The document that governs underfloor cooling in France is CSTB technical prescriptions guide no. 3164, and it does not say what it is made to say. It sets no cooling output per square metre and states plainly that indoor temperature rises once the climatic load exceeds what the floor can absorb. What it does impose is quantified: a minimum flow water temperature ranging from 18°C inland to 22°C on the Mediterranean coast, an independent manually reset safety device that cuts cooling at 12°C, and a floor covering whose thermal resistance stays below 0.09 m².K/W. NF DTU 65.14, in its 2023 edition, carried that temperature table over.
What a cooling floor does, and what it will not do
The reference guide promises no output
This is the point most sales pitches skip. Guide 3164 states that absorption capacity is limited by the design of the pipe grid, taking comfort requirements and condensation risk into account, and it provides that where the climatic load is higher, a rise in indoor temperature is accepted. In other words, the reference text assumes the floor will not hold setpoint on a heatwave day. Institutional publications put the order of magnitude of a cooling floor below 30 W/m², without that being a French normative value, against the 30 to 60 W/m³ used in the cooling capacity calculation for air conditioning.
The difference from air conditioning, in one sentence
A cooling floor works on sensible heat alone, over a horizontal surface, with a surface temperature bounded by condensation risk. It lowers operative temperature by a few degrees and it removes the warm-surface effect. It does not dehumidify and it does not absorb a peak. A fan coil or an indoor unit treats the load, the floor treats the background condition.
What belongs on the quotation
That the works provide cooling and not air conditioning, that indoor temperature may rise above setpoint when the load exceeds absorption capacity, and that this limit is the one set by the technical framework for the system. A quotation line that sells summer comfort without that reservation is an August complaint waiting to happen. For strategies that genuinely address overheating, see passive cooling and night-time overventilation.
Water temperatures: the real protection against condensation
The table by geographic zone
The French anti-condensation strategy does not rest on a sensor, it rests on a minimum flow temperature that depends on the outdoor humidity typical of the zone. A limiting device has to hold it at the panel inlet.
| Geographic zone | Minimum flow temperature |
|---|---|
| Inland | 18°C |
| Channel, North Sea and Atlantic coast north of the Loire estuary, 30 km band | 19°C |
| Atlantic coast between the Loire and Gironde estuaries, 50 km band | 20°C |
| Atlantic coast south of the Gironde estuary | 21°C |
| Mediterranean coast, 50 km band | 22°C |
Source: CSTB, technical guide 3164, table 3. Carried over into NF DTU 65.14 of July 2023.
The safety device that has to exist alongside the controls
The guide requires a safety device independent of the controls, manually reset, working even in the absence of power or motive fluid, which must cut the supply of cooling at the panels once fluid temperature reaches 12°C. Independent means the comfort controls cannot stand in for it, and manually reset means a trip leaves a trace. It is a component to identify on the hydraulic schematic and to verify at commissioning.
Dew point, clarified
EN 1264-3 does express the cooling mode limit in dew-point terms, and that is a dimensioning rule. The French guide prescribes neither a humidity sensor nor a setpoint computed in real time. Fitting a room humidity sensor remains good practice, particularly in heavily glazed homes, but it replaces neither the limiter, nor the 12°C cut-out, nor the zone table. Do not put it on the quotation as a compliance item, put it there as a comfort option.
The limits on the layers above the pipe
Resistance and mass: reversible is stricter than heating alone
| Quantity | Reversible limit | Reference |
|---|---|---|
| Thermal resistance of the covering, including acoustic layers above the heating elements | 0.09 m².K/W | CSTB technical guide 3164 |
| Total thermal resistance above the pipe | 0.13 m².K/W | CSTB technical guide 3164 |
| Thermal resistance of the slab alone | 0.04 m².K/W | CSTB technical guide 3164 |
| Surface mass of the covering works above the insulation | 160 kg/m², about 7 cm | CSTB technical guide 3164 |
| Cover above the pipe, reversible floor | 30 mm at any point | CSTB, guide 3578_V4 |
The guide states explicitly that laying rugs substantially changes the thermal resistance of the build-up and makes the covering limit impossible to meet. That sentence is worth repeating in the handover documentation, because the rug arrives after you leave.
Insulation beneath is not an adjustment variable
The guide sets a minimum thermal resistance for the insulation beneath of 0.75 m².K/W over a heated room and 1.00 m².K/W over a cellar or ground-bearing slab, rising to 1.25, then 1.50, then 2.00 m².K/W as the design minimum outdoor temperature falls to 0, minus 5 or minus 15°C. In renovation, this is often the item that decides whether the project works at all, before the question of available height even comes up.
In heating mode, the cap is still the statutory one
The heating mode of a reversible floor follows the same rules as a conventional heated floor, starting with the 28°C at the finished floor imposed in France by article 35 of the order of 23 June 1978. Sizing the heating side uses the same method as on a non-reversible system, set out in our article on sizing and pipe spacing for a low-temperature floor.
Summer operation and commissioning
The operating rules the guide imposes
The summer setpoint of the room thermostats must never drop below 24°C. Wet-room circuits are closed in summer. All exposed pipework must be insulated to rule out any condensation risk, manifolds included, and where antifreeze is used its dosage is never below 25%. These four points are checked at commissioning and written down, because they become invisible once the job is closed.
What is set on the generator
The natural pairing is a reversible air-to-water heat pump, able to produce chilled water within a range compatible with the zone limiter. A boiler covers heating perfectly but requires a dedicated cooling generator for the summer side, with the hydraulics and controls that go with it. Check that the machine can hold a high water setpoint, around 18 to 22°C depending on the zone, which is not a chiller's most common operating point: the operating range is read off the manufacturer's data, which Argile keeps in equipment catalogues verified brand by brand.
The handover file
The pressure test report, the loop mapping, verification of the limiter and of the 12°C safety device, manifold insulation, the thermal resistances of the build-up above the pipe, and the operating instructions carrying the reservation on output. On the site work itself, pressure testing, pipe layout and first heat-up, the method is the same as for a conventional heated floor and we set it out in the site sequence for a heat pump on underfloor heating. A handover documented on these points is what protects you the day someone reports a cold floor or a damp mark.
