The sun was the first heated ceiling

The reference nobody argues with

The sun has heated the Earth for as long as there has been an Earth, with no fan, no ducting and no servicing. It sits above, it emits infrared radiation, and it barely heats the air it crosses. Three characteristics no radiator brings together, and which a ceiling can reproduce.

What height changes

The sun does not heat the same way at noon and at the horizon. The higher it is, the more surfaces it reaches unobstructed. A ceiling is the highest point in a room: it sees the floor, the walls, the furniture and the people, and nothing gets in the way.

The analogy with lighting

Nobody puts a bulb on the floor to light a room. It goes on the ceiling, because that is where a source reaches the most surfaces. Infrared radiation obeys exactly the same laws as visible light, since it is the same physics.

The difference between warm air and radiation

On a sunny winter day the air is cold and you are warm in the sun. The demonstration makes itself: it is not the air temperature that decides what you feel, it is the radiation you receive.

What that implies for an installation

Reproducing that effect means a large surface at low temperature rather than a hot spot, and no obstacle between the source and the occupants. Those two conditions point to the ceiling, and they rule out a unit against a wall behind a sofa.

The principle is developed on the why heat from the ceiling page, and the comparison with other systems on the page about heating modelled on the sun.