ELI5, why do we usually have radiators under windows? Doesn’t the heat just go up then straight out the window?
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Not if the window is shut, no. In buildings of an age to have radiator heat, the windows are not automatically a primary source of heat loss. With proper storm windows and good maintenance, even quite old windows are on par with most modern windows.
So placement has a few main factors.
There’s two kinds of heating systems that use radiators: steam and hot water. You can tell which kind is in your building by counting the pipes connected to each radiator. One pipe, steam; two pipes, hot water. Steam will more often be found in smaller buildings, say 3000 square feet and less.
An important factor in placement in both systems is that there has to be at least one large cast iron pipe running from the boiler to each radiator. We want to use the least pipe that’s practical, because it’s expensive, and heavy, and it all has to be heated before the radiator can get hot.
In a typical steam system, there’s a loop of quite large pipe in the basement, like 4″, and there’s a branch of 1″ to 2″ pipe from the loop to each radiator. (One reason steam is preferred over hot water for smaller buildings: less pipe total.) These pipes are all encased in a layer of insulation (warning: asbestos in older buildings!) that brings the diameter up to 4″ or more. The branch pipes have to run through the building, and in a wood framed building, that’s typically easier to do in exterior walls due to the old framing techniques in use when radiator heat was prevalent. Balloon framing makes a huge difference when pipes have to run up 2 or three floors. The pipes tend to cross joists as seldom as possible and they almost never cross studs. Sometimes they run inside an interior wall, but not all that often because the large diameters prohibit it. They don’t often share a wall cavity with plumbing because there’s not room for both. So that pretty much puts the radiators on the exterior walls, especially on upper floors. And as others have mentioned, once the radiator is on an exterior wall, it might as well fit under a window, where there’s less often going to be furniture.
Hot water systems share some principles, but scale better because each radiator has an input and an output pipe, and they’re chained together. Output from one radiator becomes input for the next. But again this tends to place the radiators on exterior walls, because the radiators are at floor level and to use the least pipe, the pipes are also at floor level. If the radiators were in the interior of the building, it would be far more difficult to arrange the pipes so as not to interfere with doorways.
My own home, a 1929 Tudor Revival, had 14 radiators originally. Only 6 were under windows. It’s not really about the windows necessarily being drafty. I’ve never noticed a difference between rooms where the radiators are under windows versus not. Covered or not covered makes a bigger difference to heat distribution within the room, but again not THAT much. But being a Tudor, there are lots of casement windows, and being 91 years old, they open inward. Can’t really put furniture there, so putting a radiator there makes lots of sense.
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