The short answer, calculated: at a 35/30 °C flow and return, 150 mm pipe spacing and a 20 °C room, the floor gives 42 W/m² under tiles, 39 W/m² under wood and 35 W/m² under carpet (EN 1264-2, HeatAlgo's calculation). Whether that is enough depends on the room's heat loss - which is why the right water temperature comes from a design, not from one number.
Ask what flow temperature underfloor heating needs and the internet answers 35 °C. It is often right. But it answers a different question from the one an installer faces on site, because flow temperature is not a setting you pick - it is the result of three things worked out together.
In short, for homeowners
There is no single right water temperature for underfloor heating. The right one is the lowest at which the floor still covers each room's heat loss. A design works it out, and the familiar 35 °C is an example, not a rule.
"Underfloor heating temperature" can mean three different numbers:
- Flow and return. 35/30 °C means water leaves the manifold for the loops at 35 °C and comes back at 30 °C. Both values apply at the design outdoor temperature - the cold day the heat loss is calculated for.
- The setting through the season. A boiler or heat pump with weather compensation lowers the flow temperature when it is milder outside, so for most of the winter the water runs cooler than the design value.
- The floor itself. The surface is far cooler than the water. In the example from the table below (35/30 °C, tile, 150 mm spacing, a 20 °C room) the calculation puts it at about 24 °C on average. The EN 1264 limits apply to the warmest spot on the floor, usually over a pipe: 29 °C where people spend their time, 33 °C in bathrooms and up to 35 °C in an edge zone, a strip up to 1 m wide along an external wall. HeatAlgo calculates with the 29 and 33 °C limits and does not model the edge zone.
Three temperatures, one floor
Water in the pipes
35/30 °C
Flow from the manifold and return to it - the design values for the cold design day.
The setting through the season
≤ 35 °C
Weather compensation lowers the flow temperature when it is milder outside.
The floor surface
about 24 °C on average
It is warmest over each pipe, and that is where the limit applies: 29 °C, or 33 °C in a bathroom.
Nor is there one maximum flow temperature for every floor. The ceiling comes from that surface limit and from the temperatures the covering, screed and pipe are rated for. The minimum design flow temperature is simply the right one: the lowest at which the floor still covers the heat loss on the design day. On milder days weather compensation goes lower still. The lower the water temperature, the more efficiently a heat pump runs.
The covering matters a lot. At 35/30 °C and 150 mm spacing, HeatAlgo calculates 42 W/m² for tile and vinyl, 39 for wood and 35 for carpet. Tile and vinyl come out the same because EN 1264-3 designs a home's floors for a covering resistance of at least 0.10 m²K/W. HeatAlgo calculates the outputs in the tables and in this summary with the EN 1264-2:2021 characteristic for pipe laid in screed - they are not measured. The calculation assumes a 16 × 2 mm PE-X or PE-RT pipe under 45 mm of cement screed (λ = 1.2 W/(m·K)) and each covering at its "In the calculation" resistance. Dry systems are calculated differently, and their output comes from the system's manufacturer.
What to read on the flooring product sheet
- Whether the manufacturer approves it for water-based underfloor heating, and how it is laid: glued to the screed or floating on an underlay.
- Its thermal resistance R in m²K/W, covering and underlay together. If the sheet gives only a thickness and a λ value, a layer's R is its thickness in metres divided by λ; add the covering's and the underlay's together. Carpet and underlay are often rated in tog: 1 tog is 0.1 m²K/W.
- The highest temperature the manufacturer allows, at the floor surface or under the covering. It can be lower than the standard's 29 °C. HeatAlgo calculates with 29 °C (33 °C in bathrooms), so a lower limit like that is for the designer to account for.
- The conditions for laying it on a heated screed, such as the screed's moisture content and its first heating.
If the sheet gives a higher resistance than the "In the calculation" column in the table below, or a highest temperature below 29 °C, the floor may deliver less than the tables show - pass the sheet to whoever designs the system. Everything else to gather before a quote is in the underfloor heating planning checklist. If you already have a design, the guide on what an underfloor heating design should include shows where to find these values.
The technical part below shows what happens when you take the room's needs, the spacing and the covering one at a time: a table of output per square metre calculated to EN 1264-2, the point where tightening the spacing stops helping, and the ceiling no setting gets past.
