There are a number of different view points on the best positions for radiators within a room. So here is a summary of where you should position your radiators and why.
The general consensus of opinion is that the best location for a radiator is under a window and there are two very practical reasons for this.
Firstly, windows provide cold spots within a room and this causes thermal currents (slight drafts) to develop within the room. This is because the window cools the air that comes into contact with it and this in turn causes that air to fall below the window and be replaced by warmer air that is sucks from another part of the room. This result in what is known as a convection current or movement of air within the room.
Placing a radiator under a window removes this cold spot and the warm air rising from the radiator counters the cool air created by close proximity with the window. This in turn removes drafts and makes a room feel warmer.
Secondly, because rooms contain doors and windows the remaining wall space tends to be utilised for furniture and human passage. By positioning a radiator below a window it automatically sits in an area that would not otherwise be used, i.e. it does not take away anymore wall space. This makes under window locations ergonomic.
If you have a large room that requires multiple radiators it is important to locate them symmetrically within the room to balance the emission of heat and minimise cool air currents.
Placing multiple radiators in one part of a room whilst omitting them from another zone will set up a cool-to-warm air flow that will result in the room continually feeling like there is an internal breeze.
Also, always opt for multiple smaller radiators rather than one large one.
Radiator covers can be effective at disguising ugly or unsightly radiators, but they also reduce the ability of the radiator to emit its heat.
This is particularly relevant to modern convection style radiators that rely on an air flow being drawn up and through the vents of the radiator for maximised efficiency. If the free passage of air around the radiator is restricted, the radiator will give off less heat.