The short answer, calculated: at 95/86 °F supply and return water, 15 cm (5.9 in, the usual 6-inch) tubing spacing and a 68 °F room, a slab floor gives 13.3 Btu/h·ft² under tile, 12.4 Btu/h·ft² under wood and 11.1 Btu/h·ft² 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 water temperature radiant floor heating needs and you get numbers that seem to contradict each other. Most of that is not disagreement. It is three different temperatures being called by the same name.
Floor temperature, water temperature and boiler setpoint are three numbers
A homeowner on HeatingHelp asked why the boiler sits at 125 °F when flooring makers give 80-85 °F. Both can be right, because they describe different things:
- The boiler setpoint. The temperature of the water leaving the boiler or water heater. With a mixing valve or injection pumping between the boiler and the floor, this can be much warmer than the water in the tubing. It tells you little about the floor on its own.
- The supply and return water temperature. 95/86 °F means water leaves the manifold for the loops at 95 °F and comes back at 86 °F. These are design values for the coldest day the heat loss is calculated for. With outdoor reset, the boiler or heat pump lowers the supply temperature on milder days, so for most of the winter the water runs cooler than the design value.
- The floor surface temperature. Far cooler than the water. In the example in the table below (95/86 °F, tile, 6-inch spacing, a 68 °F room) the calculation puts the surface at about 75 °F on average. EN 1264 limits the warmest spot on the floor, usually over a tube: 84 °F where people spend their time, 91 °F in bathrooms and up to 95 °F in an edge strip up to 3.3 ft wide along an outside wall. HeatAlgo calculates with the 84 and 91 °F limits and does not model the edge strip.
The "80-85 °F" on a flooring sheet belongs to the third group. The 125 °F belongs to the first. Only the middle one, the supply and return water in the tubing, goes into the output table.
There is also no single maximum supply water temperature for every floor. The ceiling comes from that surface limit and from the temperatures the flooring, the concrete and the tubing are rated for. The minimum design temperature is simply the right one: the lowest at which the floor still covers the heat loss on the design day.
The floor covering is part of the calculation
Between the tubing and the room sit the concrete and whatever is laid on top. All of the heat has to cross that layer, so its thermal resistance is a term in the calculation, not a finishing detail. HeatAlgo calculates floor output with the EN 1264-2:2021 characteristic: spacing, the covering's resistance, the concrete over the tubing and the tube diameter give the floor's heat transfer coefficient, multiplied by the mean water-to-room temperature difference, with the surface-temperature ceiling above all of it.
For the covering it follows EN 1264-3: floors in a home 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. Resistances are shown here in the metric units the standard uses:
| 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 (hardwood or engineered wood) | 0.12 m²K/W | 0.12 m²K/W |
| Carpet | 0.15 m²K/W | 0.15 m²K/W |
The figures below assume 16 mm (5/8 in OD, the size of 1/2-inch PEX) tubing under 45 mm (1.8 in) of concrete over the tubing, the EN 1264 "type A" floor: tubing embedded in a slab or a thin slab. If a manufacturer publishes output for a specific panel or slab, that takes precedence. What these figures show is the thing one datasheet cannot: how far the answer moves between coverings with everything else held equal.
Joist-space, staple-up, plate and panel systems under a wood subfloor are calculated differently. HeatAlgo does not calculate them; their output comes from the system's manufacturer.
Radiant floor output in Btu per square foot
Calculated with the EN 1264-2 characteristic, not measured on a test floor. Room at 68 °F, 6-inch spacing, 5/8 in OD tubing under 1.8 in of concrete, 84 °F surface limit, each covering at the resistance in the calculation column above. In Btu/h·ft².
| Supply/return water | Tile | Vinyl | Wood | Carpet |
|---|---|---|---|---|
| 86/77 °F | 7.6 | 7.6 | 7.3 | 6.3 |
| 95/86 °F | 13.3 | 13.3 | 12.4 | 11.1 |
| 104/95 °F | 18.7 | 18.7 | 17.1 | 15.5 |
| 113/104 °F | 23.8 | 23.8 | 22.2 | 20.3 |
The table 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 tile, because its resistance is higher. Advice to run cooler water under wood is about the temperature limit the flooring maker allows, a different question from how much heat the room needs. If the maker limits the temperature, look for the missing output elsewhere: tighter spacing, a lower heat loss or another emitter.
It also explains the 22-32 Btu/h·ft² (70-100 W/m²) you may see quoted for tiled floors. At 95/86 °F a tiled slab gives 13.3 Btu/h·ft². About 23.8 takes 113/104 °F water, and about 31.4 Btu/h·ft² is the ceiling set by the floor's surface temperature, which no water temperature lets a design exceed.
