Underfloor heating
Pipe layout, circuits, pipe lengths, coils and flow
How the calculation works
Choose a quick estimate or a detailed layout. Enter the pipe (diameter, wall, bending radius, maximum circuit length, coils), flow and return temperatures and the manifold position. For each room enter the heated zones as rectangles, areas without pipes, the layout (serpentine or spiral), pipe spacing, heat load and floor output from the system table. The calculator draws the layout, splits circuits that are too long and calculates lengths, coils and flow.
Formula and assumptions
Quick estimate: L = A_heated ÷ s. Serpentine: n runs (even number), L = n·run length − (n − 1)·s + (n − 1)·π·s/2. Circuit flow: m = Q ÷ (cp · ΔT), V = m ÷ ρ. Water volume: π·(d_inner/2)²·L.
Worked example
Room 5 × 4 m, spacing 200 mm, 0.1 m from walls, serpentine layout: 93.7 m of heating pipe. With leads to the manifold the circuit exceeds 100 m, so it is split into two circuits of about 59 and 64 m (leads depend on the manifold position). At 60 W/m² the power is 1200 W, 600 W per circuit and about 103 L/h at ΔT 5 K. Water in 16 × 2 pipe: about 13.9 L; one 200 m coil is enough.
More worked examples
- Bathroom 2.0 × 2.5 m with a 0.9 × 0.9 m shower without pipes: heated 5.0 − 0.81 = 4.19 m²; the quick estimate at 100 mm spacing gives 41.9 m of pipe without leads.
- A 600 W circuit with 35 °C flow and 30 °C return: mass flow 600 ÷ (4186 · 5) = 0.0287 kg/s, i.e. about 103 L/h.
Quick estimate for 20 m² of heated area (16 × 2 pipe, without leads)
| Spacing (mm) | Pipe length (m) | Water in pipe (L) |
|---|---|---|
| 100 | 200.0 | 22.6 |
| 150 | 133.3 | 15.1 |
| 200 | 100.0 | 11.3 |
| 250 | 80.0 | 9.0 |
| 300 | 66.7 | 7.5 |
What affects real consumption
- Pipe spacing and distance from walls.
- Manifold position and length of the lead pipes.
- Maximum circuit length and bending radius per system.
Common mistakes
- Length calculated only as area ÷ spacing, without the lead pipes to the manifold.
- Pipe spacing smaller than twice the bending radius.
- Maximum circuit length checked before adding the lead pipes.
Frequently asked questions
How is underfloor heating pipe length calculated?
The quick estimate divides the heated area by the pipe spacing (e.g. 20 m² ÷ 0.2 m = 100 m). The detailed layout adds up the actual runs, bends and leads to the manifold.
Why is a circuit split in two?
Every system specifies a maximum circuit length because of pressure drop. If a circuit with its leads exceeds that length, the calculator splits the zone into strips until each circuit fits.
Does the calculator work out the heating power needed?
No. Enter the room heat load and floor output from the heat-loss calculation and the manufacturer's tables; the calculator only checks whether the output covers the load and calculates the flow.