How to Fix / Troubleshooting

Safety: Allow radiators to cool before bleeding.

Check programmer and thermostats: Ensure heating is actually being called for and room thermostat is set above room temperature.
Bleed radiators: Starting from the top floor, bleed each radiator until water flows without air.
Feel flow and return pipes: At the boiler, check that both flow and return pipes warm up. If only one is hot, circulation is poor.
Check diverter valve (combi/system): If hot water is fine but heating is poor, the diverter valve may be stuck. This requires an engineer to service or replace the valve assembly.
System cleaning: Cold spots at the bottom of radiators indicate sludge; a powerflush or radiator replacement may be needed.

Boiler runs but radiators do not get hot or only lukewarm

Possible Causes