How much heat can a radiant floor put out per square foot?

A bare tile floor at about 110 degree water can deliver roughly 30 to 40 BTU/h per square foot. Output falls quickly as floor covering R-value rises, which is why carpeted rooms often cannot be heated by floor radiant alone.

Why does tube spacing change the output?

Closer on-center spacing keeps the floor surface more uniformly warm, raising the average surface temperature that drives heat into the room. Six inch spacing yields a warmer surface and higher output than twelve inch spacing at the same water temperature.

What is the maximum safe floor surface temperature?

ASHRAE recommends keeping occupied floor surfaces at or below about 85 degrees Fahrenheit for comfort and to avoid foot discomfort. If the calculator flags a higher surface temperature, lower the supply water temperature or widen the spacing.

Does floor covering really matter that much?

Yes. Floor covering R-value sits between the warm slab and the room, so even a thin carpet and pad can cut output by half or more. Tile and stone transmit heat best, while thick carpet is the worst case for radiant floors.

How do I know if the floor can heat the room?

Compare the total BTU/h output the calculator gives against the room design heat loss. If the load exceeds what the floor can deliver at a safe surface temperature, you need supplemental heat or a different emitter.

Radiant Floor Heating BTU Output Calculator

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.

Example and tips

A bare tile floor with 9 inch spacing, 110 °F supply water, and a 68 °F room reaches roughly an 84 °F surface and delivers about 35 BTU/h per square foot, so a 200 ft² room makes around 7,000 BTU/h. Add even a 1.0 R-value carpet and the surface cools, dropping output well below 20 BTU/h per square foot. Keep the surface at or under 85 °F for comfort, and if you cannot meet the room load without exceeding that, add a supplemental emitter rather than overheating the floor.