Radiators do not get hot even though the boiler appears to run

Safety first: Allow radiators to cool before bleeding to avoid scalds.

• Check room thermostat and programmer: Ensure heating is actually being called for and that thermostatic radiator valves (TRVs) are open.
• Bleed radiators: Starting from the lowest floor, bleed each radiator until water flows steadily. Top up system pressure as needed.
• Feel flow and return pipes: If the boiler flow pipe is hot but returns are cold, circulation is restricted.
• Check motorised valves (engineer task): The engineer should verify that any 2-port or 3-port valves are opening when heating is demanded.
• System flush (engineer task): If some radiators remain cold or partially warm, a chemical or power flush may be required to remove sludge.