Radiators are only one of the options for heating a room. There are many others available which can be useful in particular situations.
Fan assisted convectors can be useful if space is limited and a conventional radiator of the appropriate size cannot be fitted.
Long low level skirting heaters can be used where you don’t want to intrude on the wall space of a room.
You can even install special plastic pipe in solid floors to provide under floor heating.
How radiators work
The odd thing with radiators is that they are actually somewhat misnamed. Most of the heat which comes from them is actually by convection. Most modern radiators have fins on the back, or between its panels if it is a double. This has enabled smaller radiators to be used as the fins vastly increse the surface area available for giving off heat.
Radiators work by heating the air which flows past them. Warm air rises from the radiators and colder air in the room falls. This circulation develops a flow of air around the room sending warm air from the radiator and delivering cooler air back to be heated. So, for radiators to work well there must be adequate clearance around them so that air flow isn't restricted. Now you know why radiators are mounted off the wall a little and above the floor.
The size of radiator required for a room depends upon two factors. Firstly the temperature that you want it to be able to maintain. That is a relatively straightforward task, but you can use this table as a guide
IDEAL ROOM TEMPERATURE
Secondly, the heat loss from your room. The calculations for this are actually quite complex since they depend upon the size of windows, number of doors and, in particular, the construction materials used to build the house. Consult a local heating engineer if you need to fit radiators in a previously unheated room or a new extension.
If you are replacing an old radiator, make a note of its details and replace with one of similar heat output capability.