The short answer: the maximum length of an underfloor loop is counted including the run to the manifold, and it depends on the pipe diameter and the system. HeatAlgo warns above 80 m for 16x2 pipe and, above 90 m, tells you to split the loop; for 17x2 the thresholds are 90 m and 100 m. Your system manufacturer's figures take precedence. How many loops rooms from 12 to 40 m² need is in the table below.
Many guides to underfloor heating quote a quick formula: floor area divided by spacing, times 1.1 for good measure. It is quick and easy to remember. The trouble is that it leaves out two things that decide whether the loop fits the limit at all - and it is wrong in two opposite directions at the same time.
In short
Building or renovating a home and comparing underfloor heating quotes? You do not need to work out the loops yourself. It does help to know what the numbers in a design or a quote mean.
- Loop length is all the pipe in one loop. That is the pipe laid in the room plus the run to the manifold: the flow and return pipes between the room and the manifold. The popular area formula leaves the run out.
- Pipe spacing comes from a calculation, not from floor area. It depends on how much heat the room needs, the floor finish and the water temperature.
- Check the permitted loop length in your system manufacturer's data. The length limits in this article (80, 90 and 100 m) are HeatAlgo's own thresholds. For your project, the data of the system you choose decides.
- One loop is not a design. A complete design connects every loop to the manifold and checks that the floor in each room delivers the heat it needs.
If you are still gathering plans and details, start with the underfloor heating planning checklist. If you already have a design, see what an underfloor heating design should include. The rest of this article is more technical: where the length limits come from, and how far off the popular formula is.
Where the length limit comes from
The limit is not a convention, it is a consequence of hydraulic resistance. Water flowing through a pipe loses pressure to friction, and the loss grows with length. A circuit that is too long means three things: the pump has to overcome more resistance, the temperature difference between flow and return grows, and balancing that loop against the others on the same manifold becomes difficult or impossible.
That is why system manufacturers publish a maximum loop length, usually a different one for each pipe diameter. Check it in your own system's datasheet - the figures circulating online differ from one another by tens of metres for the same diameter. HeatAlgo applies its own thresholds and always counts the run to the manifold in them, because the water flows through the run exactly as it flows through the loop field. For 16x2 pipe it warns above 80 m, and above 90 m - its hard limit - it tells you to split the loop. For 17x2 pipe those thresholds are 90 m and 100 m. It does not block the design. They are HeatAlgo's settings - they do not replace your system manufacturer's data.
What the formula leaves out
The setback from the walls
A loop is not run right up to the wall. HeatAlgo's calculator sets it back from the walls by 10 cm. In the underfloor heating module it is the Pipe distance from wall setting in the project parameters: 100 mm by default, adjustable from 30 to 500 mm, and per section too. The module guide shows where to find it. Your system's installation instructions give the distance they recommend. The field the pipe actually covers is therefore smaller than the room's floor area - and the formula counts the whole area. This error overstates the answer.
The run to the manifold
A pair of pipes runs from the manifold to the room: flow and return. If the manifold stands 15 m away, that is 30 m of pipe that never appears in the loop field at all - and that this formula does not contain. Some guides add it separately, as twice the distance to the manifold - rightly. This error understates the answer, and the further away the manifold, the larger it gets.
Turns
At the end of every row or ring the pipe turns back, and it has to reach the run. Those stretches add a few per cent to the area divided by the spacing. The arc itself does not lengthen the pipe: it is shorter than a square turn. This is the smallest of the three corrections. Guides describe the 1.1 multiplier as a margin for bends, fittings and offcuts. In the table below it adds 10%, more than the turns add.
What the loop length includes
In this article, in the calculator and in HeatAlgo's module, loop length means all the pipe in one loop: the pipe in the loop field plus the run to the manifold and back. That total is what gets compared with the limit. It is not the pipe for the whole house - with several loops, each one has its own run. Nor does the length tell you whether the spacing is right or whether the floor will heat the room. That is checked separately, by comparing the loop's output with the room's heat demand.
How far off it is, calculated
A room 3.2 m wide and 4.0 m long, so 12.8 m². The manifold is 15 m away, and the run goes from it in a straight line to the middle of the longer, 4 m wall. The order of the dimensions matters - entered the other way round they give a different loop and different numbers. The "formula" column is (12.8 ÷ spacing) × 1.1; the "pipe in total" column is the result from real loop geometry including the run to the manifold.
HeatAlgo's geometry engine calculated the table on 23 September 2026: a 10 cm setback from the walls (as in the calculator, and the module's default), bends of 50 to 80 mm radius depending on spacing, and a straight run. It is a calculation, not a site measurement.
| Spacing | Formula | In the room | Run to manifold | Pipe in total |
|---|---|---|---|---|
| 10 cm | 140.8 m | 115.0 m | 29.7 m | 144.7 m |
| 15 cm | 93.9 m | 77.0 m | 29.7 m | 106.6 m |
| 20 cm | 70.4 m | 59.5 m | 29.7 m | 89.1 m |
| 25 cm | 56.3 m | 46.8 m | 29.7 m | 76.5 m |
| 30 cm | 46.9 m | 39.2 m | 29.7 m | 68.9 m |
At 10 and 15 cm a single loop is over 100 m, so this room needs two loops. The "pipe in total" column then shows it as one unsplit loop. After the split each loop has its own run, so the room takes more pipe in total.