Heat is driven by a temperature difference, not by the flow temperature
A floor does not emit heat "at 35 °C". It emits in proportion to the difference between the mean water temperature in the loop and the room temperature. Mean, not flow: water gives up heat along the way and comes back cooler, so at 35/30 °C in a 20 °C room the driving difference is about 12.3 K (the logarithmic mean, the way EN 1264-2 takes it), not 15 K.
Hence the first practical consequence: the temperature drop is a design parameter too. The same flow temperature with a bigger drop gives a lower mean, and therefore less power. Quoting a flow temperature without its return describes half an installation.
The covering is inside the equation, not beside it
Between the pipe and the air sit the screed and whatever was laid on top. All of the power has to cross that layer, so its thermal resistance is not a finishing detail - it is a term in the calculation. HeatAlgo's underfloor module derives deliverable output on EN 1264-2's thermal characteristic: pipe spacing, the covering's thermal resistance, the screed cover over the pipe and the pipe diameter give the floor's heat transmission coefficient, multiplied by the logarithmic water-to-room temperature difference - with the limit characteristic set by the maximum surface temperature above all of it. For the covering it follows EN 1264-3: a home's floors are designed for a covering of at least 0.10 m²K/W, and a covering's own resistance counts only where it is higher. So tile and vinyl go into the calculation at 0.10:
| Covering | Thermal resistance | In the calculation |
|---|---|---|
| Tile | 0.05 m²K/W | 0.10 m²K/W |
| Vinyl | 0.08 m²K/W | 0.10 m²K/W |
| Wood | 0.12 m²K/W | 0.12 m²K/W |
| Carpet | 0.15 m²K/W | 0.15 m²K/W |
The exception is tile in a bathroom: it is calculated at its own 0.05 m²K/W, because bathroom tiles are not replaced later. Every other room is calculated at 0.10 m²K/W.
The figures below are the standard's, for a 16 mm pipe under 45 mm of cement screed - a typical floor, not your system's datasheet. If a manufacturer publishes a thermal characteristic for a specific panel and screed, that takes precedence. What the figures below show is the thing one manufacturer's datasheet cannot: how much the answer moves between coverings with everything else held equal.
The calculated table
Calculated with the EN 1264-2 characteristic, not measured on a test floor. Room at 20 °C, 150 mm pipe spacing, 16 mm pipe under 45 mm of screed, 29 °C surface temperature limit, each covering at the resistance in the calculation column above. In watts per square metre.
| Flow/return | Tile | Vinyl | Wood | Carpet |
|---|---|---|---|---|
| 30/25 °C | 24 | 24 | 23 | 20 |
| 35/30 °C | 42 | 42 | 39 | 35 |
| 40/35 °C | 59 | 59 | 54 | 49 |
| 45/40 °C | 75 | 75 | 70 | 64 |
The table also settles something often heard the other way round. Under wood the water has to be warmer, not cooler. At the same water temperature wood gives less than tiles, because its thermal resistance is higher. Advice to run cooler water under timber is about the temperature limit the flooring manufacturer allows - a different question from how much heat the room needs. If the manufacturer limits the temperature, look for the missing output elsewhere: tighter spacing, a lower room heat loss, or a radiator.
The same table explains the "70-100 W/m² at 35 °C" you may see quoted for tiled floors. At 35/30 °C a screed floor gives 42 W/m² under tiles. Around 75 W/m² takes 45/40 °C water, and about 99 W/m² is the ceiling set by the floor's surface temperature, which no water temperature lets a design exceed.
Take the 35/30 °C row, the familiar answer. A room needing 50 W/m² is served by none of these floors: tile and vinyl give 42 W/m², 17% short, wood 39 W/m², 23% short, and carpet 35 W/m², 30% short. The gap between the first and the last is the top layer alone - the one decided latest and least often referred back to the designer.
Spacing has a ceiling
The reflex when power is short is to tighten the spacing. It works, but less and for less long than expected. The same floor, with wood:
| Flow/return | 100 mm | 150 mm | 200 mm |
|---|---|---|---|
| 30/25 °C | 25 | 23 | 20 |
| 35/30 °C | 43 | 39 | 35 |
| 40/35 °C | 60 | 54 | 49 |
| 45/40 °C | 77 | 70 | 64 |
At 35/30 °C, going from 200 to 150 mm buys 4 W/m² and 150 to 100 mm another 4. That is 8 W/m² for twice the pipe - and it still lands at 43 W/m², below the 50 required. At that flow temperature no spacing exists that serves this room under wood. The temperature has to move: roughly 39/34 °C, and then 150 mm is enough.
