They're not running now, but in a few months, the radiators in my new apartment will become quite hot!
Practically, I just don't want them to get burned and I don't mind them sitting on or jumping off the radiators to get at the windows year-round.
If you are talking about the standard radiators used in most of Europe, i.e. central heating with hot water, those are perfectly safe for cats.
They get hot enough to make it uncomfortable to touch them for prolonged periods of time, but typically not hot enough to cause burns, at least not from touching until you notice it getting uncomfortable.
The ancestors of our cats are desert dwellers, so their paws are slightly more resilient to hot surfaces than our feet and hands to begin with, and their fur serves as an insulator, making it a little more difficult to burn the skin when briefly touching a hot surface.
Generally, cats will not walk or lie down too much on most radiators in my experience, as it isn't very convenient to do, but in the colder months they might make a habit of sleeping on the window sill above the radiators, or on the ground below the radiators.
There are radiators that can be dangerous, the ones that are gas powered and heat up a metal mesh until it is red-hot. If those do not have a proper safety grating, a pet might burn itself, especially dogs are in danger of burning their tails.
Another concern with these is that the heating elements do get hot enough to set stuff on fire, so long-haired pets (or humans, especially children) might be at risk even if the unit has a proper safety grating that sufficiently prevents direct touch.