Take the 95/86 °F row and a room that needs 15.9 Btu/h·ft² (a stated example load). None of these floors serves it: tile and vinyl give 13.3, 17% short, wood 12.4, 23% short, and carpet 11.1, 30% short. The gap between the first and the last is the top layer alone, the one decided latest and least often sent back to the designer.
Tighter tubing spacing has a ceiling
The reflex when output is short is to tighten the spacing. It works, but less, and for less long, than expected. The same floor, with wood, at 4, 6 and 8 in on center (100, 150 and 200 mm):
| Supply/return water | 4 in | 6 in | 8 in |
|---|---|---|---|
| 86/77 °F | 7.9 | 7.3 | 6.3 |
| 95/86 °F | 13.6 | 12.4 | 11.1 |
| 104/95 °F | 19.0 | 17.1 | 15.5 |
| 113/104 °F | 24.4 | 22.2 | 20.3 |
At 95/86 °F, going from 8 in to 6 in buys about 1.3 Btu/h·ft² and 6 in to 4 in about the same again. That second step takes far more tubing, and it still lands at 13.6 Btu/h·ft², below the 15.9 required. At that water temperature no spacing serves this room under wood. The temperature has to move: roughly 102/93 °F, and then 6 in is enough. How spacing turns into feet of tubing and number of loops is in the guide on radiant floor tubing spacing and loop length.
Required Btu/h per square foot do not determine the spacing on their own. They determine it only together with the water temperature and the covering, and sometimes not at all, because at the assumed temperature no spacing is enough.
The surface temperature ceiling
Above all of this sits a limit harder than spacing and temperature combined: the floor surface temperature. Occupied rooms are held to 84 °F, bathrooms to 91 °F, and those are the two cases HeatAlgo models. That is a comfort limit, not a controller setting.
For an 84 °F limit in a 68 °F room, EN 1264-2's limit curve tops out at about 31.7 Btu/h·ft² for a floor at one uniform temperature. A real floor is warmer over the tube than between tubes, so its ceiling sits lower: about 31.4 Btu/h·ft² at 6 in and 30.7 at 8 in, for the 0.10 m²K/W the calculation takes for tile.
The table stays under that ceiling: at 113/104 °F tile gives 23.8 Btu/h·ft² at 6 in and 26.6 at 4 in. At 4 in and 122/113 °F it binds: the characteristic would give 32.3, but a design may count only just under the ceiling. From there on the surface limit does the limiting, not the spacing.
Bathrooms are different. A bathroom does not set the water temperature for the system, so it gets the water the other rooms need, and its warmer air shrinks the temperature difference. At 95/86 °F a bathroom held at 75 °F gets about 9.8 Btu/h·ft² at 4-inch spacing, and the surface limit plays no part. Whether the floor alone can heat a bathroom is answered in is radiant floor heat enough for a bathroom?
If a room's heat loss is above the ceiling, the answer is not more tubing. It is another emitter, or a lower heat loss.
The order that works
- Get the room's heat loss, room by room, from the contractor's room-by-room load calculation, not from a Btu-per-square-foot rule of thumb.
- Divide it by the heated floor area to get the Btu/h·ft² the floor has to deliver.
- Settle the floor covering before choosing a temperature. If it is undecided, design for the highest-resistance covering you are willing to allow. Raising the water temperature after the pour is easy; relaying tubing is not.
- Find the lowest water temperature that works at a realistic spacing.
- Check the surface ceiling. If the required output sits above it, go back to the heat loss, not to the spacing.
- Only then work out loop lengths, because only then is the spacing known. Maximum loop lengths come from the tubing manufacturer.
Boiler, water heater or heat pump
A radiant floor does not care what heats the water. Boilers, including mod-cons, and water heaters both supply radiant floors, usually through a mixing valve or injection loop that brings the water down to the supply temperature the floor needs. The table above is about that supply temperature, whatever produces it.
A radiant floor heat pump is where the number matters most. A heat pump does not create heat, it moves it, and the effort rises with the gap between the outdoor source and the water temperature it has to reach. Every degree off the supply temperature improves its seasonal efficiency, for the whole winter, not just on design days.
So the spacing decision is also an operating decision. Tighter spacing costs once, at installation. The warmer water forced by wider spacing or a wood floor is asked of the heat pump every day of the season. Sizing the heat pump itself is a separate step, and any figure for it is preliminary; why a bigger unit is not safer is in oversized heat pump and short cycling.
The check at the end
The water temperature is not a result in itself. The result is whether every room gets as much heat as it loses. So the last step compares deliverable floor output with the heat loss, room by room, because a single room with carpet or a wood floor can set the temperature for the whole system.
In HeatAlgo's radiant floor heating module that comparison happens on its own: each floor area carries its own spacing and covering, and HeatAlgo flags a shortfall wherever the floor will not cover the loss.
Check your floor for freeA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.