That is the point of this article. Required watts per square metre do not determine the spacing. They determine it only together with flow temperature and covering - and sometimes they do not determine it at all, because at the assumed temperature no spacing is sufficient.
The ceiling you cannot beat
Above all of this sits a limit harder than spacing and temperature combined: the floor surface temperature. Occupied rooms are held to 29 °C, bathrooms to 33 °C - and those are the two cases HeatAlgo models. That is a comfort and physiology limit, not a controller setting.
Turning that limit into watts per square metre is what the limit characteristic in EN 1264-2 does, and it is what HeatAlgo computes on. For a 29 °C limit in a 20 °C room its maximum is about 100 W/m² - for a floor at one uniform temperature. A real floor is warmer over the pipe than between the pipes, so it reaches 29 °C over the pipe first and its ceiling sits lower: about 99 W/m² at 150 mm and 97 at 200 mm, for the 0.10 m²K/W the calculation takes for tile.
The table stays under that ceiling: at 45/40 °C tile gives 75 W/m² at 150 mm and 84 at 100 mm. At 100 mm and 50/45 °C it binds: the characteristic would give 102 W/m², but a design may count only just under 100. From that point on it is the permissible surface temperature doing the limiting, not the spacing. The floor does not hold itself back, though: if water that warm really ran through the loop, the surface over the pipes would go above 29 °C.
Bathrooms are more interesting than they look. Held at 24 °C, a 29 °C limit would allow only 52 W/m² at 45/40 °C and 100 mm, and 150 or 200 mm would change little. Permitting 33 °C lifts the ceiling to about 99 W/m², and the floor at 45/40 °C then delivers 87 W/m². With water that warm, it is the higher limit, not tighter spacing, that gives a bathroom its extra output.
With low-temperature water it is different. A bathroom does not set the flow temperature for the system - EN 1264-3 leaves it out of that choice - so it gets the water the other rooms need. At 35/30 °C the same bathroom gets about 39 W/m² at 100 mm spacing: the warmer room shrinks the temperature difference, and the surface limit plays no part at all. Whether the floor alone can heat a bathroom depends on its heat loss, the free floor area and the water temperature, and sometimes a towel radiator is needed as well.
If a room's heat loss exceeds the ceiling, the answer is not more pipe. It is another emitter, or a lower heat loss.
The order that works
- Calculate the room's heat loss - room by room, from a proper heat loss calculation, not from a watts-per-square-metre rule of thumb. Without that number the rest is guesswork. What a heat loss calculation is, worked through on one house, is in the heat loss guide. A HeatAlgo report is not an MCS heat loss calculation.
- Divide it by the heated area to get required watts per square metre. That is the number the floor has to deliver.
- Settle the covering before choosing a temperature. If the client has not decided, design for the worst case you are willing to allow - raising the temperature after the screed is poured is cheap, relaying pipe is not.
- Find the lowest temperature that works at a realistic spacing. Lowest, because every degree down is a lower bill for the whole season.
- Check the surface ceiling. If the required output sits above it, go back to the room's heat loss, not to the spacing.
- Only now work out loop length, because only now do you know the spacing. How to do that, and why the popular formula is wrong in two directions at once, is covered in a separate article on spacing and loop length. Worked examples for rooms from 12 to 40 m² show how many loops and how much pipe each spacing takes.
What this means for a heat pump
A heat pump does not create heat, it moves it, and the cost of moving rises with the gap between the source and the water temperature it has to reach. Every degree the flow temperature comes down improves the unit's seasonal efficiency - for the whole winter, not just on design days.
So the spacing decision is a running-cost decision. Tighter spacing costs once, at installation. The higher flow temperature forced by wider spacing or a wooden floor costs every month for the life of the system. It is the same relationship behind sizing a heat pump - a matched installation and a matched unit are one job, not two.
Short answers
Single questions, each with one number calculated in HeatAlgo:
- Is underfloor heating enough in a bathroom? - the same floor in a 24 °C room.
- Does a thicker screed reduce underfloor heating output? - from 30 to 80 mm of screed over the pipe.
The check at the end
Flow temperature is not a result in itself. The result is whether every room receives as much power as it loses. So the last step is comparing deliverable output against the calculated heat loss, room by room - because a single room with a wooden floor can set the temperature for the entire circuit.
In the underfloor heating design tool that comparison happens on its own: every section carries its own spacing and covering, and HeatAlgo flags a deficit wherever the floor will not cover the loss.
Where those values belong in the paperwork is covered by the guide on 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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