What is radiant heating and cooling?

Warming or cooling floor, ceiling or wall surfaces so they exchange heat with occupants and objects mainly by radiation. The surface-to-room coefficient is about 5 W/m²K radiative plus a few W/m²K convective — roughly 10 W/m²K in total for floor heating or ceiling cooling. With no air blown, draughts and noise are low.

How much capacity do radiant systems have?

Surface temperature limits set it. Floor heating is limited to about 29 °C so feet don't overheat, giving about 100 W/m² at 20 °C room. Ceiling cooling must stay above dew point (e.g. 17 °C), giving about 100 W/m² at 26 °C. Floor cooling transfers less because cool air pools at the floor: about 49 W/m².

Why can radiant heating use a lower air temperature?

Perceived warmth depends on both air temperature and surrounding surface temperatures (mean radiant temperature); their average is the operative temperature. In a 5×4 m room with a 26 °C floor, mean radiant temperature rose about 1.6 K, so 21 °C operative needed only 21.9 °C air (23.5 °C with radiators).

Radiant Heating and Cooling Systems

Category: Thermal – HVAC & Buildings | Revised 2026-10-01

CAE visualization for radiant heating cooling theory - technical simulation diagram

Radiant heating and cooling system

Theory: conditioning with surface temperature

Overview

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How do floor heating and ceiling cooling differ from air conditioning?

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Air conditioning changes air temperature to warm or cool people; radiant systems change floor or ceiling surface temperature and exchange heat with people mainly by radiation. The surface-to-room coefficient is small, about 10 W/m²K, so capacity is set almost entirely by surface temperature. Floors too hot are uncomfortable underfoot and ceilings too cold condense, so the usable range is narrow. On the other hand, perceived temperature averages air and surrounding surfaces, so heating can keep air a little cooler. Let's calculate floor and ceiling capacity and the operative temperature used for comfort.

Capacity characteristic curves

$$ q_{floor,heat} = q_{ceil,cool} = 8.92\,\Delta T^{1.1},\qquad q_{floor,cool} \approx 7\,\Delta T,\qquad q_{ceil,heat} \approx 6\,\Delta T\quad[\mathrm{W/m^2}] $$

ΔT is surface minus room temperature. Floor heating and ceiling cooling put the warm surface below or the cool one above, promoting convection and higher coefficients. The reverse (floor cooling, ceiling heating) stratifies the air and gives less capacity.

Operative temperature

$$ t_{op} \approx \frac{t_a + \bar t_r}{2},\qquad \bar t_r \approx \frac{\sum_i A_i t_i}{\sum_i A_i},\qquad h_r = 4\varepsilon\sigma T^3 \approx 5\ \mathrm{W/m^2K} $$

Coffee Break Trivia

Roman underfloor heating

Radiant heating is ancient. Roman baths and villas used the hypocaust, passing hot furnace gases under floors and through walls. Korea's ondol likewise warms floors by running stove smoke beneath them. In the 20th century, hot-water pipes embedded in floors and concrete spread, and today whole building structures are sometimes heated and cooled with water.

Worked examples

Example 1: surface temperature and capacity

Mode	Conditions	Capacity
Floor heating	29 °C surface, 20 °C room	100 W/m²
Floor heating	27 °C surface, 20 °C room	76 W/m²
Ceiling cooling	17 °C surface, 26 °C room	100 W/m²
Floor cooling	19 °C surface, 26 °C room	49 W/m²
Ceiling heating	30 °C surface, 20 °C room	60 W/m²

The radiative coefficient at emissivity 0.9 and 20 °C is about 5.1 W/m²K.

Example 2: air temperature for 21 °C operative (5×4×2.6 m room)

Heating	Surfaces (floor, ceiling, walls)	Mean radiant	Air needed
Radiators	19 / 19 / 18 °C	18.5 °C	23.5 °C
Floor heating	26 / 19 / 18 °C	20.1 °C	21.9 °C

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Just a 26 °C floor lets the air be 1.6 K cooler with the same warmth.

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The body loses heat by convection to air and radiation to surrounding surfaces. A warm floor reduces radiative loss, so slightly cooler air feels the same — and less heat escapes through leaks and ventilation. In this room, ventilation loss (0.5 air changes per hour, 0 °C outside) drops from 205 W to 191 W, about 7%. But the effect depends on surface area and temperature, and floor temperature is capped by foot comfort, so capacity is limited. Poorly insulated buildings often need supplementary heating beyond floor heating.

Design tips

Compare the required load with the capacity allowed by surface temperature limits.
For cooling, control indoor dew point and keep surfaces above it (combine with dehumidification).
Subtract area covered by furniture and rugs.
Assess comfort with operative temperature or PMV, not air temperature.
Concrete-embedded systems respond slowly; use predictive control.

Coffee Break Trivia

“The ceiling sweated”

In one office with ceiling cooling, condensation formed on the ceiling on a rainy-season morning and water dripped. Ventilation had stopped overnight, and when staff arrived humidity rose until the dew point exceeded the ceiling surface temperature. The fix added control keeping chilled-water temperature above the measured dew point and pre-occupancy dehumidification. Radiant cooling needs humidity control as much as temperature control.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Ignoring surface limits	Capacity overrated	Check limits and dew point
Heating coefficients for floor cooling	Capacity doubled	Mode-specific curves
Comfort by air temperature only	Over-heating	Use operative temperature
No humidity control	Condensation	Dew-point control
Ignoring covered area	Under-capacity	Use effective area

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