Sizing an air-to-water heat pump on underfloor heating is not choosing an output, it is verifying that an output is reachable under two ceilings. Article 35 of the French order of 23 June 1978 forbids exceeding 28°C at the finished floor, and NF DTU 65.14 caps flow water at 50°C, a figure repeated in the AQC's E.04 pathology sheet. EN 1264-3 builds its dimensioning on limiting curves referred to a 20°C reference room temperature. The whole calculation consists of fitting the heat loss under those bounds. The site work, pressure testing, first heat-up and commissioning, is covered in our article on the installation sequence.
What the floor imposes on the sizing of the machine
A surface temperature cap is an output cap
This is the mental inversion the heat pump plus floor pairing demands. On radiators, high demand is met by raising water temperature. On a floor, surface temperature is bounded, so the driving difference against the room is bounded, so output per square metre of active floor is bounded too. The variable left open is area, not temperature. If a room's heat loss does not fit, the answer is insulation, a supplementary emitter, or active area won back, never a raised flow temperature.
Active area is not room area
You take out of the calculation the areas under fixed furniture, the kitchen island, the shower tray, the bath and the wardrobes. A demand of 1,000 W over 20 m² of active floor is 50 W/m² to cover, whereas the same demand spread over a 25 m² room would read 40 W/m² and produce a calculation that passes on a job that will not. It is the first thing to check when you pick up a study produced by someone else.
The bounds of the calculation, in one table
| Quantity | Value | Source |
|---|---|---|
| Temperature at the finished floor | 28°C at any point, design conditions | Order of 23 June 1978, article 35 (France) |
| Flow water temperature | 50°C maximum | NF DTU 65.14, repeated in AQC sheet E.04 |
| Design surface temperature, occupied zone | 29°C for a 20°C room | EN 1264-2 limiting curves |
| Design surface temperature, bathroom | 33°C for a 24°C room | EN 1264-2 limiting curves |
Where a national rule is stricter than the standard, the national rule governs. The detailed method for sizing the emitter itself, spacing and loop lengths included, is set out in sizing a low-temperature underfloor heating system.
The calculation chain, in order
From heat loss to flow rate
| Step | What it determines | What frames it |
|---|---|---|
| 1. Room-by-room heat loss | The demand in watts, for each room | Real fabric, ventilation, thermal bridges |
| 2. Active area per room | The watts per square metre to deliver | Pipe layout, no-heat zones under fixed furniture |
| 3. Surface temperature reached | Whether the demand is feasible at all | Cap at the finished floor, EN 1264 limiting curves |
| 4. Water regime and flow-return difference | The flow temperature adopted | 50°C maximum, NF DTU 65.14 |
| 5. Flow rate per loop and pressure loss | The pump and the balancing | System instructions, manufacturer output charts |
| 6. Minimum modulated output of the machine | Mid-season behaviour | Manufacturer's instructions |
Each step is read in order and each one can invalidate the one before it. Skipping step 3 is the most common cause of installations that never reach setpoint in one or two rooms, with nothing wrong with the machine. For the heat loss calculation itself, see the heat loss method.
What the calculation must make visible
The calculation must show, room by room, the demand, the active area adopted, the resulting output per square metre and the associated surface temperature. That is the level of detail of a sizing note produced to NF EN 12831-1 from the survey records. A calculation that stops at a total in kilowatts proves nothing and protects nothing. That level of detail is also what lets you refuse a line the client wants to drop, such as insulating a ground floor over a cellar, by showing what it costs in flow temperature.
The floor covering belongs in the calculation, not in the finishes
The covering is a thermal resistance in series with the emitter, and it is chosen at the sizing stage. On a reversible floor, the CSTB caps that resistance at 0.09 m².K/W for the covering and 0.13 m².K/W for everything above the pipe. For a heating-only floor, no institution publishes a ceiling, so the reference is the system instructions and the covering's own document. Fix the covering on the quotation, otherwise a last-minute change moves your flow temperature.
Choosing the machine on the right criteria
Minimum modulated output before nominal output
An oversized heat pump on underfloor heating is penalised twice, by the slab's thermal mass which lengthens response times and by a modulation floor too high to avoid cycling at mid-season. The figure to compare against demand is not nominal output at the design point, it is minimum modulated output at average outdoor temperatures. Everything else, including the real performance measured by SCOP, follows from that choice.
Water volume and buffer tank are calculated
A buffer answers a specific problem, a water volume too small relative to the minimum modulated output, or zoning that regularly closes a large share of the loops. The litres-per-kilowatt figures in circulation are not traceable to any verifiable source. The minimum volume and nominal flow rate required are in the machine's instructions, they are checked there and copied onto the quotation. The cases where it genuinely helps are set out in our article on the buffer tank.
The heating curve is set at the sizing stage, not at commissioning
The slope of the heating curve follows from the pairing of design flow temperature and design outdoor temperature, which is to say from your calculation. Arriving on site with a slope still to be found by trial condemns you to two weeks of return visits. Set it on paper, with the outdoor sensor, then fine-tune on site.
Special cases and the limits to write down
Reversible mode changes the framework
If the project includes cooling through the floor, sizing no longer follows heating rules alone. Minimum flow water temperatures by geographic zone, the safety cut-out and the limits on the covering's thermal resistance are all tighter. They are set out in our article on underfloor cooling, together with the reservation to carry on the quotation regarding the cooling output genuinely available.
Partial renovation, floor plus radiators
Where underfloor heating coexists with radiators, the design point is the circuit that demands the highest temperature, and that is almost always the radiator circuit. Either you resize the radiators down to the floor's regime, or you accept two regimes with a mixing valve on the floor, which costs a component, a control and a maintenance item. That choice is settled at the sizing stage, not at connection time. The subject is developed in low temperature versus high temperature on the emitter side.
What you commit to by signing
A room-by-room calculation, the water regime adopted, the genuinely heated active area, the covering fixed, and the exact machine reference with its minimum modulated output. Those are the five lines that make the difference in a dispute, and the same five that let you justify a price against a cheaper bid sized on floor area. The site sequence that follows, pressure test, heat-up and settings, is the subject of our article on the installation.
