Short answer: HeatAlgo's reference house, 102.2 m² with underfloor heating in every room at 15 cm spacing, has 798.2 m of 16×2 pipe in the floor in 11 loops - 7.8 m per m², not the rule's 6.7 m/m². The materials list orders 863.9 m (8.5 m per m²), and at the merchant you buy 1000 m in three coils.
"15 m² / 0.10 = 150 m" is how many forum threads work out their pipe. That formula tells you how much pipe lies in the loop field, and overstates even that. It does not tell you what to buy: it leaves out the water's route from the manifold to the room and back, once for every loop.
From the rule to the order
Pipe in a 102.2 m² house - from the rule to the coils
Rule: area ÷ spacing681.3 m
Laid in the floor798.2 m
loopsrunsMaterials list863.9 m
Bought: 600 + 200 + 200 m1000 m
offcuts
- pipe in the rooms 615.6 m
- runs to the manifold 171.6 m
- reserve for connections 11.0 m
- top-up to the rule 65.7 m
The reference house has eight rooms but 11 loops: the 28 m² living room is split into three and the bedroom into two, because one loop would be too long. Each loop has its own run, 15.6 m there and back, so the house pays for runs eleven times, not eight. That is 171.6 m the rule never sees.
The pipe in the loop field alone is 5.6 to 6.1 m per m² in this house, less than the rule, because the pipe does not run right against the walls. Why is in the guide underfloor heating loop length and pipe spacing.
Why the list orders more than is laid
HeatAlgo's materials list works out each loop so that it never orders short. It takes the larger of two values: the drawn loop length, or the area divided by the spacing. Then it adds the run and 1.0 m of reserve. In this house area ÷ spacing wins in every room, so the list adds 65.7 m on top of what was drawn. That is a margin for cutting and for a route changed on site, not pipe the house needs.
| Room | Loops | Drawn | Area ÷ spacing | Runs and reserve |
|---|---|---|---|---|
| Living room, 28 m² | 3 | 170.8 | 186.7 | 49.8 |
| Kitchen, 10 m² | 1 | 58.7 | 66.7 | 16.6 |
| Bedroom, 16.1 m² | 2 | 96.5 | 107.3 | 33.2 |
| Room 1, 11.9 m² | 1 | 72.4 | 79.3 | 16.6 |
| Room 2, 11.9 m² | 1 | 72.4 | 79.3 | 16.6 |
| Bathroom, 8.1 m² | 1 | 48.9 | 54.0 | 16.6 |
| Hall, 10.2 m² | 1 | 62.3 | 68.0 | 16.6 |
| Plant room, 6 m² | 1 | 33.6 | 40.0 | 16.6 |
| Total, 102.2 m² | 11 | 615.6 | 681.3 | 182.6 |
The order is the "Area ÷ spacing" column plus "Runs and reserve": 681.3 + 182.6 = 863.9 m.
Why 1000 m, not 863.9 m
Loops in a screed are never joined from pieces: each one runs as a single length. So pipe is bought in coils, and the loops are allocated so each fits in one. For this house HeatAlgo chose a 600 m coil and two of 200 m. Seven loops fit in the 600 m coil and two in each 200 m one. That leaves 135.6 m in offcuts, too short for another loop.
This table is something to take to the merchant: a list of loop lengths lets you ask for coils that fit them, rather than "the usual amount".
How to work out your own house
For one room, the free loop calculator is enough: enter the dimensions, the spacing and the distance to the manifold, and you get the pipe in the room, the run and the number of loops. For a whole house, HeatAlgo builds the materials list from your underfloor heating design, with the pipe, coils, manifolds and insulation. The runs depend on how far the manifold is: this house assumes 8 m from every room.
Work out your house