central heating switched to off, some radiators red hot all the time ?

we have a 20 yr old potterton ep2000 boiler. even when we switch the central heating to off, there are 3 radiators red hot all the time. Even turning the individual radiator thermostats doesn't help. People have mentioned some sort of valve - can anyone give me more details? I am dying of heat exhaustion and it's only april!!! Please help???