The warmest place in a room is between the radiator and the wall behind it.
A vast amount of heat from a radiator is lost heating the wall behind it, or the window above it, and often the world outside.
With a double radiator more of the heat will get into the room but, the comfort factor relies on radiant heat, while most of the heat is convected rising to heat the walls and ceiling and this is lost to you. A better idea is to follow the continental system, which is to fit radiators sized 50% bigger than you expect to need.
In real life the cost difference cannot be certain, but the comfort will be noticed.
The original heating design for your property would take into consideration the radiator sizes and and KW requirement of the boiler to supply the calculated demand. Meaning that the boiler may only have sufficient capability to supply the original radiarors.
Shoiuld you double the heating output of the radiators then you may be demanding a load that the boiler cannot supply and cause it to run indefinately at great cost in fuel.
Should the boiler be big enough and you go ahead with rad changes then take advice from htg engineer and fit thermostatic rad valves.
Difficult to calculate cost as there are a few factors to be considered. Like programmed on times, time taken to reach required room temps, boiler flow temperature setting, boiler size etc,etc,
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