Radiant floor output depends far more on the floor surface temperature than on the water temperature alone, because the slab and floor covering sit between the tube water and the room. This calculator uses the ASHRAE radiant panel model to turn supply temperature, tube spacing, and covering resistance into a realistic BTU/h per square foot.
How it works
The panel correlation ties output to how much warmer the floor surface is than the room air:
q (BTU/h·ft²) = 2.0 × (Tsurface − Tair)^1.1
Tsurface = Tair + (Twater − Tair) × ε
ε = spacingFactor / (1 + coveringR × 1.8)The effectiveness factor ε captures two real losses: wider tube spacing leaves
cooler stripes between tubes, and a higher floor-covering R-value insulates the
surface from the warm slab. The spacing factor is 0.62 at 6 inch, 0.55 at 9 inch,
and 0.48 at 12 inch on-center.
Worked example: comparing floor coverings
Room conditions: 68 °F air temperature, 110 °F supply water, 9-inch tube spacing.
| Floor covering | Approx R-value | Approx surface temp (°F) | Approx BTU/h·ft² |
|---|---|---|---|
| Bare tile or stone | 0.05 | ~84 | ~35 |
| Hardwood (3/4 in.) | 0.68 | ~79 | ~27 |
| Engineered wood | 0.40 | ~81 | ~30 |
| Carpet + thin pad | 1.5 | ~72 | ~16 |
| Carpet + thick pad | 2.5 | ~67 | ~9 |
These are illustrative values. Even a modest carpet-and-pad combination at R-1.5 cuts output by more than half compared to bare tile. A 200 ft² room that needs 30 BTU/h·ft² (6,000 BTU/h total) to meet its design heat loss can satisfy that requirement with bare tile or hardwood, but not with carpet.
The 85 °F surface temperature limit
ASHRAE sets the comfort limit for occupied floor surfaces at approximately 85 °F. Above that, bare feet feel uncomfortably warm and prolonged contact can cause minor burns in vulnerable occupants. The limit also protects hardwood flooring from moisture-related movement — most wood flooring manufacturers specify a maximum surface temperature, commonly 80–85 °F.
If the calculator shows a surface temperature above 85 °F, lower the supply water temperature using a mixing valve or reduce the number of modules in series. Never try to compensate by widening the tube spacing at a fixed high supply temperature — the surface temperature will still exceed the limit in the zone directly above the tubes, even if the average is lower.
When the floor cannot carry the full load
Compare the total BTU/h output from the calculator against the room’s design heat loss. If the floor cannot deliver enough heat while staying at or under 85 °F surface temperature, the common solutions are:
- Increase panel area — if the room layout allows it, extending heated floor into non-occupied zones (under furniture, closets) adds surface area
- Lower the floor covering R-value — switching from carpet to tile is often the single biggest change available
- Add a supplemental emitter — a small panel radiator, fan coil, or baseboard unit can carry the deficit, allowing the floor to run at a comfortable surface temperature
In bathrooms and tiled areas with low design loads (well-insulated, small rooms), radiant floor often handles the full load easily. In carpeted living rooms with high glazing areas, supplemental heat is frequently needed.