Short answer: usually not on the same water as the rest of the house. At a 35/30 °C flow and return, 15 cm spacing and tiles, the floor that gives 42 W/m² in a 20 °C room gives only 28 W/m² in a bathroom heated to 24 °C. The bathroom in HeatAlgo's reference house needs 40.6 W/m², so its whole floor covers it only at 40/35 °C (45 W/m²).
"We can't seem to get it up to 23 degrees," one homeowner writes on a UK forum, and the cold-bathroom version of that question recurs across self-build forums. The replies are about pump speed, the mixing valve and patience. Hardly anyone compares what the bathroom loses with what its floor can give. That comparison settles it.
Why the same floor heats a bathroom less
A floor gives off heat in proportion to the difference between the water and the room. Water at 35/30 °C is on average 12.3 K warmer than a 20 °C room, but only 8.2 K warmer than a 24 °C bathroom. The same pipe in the same screed therefore gives a third less. That is how the EN 1264-2:2021 characteristic behind HeatAlgo's engine works.
Same water, two rooms
The bathroom's higher floor surface limit - 33 °C instead of 29 °C - does not help here. At 35/30 °C the floor never reaches it. The water limits it, not the cap.
How short the example bathroom is
HeatAlgo's reference house has an 8.1 m² bathroom heated to 24 °C. Its design heat loss to EN 12831-1:2017 is 329 W, or 40.6 W/m². At 35/30 °C the whole floor gives 225 W. That leaves 104 W missing.
In a real bathroom not all of the floor is heated. A 170 × 75 cm bath and a 90 × 90 cm shower tray take 2.1 m² between them, leaving 6.0 m² of heated floor:
| Heated floor | 35/30 °C | 40/35 °C | Needed |
|---|---|---|---|
| 8.1 m² (all of it) | 225 W | 364 W | 329 W |
| 6.0 m² (bath and tray out) | 167 W | 270 W | 329 W |
With the bath and the tray in, even 40/35 °C is not enough. The floor alone would cover this bathroom only at 43/38 °C. Closer spacing changes little: 10 cm instead of 15 cm gives 31 W/m² at 35/30 °C. Why spacing has a ceiling is in the guide underfloor heating flow temperature.
The towel radiator catch
The natural answer is "add a towel radiator". But a radiator's catalogue output is rated to EN 442 at 75/65 °C water and a 20 °C room. On 35/30 °C water, in a 24 °C bathroom, it gives about 10% of that. A 500 W towel radiator then gives about 48 W. Covering the missing 104 W would take one rated at about 1100 W.
What to do about it
- An electric element in the towel radiator. The simplest fix: it heats whatever the water temperature.
- Warmer water for the bathroom. A manifold delivers one flow temperature. A warmer bathroom loop means its own mixing set, or the whole house at 40/35 °C, which lowers the heat pump's efficiency in every room.
- A smaller heat loss. A better window or an insulated outside wall cuts the 329 W at the source.
Which one fits shows in a room-by-room heat loss calculation and an underfloor heating design that calculates the bathroom on its own, with its real heated area.
The 329 W is for a Warsaw design day at -20 °C; the same bathroom in the UK or Ireland loses less. The 24 against 20 °C gap, and the third less that follows from it, are the same everywhere. The figures are for wet underfloor heating in a screed. The radiator 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. A HeatAlgo report is not an MCS heat loss calculation.
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