There is a pattern here. In every row the formula overstates the pipe inside the room - by 25.8 m at 10 cm, by 7.7 m at 30 cm. And in every row it understates the total, because the run to the manifold is missing from it. The two errors point in opposite directions, so they partly cancel: at tight spacing the wall-setback error nearly makes up for the missing run and the formula looks about right, while at wide spacing the run dominates and the formula is clearly too low.
The row that matters most is 15 cm spacing. The formula gives 93.9 m; the real circuit is 106.6 m - the formula lands 12.7 m short. That is the whole point: 12.7 m is more than the 10 m between HeatAlgo's successive thresholds (80, 90 and 100 m). Against 100 m, HeatAlgo's hard limit for 17x2 pipe and the one the calculator uses, the formula says "one loop is enough" and the correct answer is two. Against any other limit the same error can put you on the wrong side of it just as easily - because what is missing is the entire run to the manifold, and here that run is 29.7 m.
The difference is not academic: it is a decision about the number of circuits, taken on site from a number that omits the distance to the manifold.
Work out your own roomHow to choose the spacing
Spacing is not a matter of taste, nor of floor area alone. It follows from how much heat the floor has to deliver into the room. Pipe laid more tightly gives more watts per square metre at the same flow temperature - and conversely, wider spacing needs a higher temperature to deliver the same output.
Hence the practical order:
- Start from the room's heat demand. Without it, choosing a spacing is guesswork. That means a heat loss calculation room by room, not a per-square-metre rule of thumb. A HeatAlgo report is not an MCS heat loss calculation.
- Check the floor can carry that output at the intended flow temperature and with the chosen covering. Tile conducts heat far better than wood or carpet.
- Consider a tighter spacing in an edge zone, along external walls and under large windows where losses are highest. In HeatAlgo an edge zone is part of the same loop, laid more tightly.
- Check the length - only now, because only now do you know the spacing.
The calculator offers five spacings: 10, 15, 20, 25 and 30 cm. In the underfloor heating module you can also type your own spacing from 50 to 300 mm in 5 mm steps, and Apply the recommended spacing picks the widest of 200, 175, 150, 125 and 100 mm at which the floor covers the room's heat demand. The details are in the module guide.
The lower the flow temperature, the more a tighter spacing pays for itself - and that is the direct link to a heat pump, whose seasonal efficiency rises when it can run cooler. The same output delivered at 30 °C rather than 40 °C is a different electricity bill across a whole winter. How water temperature, floor covering and spacing together set a floor's output is explained in the guide to underfloor heating flow temperature.
When to split into two loops
Usually for one of two reasons.
Length. One circuit including the run to the manifold exceeds the permitted figure. Split the field into two sections of similar area - similar, because two loops of very different length are hard to balance on one manifold. Each section is then a separate loop with its own flow and return to the manifold: splitting divides the pipe in the room, never the run. So two loops need more pipe in total than one. Each has to fit the limit on its own, and a long run can call for a third loop or more. If the run alone comes close to the limit, splitting stops being the answer - move the manifold closer, or add a second one.
Shape. L and T shapes, narrow necks, alcoves and obstacles can make a single loop impossible to route without crossing pipes or leaving a patch uncovered. That is not a question of metres, it is geometry. The honest answer there is "split", not "draw something that cannot be laid".
How many loops per room - six examples
The same calculation for six typical rooms. Assumptions: the manifold 8 m from the room, pipe set back 10 cm from the walls, a limit of 100 m per circuit, as in the calculator. "One loop" is the whole pipe if the room had a single circuit, run included. Where it exceeds 100 m the room gets more loops, each with its own run. Open a room to see its loop drawing and every spacing.
| Room | Size | One loop at 150 mm, with run | Loops at 150 mm | Loops at 200 mm |
|---|---|---|---|---|
| Room 12 m² | 3 × 4 m | 89.1 m | 1 | 1 |
| Room 16 m² | 4 × 4 m | 114.5 m | 2 | 1 |
| Room 20 m² | 4 × 5 m | 139.5 m | 2 | 2 |
| Room 25 m² | 5 × 5 m | 172.1 m | 2 | 2 |
| Room 30 m² | 5 × 6 m | 203.1 m | 3 | 2 |
| Room 40 m² | 5 × 8 m | 265.1 m | 3 | 3 |
So a 30 m² living room takes three loops at 150 mm and two at 200 mm. The spacing still follows from the output the room needs, not from the loop count: a wider spacing gives less heat at the same water temperature.
Short answers
Single questions, each with one number calculated in HeatAlgo:
- How much underfloor heating pipe for a 100 m² house? - a whole house, with runs and coils.
The check at the end
Loop length is not a result in itself. The result is the answer to whether this floor covers the room's demand. So the last step is comparing the loop's output against the room's heat loss from the heat loss calculation - and if it falls short, you go back to the spacing or the flow temperature before anything reaches the site.
That is also why the calculator in this article handles one rectangular room and nothing more. A real project is several rooms on one manifold, each with its own heat loss and its own spacing, plus the balance of the whole. L and T shapes, slanted walls, obstacles and multiple floors are handled in the underfloor heating design tool.
If you already have a design and want to check that these quantities are in it, read what an underfloor heating design should include.
These figures come from the same engine that calculated our reference house, with every input published for checking. See the method: a reference house calculated in full.
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