Heat pump output: sizing by heat loss

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 base outdoor temperature, colder in zone H1 than in H3. It 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 audit, a DPE or a room-by-room calculation. Multiply it by the base temperature difference (target indoor minus base outdoor). You get watts, then divide by 1,000 to get kW. Watch out for infiltration, ventilation and thermal bridges, which are often underestimated.

Domestic hot water, reheat and intermittent operation: when to add a margin without oversizing

If the heat pump also handles domestic hot water, avoid the reflex of a blanket "+20%". Prefer a small margin and a DHW-priority strategy, or a dedicated backup. 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 renovation, 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 DPE 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 RGE files with Argile (sizing + financial support)

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 RGE file gains in consistency.

Pre-quoting: fold heat pump output, MaPrimeRénov' and CEE 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 available financial support (MaPrimeRénov' and CEE) under the rules in force in 2026. The result: an offer that's ready to sign, and better-organised paperwork.