eli5: Why are radiators in houses often situated under a window- surely this is the worst place and the easiest way to lose all the heat?
Not necessarily – between my century home with oil fired steam radiators and several older apartments, just because windows are OLD, doesn’t mean they can’t be relatively good insulators (my 100+ yr old house had original double pane windows that were surprisingly good) – yes, a lot of old windows suck, but not all.
In any case, the positioning of a radiator is always going to fight with wall-space for tables, sofas/chairs and bookshelves, shelving units, dressers etc. Modern heat and return air vents can go UNDER or BEHIND these things with some limits. You _tend_ not to put these things right in front of windows, so the wall space under the window isn’t going to be occupied, so why not put the radiator there?
Radiators used to be placed under windows to help the flow of warm air around the room as it hits the cold air from the window. This is less common in newer homes as windows are better than they used to be. (Triple pane, etc)
Don’t forget you also need to fight the condensation on the windows. Radiators under the window help with that, both before it forms and for evaporation afterwards.
It’s because the cold windows are the greatest heat loss from the room and the radiator under the window causes a rising current of warmed air in front of the window. It counteracts the cold down draught generated by the cold window.
If you were to put the radiator on the opposite internal wall (as seems logical) then you’d get the warm air rising to the ceiling, flowing across the ceiling and a descending draught of cold air flowing down past the window and across the floor. The warm up-draught from the radiator and the cold down-draught from the glazing would reinforce each other, The strong cold draught flows across the floor. People are most sensitive to cold draughts around their ankles.
It was more important in the days of single glazing and steel window frames.
TLDR: Radiators are placed under windows so that the rising warm air will counteract the cold down draught generated by the glazing. If the radiator were on the opposite internal wall, the cold down-draught and the warm up-draught would reinforce each other, which may prove uncomfortable.
It’s actually the opposite. You’re right that heat is lost through the window, and transversely cold air comes in through the window and creates a cold pocket in the room. The radiator under the window heats up that air pocket, which heat rises and carries it it the ceiling and spreads out through the room.