A steam or hot water heating system will have radiators, in older homes they will be the cast iron dinosaurs that you are familiar with. In homes build from around 1960 on hot water heat is circulated thru "fin-tube" radiators which are just around the baseboards on the outside walls of the rooms. You need a certain number of square feet of radiating surface per cubic foot of room so you can either put it in a big radiator or two under a window or you can string it out around the circumference.

A forced air heating system delivers hot air from a central furnace through a system of duct work and vents with cold air return vents to each room. this is used where cooling is also needed as part of the central system where as radiator systems are more used in cold climates that don't need central air for summer time comfort.

Older homes have them. They are used with steam heating and we see steam heating as mostly obsolete. Newer homes are built with 'forced air' heating, so instead of radiators we have heat registers (air ducts). In many newer homes the same ducts are used for both heating and air conditioning.

Yes, heat registers are under windows, because windows are cold.

You can most homes have a heater with vents etc But yes some homes still have them

In the UK, almost every home will have a radiator positioned under it's window. Is this the case in America? What is the most common way of heating a home? If it is not with a radiator, is it still with convection currents - and is the heating device normally placed under the windows?

Thanks very much,

Pete