Short answer: usually not on the same water as the rest of the house. At 95/86 °F supply and return water, 15 cm (5.9 in, the usual 6-inch) tubing spacing and tile, the radiant floor that gives 13.3 Btu/h·ft² in a 68 °F room gives only 8.9 Btu/h·ft² in a bathroom kept at 75 °F. The example bathroom below needs 12.9 Btu/h·ft², so its whole floor covers it only with 104/95 °F water (14.3 Btu/h·ft²).
This is about hydronic radiant floor heat: warm water in PEX tubing embedded in a slab or thin slab, meant to heat the room. Most heated bathroom floors in the US are electric floor warming, a mat under the tile that makes the floor pleasant to stand on but is not sized to heat the room. HeatAlgo does not calculate electric mats, and it does not calculate staple-up or joist-space tubing under a wood subfloor either.
A homeowner on HeatingHelp asked whether a hydronic towel warmer could simply run as one more zone of the floor heat. The replies are about piping and controls. Hardly anyone compares what the bathroom loses with what its floor and towel warmer can give on that water. That comparison settles it.
Why the same radiant floor heats a bathroom less
A floor gives off heat in proportion to the difference between the water and the room. Water at 95/86 °F is on average 22.1 °F warmer than a 68 °F room, but only 14.8 °F warmer than a 75 °F bathroom. The same tubing in the same slab therefore gives a third less. That is how the EN 1264-2:2021 characteristic behind HeatAlgo's floor output works.
The bathroom's higher floor surface limit, 91 °F instead of 84 °F, does not help here. At 95/86 °F the floor never gets near it. The water limits the output, not the cap.
How short the example bathroom is
The example is the bathroom in HeatAlgo's reference house, a 102.2 m² (1,100 ft²) house calculated for a Warsaw design day of -20 °C (-4 °F). It is 8.1 m² (87 ft²), kept at 75 °F, and its heat loss to EN 12831-1:2017 is 329 W (1,123 Btu/h), or 12.9 Btu/h·ft². Treat that as a stated example load, not a sizing of a US bathroom. At 95/86 °F the whole floor gives 768 Btu/h, which leaves 355 Btu/h missing.
In a real bathroom not all of the floor is heated. A 67 × 30 in tub and a 35 × 35 in shower pan take 23 ft² between them, leaving 65 ft² of heated floor:
| Heated floor | 95/86 °F water | 104/95 °F water | Needed |
|---|---|---|---|
| 87 ft² (all of it) | 768 Btu/h | 1,242 Btu/h | 1,123 Btu/h |
| 65 ft² (tub and shower out) | 570 Btu/h | 921 Btu/h | 1,123 Btu/h |
With the tub and shower in, even 104/95 °F is not enough. The floor alone would cover this bathroom only at 109/100 °F. Closer spacing changes little: 4 in on center instead of 6 in gives 9.8 Btu/h·ft² at 95/86 °F. Why spacing has a ceiling is in the guide on radiant floor heating water temperature.
The towel warmer catch
The natural answer is "add a hydronic towel warmer". But a towel warmer's catalog output, when rated to the European EN 442 standard, assumes 167/149 °F water in a 68 °F room. On 95/86 °F water in a 75 °F bathroom it gives about 10% of that. A towel warmer rated at 1,706 Btu/h (500 W) then gives about 164 Btu/h. Covering the missing 355 Btu/h would take one rated at about 3,750 Btu/h.
What to do about it
- An electric element in the towel warmer. The simplest fix: it heats whatever the water temperature.
- Warmer water for the bathroom. A manifold delivers one supply temperature. A warmer bathroom loop means its own mixing, or the whole house at 104/95 °F, which lowers a heat pump's efficiency in every room.
- A smaller heat loss. A better window or an insulated outside wall cuts the load at the source.
Which one fits shows in the contractor's room-by-room load calculation and a radiant floor design that treats the bathroom on its own, with its real heated area.
The 1,123 Btu/h is for the Warsaw design day; a bathroom in a milder climate loses less, a colder one more. The 75 against 68 °F gap, and the third less that follows from it, are the same everywhere. The floor figures are for hydronic tubing under 45 mm (1.8 in) of concrete. The towel warmer output is an estimate with the exponent of 1.3 that HeatAlgo's heat pump module uses; the manufacturer gives the real low-temperature output.
Check your bathroom floor for freeA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.