The Physics of Radiant Heat Transfer

Radiant heating and cooling — where a floor or ceiling panel warms or cools a room mostly by giving off or absorbing radiant heat rather than blowing air — has long been common in cold climates. As more of those homes add cooling systems (partly due to warming and wildfire smoke), and as the approach spreads to other climates, it helps to know what limits how much heating or cooling a radiant panel can actually deliver.

The underlying physics comes down to two things: how big the panel is, and how different its surface temperature is from the room's air temperature. A bigger panel or a bigger temperature gap moves more heat. Working through the numbers, a radiant cooling panel with a 10°F difference between panel and room can deliver roughly 2 tons of cooling for every 1,000 square feet of panel — in the range of 15,000 to 25,000 BTU/hr per 1,000 square feet. Radiant heating panels, run at a bigger temperature difference, do somewhat better: about 20,000 to 36,000 BTU/hr per 1,000 square feet.

For a homeowner, the practical point is that radiant systems have real capacity limits tied to panel size and temperature, so a radiant floor or ceiling can't just be cranked up indefinitely to cover a big heating or cooling load — the panel area and the design temperatures have to be sized to match the room. A separate follow-up will look specifically at whether radiant cooling makes sense in humid climates.