Low-temperature underfloor heating: sizing and pipe spacing (2026 guide)

Checking the prerequisites before sizing your underfloor heating

Clarifying the use case: new build or renovation, room by room, temperature setpoints

Before pulling out the calculator, set the framework. In new build or renovation, available height, screed type and floor covering don't give you the same margins. List the rooms, their uses, and your setpoints. A lightly heated bedroom isn't sized the same way as a bathroom. This sorting is what avoids overloading some loops while underfeeding others.

Estimating heating demand: heat loss, insulation level and ventilation

Underfloor heating is sized on actual heat loss. So wall insulation, roof, ground floor, window quality, thermal bridges. Add ventilation, since mechanical ventilation and airtightness weigh on the output required. In renovation, a thermal study or energy audit avoids "rough estimates" and limits oversizing.

Choosing the flow temperature: low-temperature logic and comfort limits

Aim for low temperature whenever possible. The lower the flow temperature, the more smoothly the heat pump or condensing boiler operates. Also check comfort. Floor surface temperature must stay moderate, with more tolerant zones in bathrooms. If you have to go too high, the signal is clear: there's not enough insulation, or the heating curve isn't right.

Carrying out the sizing: output, active surfaces and water regime

Calculating useful output per zone: W/m², actually heated surfaces

You start from the heat loss per room. Useful output is read in W, then converted to W/m² by dividing it by the surface that's actually heated. On underfloor heating, you exclude areas under fixed furniture, kitchen islands, showers, bathtubs or wardrobes. Result: a demand of 1,000 W over 20 m² of active surface is 50 W/m² to cover.

Defining the water regime: flow/return, ΔT and impact on the emitter

Set a flow/return regime consistent with low-temperature production (often 35/30 or 40/35). The ΔT (flow-return gap, often 5 K) drives the flow rate. The lower the flow temperature, the more active surface and suitable pipe spacing you need to deliver the same output, while keeping the floor comfortable.

Adapting the sizing to the floor covering: tile, wood, resilient flooring

The floor covering acts as an insulating layer. Tile lets heat through easily. Wood flooring and resilient coverings increase thermal resistance and limit available output. Check the manufacturer's limits and permissible surface temperatures. If needed, tighten the pipe spacing or revisit the room-by-room requirements.

Determining the pipe spacing and loop length without getting it wrong

Choosing the pipe spacing: 10, 15 or 20 cm depending on the output required

On underfloor heating, pipe spacing drives output per m² and evenness. In practice, 20 cm spacing is often enough in a well-insulated home. Move to 15 cm for most rooms. Reserve 10 cm for high demand or perimeter zones, to avoid the sensation of "warm floor in the centre, cool at the edge".

Setting the maximum circuit length: pressure loss, balancing and comfort

The longer a loop, the higher the pressure loss climbs, and the trickier balancing becomes. Keep circuits of similar length, and split them if needed. A common rule of thumb on site is 100 m max per circuit in standard pipe, to be adjusted for diameter, flow rate and circulator. The goal is a flow rate that's easy to adjust at the header.

Positioning reinforced zones: cold façades, glazed bays, corners

Treat heat loss like draughts. Tighten the spacing over 0.5 to 1 m along cold façades, glazed bays and corners. This reinforced zone improves comfort without overheating the rest. Also plan for constraints, expansion joints, door thresholds, and avoid loops that are too "twisted".

Choosing the right emitter and ensuring compatibility with the heat source

Underfloor heating as the main emitter: typical cases and points to watch

Underfloor heating is often the right choice in a well-insulated house, in an extension, or after a major renovation. It operates at low temperature, with high thermal mass. Points to watch: take care with sub-slab insulation, plan for coherent zones, avoid highly insulating floor coverings. Keep the flow temperature moderate for comfort, and to limit overheating risk.

Heat pump / boiler compatibility: temperature, flow rate, heating curve

A heat pump performs better when the emitter accepts low-temperature water. Check radiator output at 45°C or below, otherwise plan for resizing or a boiler backup. Flow rate needs to follow the demand of the loops and radiators. Set the heating curve according to insulation and exposure, to avoid jolts and short cycles.

Controls and headers: actuators, flow meters, thermostats and balancing

At the header, flow meters are used to balance the loops. The actuators, often solenoid heads, open or close based on zone thermostats. Plan for a suitable circulator, careful air bleeding, and gradual adjustments after heating start-up. Good balancing makes heat circulate like light, everywhere, with no cold spots.

Site checks and settings in 2026: avoiding callbacks and disputes

Tests before the screed: pressurisation, tightness and circuit mapping

Before pouring the screed, secure your underfloor heating with pressurisation maintained during the pour. The goal is to detect any leak, avoid crushing a pipe, and lock in accurate reference points. Photograph the pipe layout, record loop lengths, and leave a simple plan in the site file. This is often what saves a repair when a hole needs drilling later.

Bringing the screed up to temperature: schedule, steps and precautions for floor coverings

The first heat-up is managed like a gentle ramp. Follow a heating curve in steps, according to the DTU, the screed, and the manufacturer's instructions. Check residual moisture before installing the flooring, especially for wood or PVC. A written, signed schedule avoids disputes if a floor later lifts or cracks.

Final settings: flow rates at the header, temperatures, customer documentation

At commissioning, carry out flow-rate balancing at the header, check flow and return temperatures and room-by-room controls. Hand over a clear as-built file, test reports, manuals, and a "good practices" sheet for the customer. In 2026, traceability is your best insurance.