There are many possible reasons why your crown boiler radiator may not be heating properly. Possibilities include defective zone valve, trapped air in the radiator, defective circulator, or faulty pump seal. The most common problem is failed zone valves; if this component fails, heated water can't escape from the boiler and the system will overheat. Repairing a failing zone valve typically involves replacing the entire unit. Other potential causes of no heat include blocked pipes near the boiler or defective radiators, which must be replaced as a whole. Finally, if there's an issue with the circulator or pump seal, these components just need to be lubricated and/or replaced as necessary.