss coefficient in W/K taken from an assessment, an EPC or a room-by-room calculation. Multiply it by the design temperature difference (target indoor minus design outdoor). You get watts, then divide by 1,000 to get kW. Watch out for infiltration, ventilation and thermal bridges, which are often underestimated.
The bracket reads directly off the calculated design heat loss, the figure recorded in the sizing note drawn up to NF EN 12831-1. The output to compare is the one delivered at the design outdoor temperature and at the planned flow temperature, never the catalogue nominal figure, which is measured at +7 °C outside.
| Design heat loss | Heat pump alone | Heat pump plus back-up, minimum |
|---|---|---|
| 5 kW | up to 5.0 kW | 6.0 kW |
| 7 kW | up to 7.0 kW | 8.4 kW |
| 9 kW | up to 9.0 kW | 10.8 kW |
| 12 kW | up to 12.0 kW | 14.4 kW |
| 15 kW | up to 15.0 kW | 18.0 kW |
The room-by-room method behind that figure is covered in our article on BS EN 12831.
Calculation
The design outdoor temperature for your site
Enter the address: the calculation reads the department from the commune code, takes the ground elevation at that point from the national mapping service, then applies the tables of the French national annex. This is the value that sets the temperature difference your heat-loss calculation runs on, and therefore the output of the machine.
The standard defines neither the coast nor how to measure it: the installer declares it. The correction only applies below 200 m.
Four worked examples while you type
The result depends on the department and on the elevation of the point, not on the town alone. Compare the last two rows: the same department value of −10 °C, and nine degrees apart, purely because of elevation.
| Town | Department θe,D | Elevation | θe used |
|---|---|---|---|
| Brest | −4 °C | 47 m | −4 °C |
| Strasbourg | −15 °C | 143 m | −15 °C |
| Bourg-en-Bresse | −10 °C | 237 m | −11 °C |
| Briançon | −10 °C | 1214 m | −20 °C |
θe is the temperature you size on
Heat losses are calculated on this value, and it is what sets the output of the machine. What it is not is a record low: the standard takes the extreme night temperature reached on at least five days a year over thirty years. Sizing on an exceptional cold snap is what oversizes the whole installation.
Address resolved by the Base Adresse Nationale, elevation by the IGN altimetry service. The result follows tables D.1a, D.1b and D.1c of NF P 52-612/CN, the French national annex to EN 12831-1.
An Argile tool
Domestic hot water, reheat and intermittent operation: when to add a margin without oversizing
If the heat pump also handles domestic hot water, the reflex of a blanket "+20%" lands in the wrong place: the margin belongs to the heat pump plus backup combined, not to the heat pump alone. Prefer a DHW-priority strategy and a backup sized to make up the difference. For reheat after a night-time setback, a moderate margin and a good weather compensation curve beat oversizing that causes cycling.
Monovalent, bivalent, boiler back-up: choosing the most coherent output strategy
Go monovalent if the envelope is sound and the emitters are suitable. Go bivalent for high-temperature radiators, limited electrical capacity, or marked cold. When backing up a boiler, set a realistic bivalence temperature so the heat pump does most of the work without losing comfort. To refine the system choice, also see how to choose between an air-to-air and an air-to-water heat pump depending on the emitters and usage.
Avoiding classic heat pump sizing mistakes (and their impact)
Oversizing: short cycling, wear, noise and rising consumption
A heat pump that's too powerful quickly hits its setpoint and then keeps restarting. This short cycling degrades seasonal efficiency, multiplies compressor starts, and can increase perceived noise, especially in mid-season. The result is a bill that doesn't fall as much as expected and a unit that wears out faster.
Undersizing: discomfort, electric backup and customer disputes
A heat pump that's too weak struggles during cold peaks. The home can't hold its temperature, the occupant overworks the thermostat, and the system falls back on electric backup if fitted. Consumption rises, comfort drops, and you end up handling customer complaints, or even a dispute over whether the sizing was correct.
Whole-house renovation in 2026: size after insulation, not before
In a whole-house retrofit, heating needs drop after insulation, air-leak sealing and ventilation tuning. In 2026, protect your sale by sizing the heat pump on the post-works heat loss. Otherwise, you risk installing a heat pump that's too big once the envelope has been improved, with all the unwanted effects that come with it. To go further, see our article on sizing and EPC estimation.
Adapting the sizing to the reality of the job: emitters, hydraulics and constraints
Radiators vs underfloor heating: flow temperatures and delivered output
With radiators, the flow temperature often targets 45 to 55°C, sometimes higher on an older installation. With underfloor heating, you work more around 30 to 40°C. The lower the temperature, the easier the heat pump breathes. Before confirming the output, check the radiator sizes, loop spacing and the comfort setpoint room by room.
Hydraulics and layout: water volume, balancing, buffer tank and controls
A heat pump doesn't like unstable flow rates. A small network, thermostatic heads closing off, or a single emitter running. In these cases, plan for a sufficient water volume, good balancing, and, if needed, a buffer tank or hydraulic separation. On the controls side, a well-tuned weather compensation curve limits cycling and improves comfort.
On-site constraints: local rules, noise, clearances and access (common cases)
At the address, obstacles are often obvious once you're on site. Co-ownership rules, protected areas, noise-sensitive neighbours. Add the minimum clearances around the outdoor unit, the condensate drain, and real access for handling and maintenance. Spotting these early avoids oversizing forced by a constrained layout.
Saving time and de-risking your files with Argile (sizing + funding)
Energy diagnosis in under 5 minutes: structure heat loss and compare scenarios
In 5 minutes, Argile helps you structure heat loss (walls, loft, floors, ventilation) and compare several scenarios. You quickly see what really weighs on the bill, and you can argue your case on clear grounds, without an endless spreadsheet.
Feasibility analysis by address: spot technical constraints before the site visit
From the address alone, Argile flags the points to watch before you travel. Access, building type, likely constraints for a heat pump, unit placement, condensate drainage. You arrive at the visit with a checklist, and your compliance file gains in consistency.
Pre-quoting: fold heat pump output and funding into a clear offer in 2026
Argile turns your technical choices into a readable pre-quote. Heat pump output and options, associated insulation work, then an estimate of the funding available under the rules in force in 2026. The result: an offer that's ready to sign, and better-organised paperwork.
