HesapHanem

Underfloor Heating Output Calculator

Enter the floor area, flow and return water temperatures and room temperature to estimate the heat output.

How it is calculated

The tool uses a simple linear model: Tm = (Tflow + Treturn) / 2, ΔT = Tm − Troom, q = k × ΔT and Q = q × A. Temperatures are in °C, k is the floor factor in W/m²K that you supply (it bundles pipe spacing, screed and floor covering), q is the output per square metre in W/m², A is the floor area in m² and Q is the total output in W (also shown in kW). The mean water temperature is the plain average of flow and return. All five fields are required. No result is shown if area or factor is zero or negative, if the return temperature is higher than the flow temperature, if the water is outside the liquid range (flow above 100 °C or return below 0 °C), or if the mean water temperature is not above room temperature.

Worked example

A 20 m² room, flow 40 °C, return 35 °C, room 20 °C, floor factor 8 W/m²K. Mean water temperature = (40 + 35) / 2 = 37.5 °C. ΔT = 37.5 − 20 = 17.5 K. Output per square metre = 8 × 17.5 = 140.0 W/m². Total output = 140 × 20 = 2,800 W, which is 2.80 kW, and the mean water temperature is shown as 37.5 °C.

Try this example →

Common mistakes

  • Using the flow temperature instead of the mean of flow and return as the water temperature. The calculation needs the average, which is lower.
  • Entering the floor factor in the wrong unit. It is W per square metre per kelvin, not a total in watts and not a W/m² figure.
  • Swapping flow and return, which makes the return higher than the flow, so no result is given. A flow temperature above 100 °C is also refused.
  • Entering the design outdoor temperature or a thermostat setback value as the room temperature instead of the target room temperature; if it is at or above the mean water temperature there is no result.
  • Treating the total as the room's heat loss. It is the most the floor can emit at these settings, so compare it with the heat loss of the room separately.

About this calculator

The heat a water-based underfloor heating (UFH) system delivers depends on the temperature difference between the circulating water and the room, and on the floor construction — the general relationship, formalized in the EN 1264 standard, is that heat output (W/m²) scales with the mean water temperature minus room temperature (ΔT), with the exact output-per-degree factor depending on pipe spacing, floor covering and screed depth. A screeded system with tile flooring and tight pipe spacing delivers noticeably more watts per square metre at the same ΔT than one under thick carpet with wide spacing, because floor covering resistance limits how fast heat can pass through.

Heating engineers and self-builders use output calculations like this to check whether a proposed UFH design can actually meet a room’s heat loss on the coldest day of the year — undersizing pipe spacing or running water too cool relative to floor covering resistance is one of the most common reasons UFH systems feel inadequate in practice, especially in rooms with high heat loss like those with large windows.

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