How it works
From room size to a laid-out floor
-
01
Pipe for the area
A floor takes about its area divided by the spacing in pipe: 10 m per m² at 100 mm centres, 6.7 m at 150 mm, 5 m at 200 mm.
-
02
Loops under the limit
Every loop, tails included, stays under the maximum length. When a room needs more, it is split into side-by-side zones along the wall the pipes come in through.
-
03
Drawn as it’s laid
Each zone is a counterflow spiral or a double meander, set back from the walls. The lengths are measured on the drawing, so the numbers and the picture agree.
-
04
Heat and flow
Output follows EN 1264 for your screed, covering and temperatures. Each loop’s flow carries its share of the room’s heat, and its pressure drop is worked out for the pipe size.
Heat output
Underfloor heating output at heat-pump temperatures
Watts per square metre for 16 mm pipe at 150 mm centres in a 65 mm sand and cement screed, with the room at 21 °C and a 5 K drop from flow to return. Figures in copper put the floor surface over 29 °C, the comfort limit for living areas.
| Floor covering | 35 °C flow | 40 °C flow | 45 °C flow | 50 °C flow |
|---|---|---|---|---|
| Tile or stone | 57 W/m² | 83 W/m² | 108 W/m² | 133 W/m² |
| Vinyl or LVT | 52 W/m² | 76 W/m² | 99 W/m² | 122 W/m² |
| Laminate on underlay | 42 W/m² | 61 W/m² | 80 W/m² | 98 W/m² |
| Engineered wood | 37 W/m² | 54 W/m² | 71 W/m² | 87 W/m² |
| Carpet, 1.5 tog with underlay | 32 W/m² | 46 W/m² | 60 W/m² | 74 W/m² |
Spacing matters as much as temperature. On tile at 35 °C flow the same floor gives 66 W/m² at 100 mm, 57 W/m² at 150 mm, 50 W/m² at 200 mm and 38 W/m² at 300 mm . A well-insulated new build often needs 30–50 W/m²; an older house can need twice that.
How long can an underfloor heating loop be?
Most makers allow about 100 m for 16 mm pipe, 80 m for 12 mm and 120 m for 20 mm, counting the tails to the manifold. Past that, the pump struggles to push enough water through and the far end of the loop runs cool. Keep loops on one manifold within about 20% of each other, or set their flow meters carefully.
How much floor does one loop cover?
Take the tails off the maximum and multiply by the spacing. A 100 m loop with 10 m of tails covers about 9 m² at 100 mm centres, 13.5 m² at 150 mm and 18 m² at 200 mm. Bigger rooms need more loops, and each loop needs its own pair of manifold ports.
FAQ
Questions self-builders ask
How many underfloor heating loops does a room need? +
Divide the floor area by the pipe spacing to get the pipe length — 20 m² at 150 mm centres is about 133 m — add both tails to the manifold, and split it into loops that each stay under the pipe maker’s maximum, usually 100 m for 16 mm pipe. That room needs two loops. The planner does the split and draws each loop.
What is the maximum length of an underfloor heating loop? +
It depends on the pipe and its maker. Common limits are about 80 m for 12 mm pipe, 100 m for 16 mm and 120 m for 20 mm, tails included. Longer loops need more pump pressure and lose more temperature along the way.
What flow temperature do I need with a heat pump? +
Usually 35–45 °C; the lower the temperature, the more efficient the heat pump. The output table below shows what each floor covering gives at each temperature. If the floor falls short at 35 °C, closer pipe spacing or a thinner covering usually costs less in running costs than a higher flow temperature.
Spiral or meander? +
A counterflow spiral alternates flow and return pipes, so the floor warms evenly; it is the usual choice for screed floors. A meander is quicker to lay and suits long, narrow rooms. The planner draws a double meander, which runs flow and return side by side to even out the temperature.
Does it work for overlay boards or spreader plates on joists? +
The layout, loop lengths and flow settings apply to any wet system. The heat-output figures are for pipes in screed (EN 1264 type A). Low-profile boards and spreader plates usually give less, so use your supplier’s output table for those.
How accurate are the heat-output figures? +
They follow the EN 1264-2 calculation for pipes in screed. A particular maker’s tested figures can differ by about 10%, either way. Check them against your supplier’s data, and size the system against a room-by-room heat-loss calculation.
Is it free? +
Yes. The one-room planner is free, with no signup and nothing to install. Whole-house layouts from a floor plan are a planned paid feature; you can ask to be told when it is ready.
Method and limits
Heat output uses the EN 1264-2 characteristic for system types A and C (pipe in screed), with B = 6.7 W/m²K for plastic pipe with a 2 mm wall, the standard’s factors for spacing, covering, screed cover and pipe diameter, and the log-mean water-to-room temperature difference. Floor surface temperature uses the EN 1264 base curve. Flow rates assume water and the temperature drop you set. Pressure drop is Darcy–Weisbach for straight pipe with a 2 mm wall, without the manifold and valves. Heat lost downwards and from the tails is not counted. Use it to check a design, not to replace a heat-loss calculation or your pipe maker’s data.