You seem to need a radiator every 4 or 5 squares in corridors. Every room needs at least one, bigger rooms 2 or 3. I think it works on a radius, at least in the corridors. Something like the character has to be within 3 spaces of a radiator or they start getting colder so make sure to have them near any waiting areas.
There is also supposed to be a setting to turn up the heating but I never found it. You need a whole lot of radiators at any rate.