Cast iron radiators UK have a greater capacity than traditional radiators. That is, for two radiators of the same size, a cast iron one will have more water than a modern flat radiator. This leads to two contradictory issues:
1.On the one hand, they take a long time to heat up. This makes them less efficient because you have to spend more to heat them up…initially. But…
2.They retain their heat for much longer than other radiators do.
So, while the initial cost may be more expensive, once the radiator has reached temperature, it is actually far more efficient. By around the third hour, the cost comparison between cast iron radiators UK and other styles moves in favour of the cast iron unit i.e. after three hours, the cast iron radiator has become cheaper to run.
In case you're having trouble understanding this, let’s look at this example:
Cast iron radiator: costs £0.50 to get to temperature, then £0.15 for each subsequent hour. By the end of hour 3, it costs £0.95.
Modern radiator: costs £0.40 to get to temperature, then £0.25 for each subsequent hour. By the end of hour 3, it costs £1.15
Overall, then, providing you have your central heating system on for a prolonged period. The cast iron radiator is more efficient. Furthermore, once you switch it off, it will continue to produce heat for at least another hour or two, while the modern radiator will cool within half an hour. So, you get two free hours of heat on top.