On 13/01/2021 08:55, alan_m wrote:

The reason for radiators under windows is that you get a cold downward
draft from the window which even in a well heated room makes your feet
feel cold and you perceive that the room is much colder than it actually
is. Putting a radiator under the window counteracts this downward cold
draft.

The main reason is that it's generally otherwise unused space.

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Cheers
Clive