It's a boiler fitted in 1986 by prev. owner boiler type is a Glow-worm fuelsaver 'F', 8 rads are fed from it aswell as tap hot water. the boiler stat has a range of 1-5, if I try and turn up boiler stat from it's current setting of 2 upto 3+ then the rads start to go cold and tap hot water heat is increased to unbearable temp. Do not understand why as boiler turns on and off correctly and supplies the water correctly at setting 2. The hot water tank has an independant stat that is new. Any ideas what could be causing this problem as when weather gets colder aren't able to increase rad water temp? Thanks oldboiler.