Boiler runs but radiators remain cold or only lukewarm, while the boiler flow pipe is hot.

Safety: Allow radiators to cool before handling valves.

• Check radiator valves: Ensure both the thermostatic radiator valve (TRV) and lockshield are open on cold radiators.
• Feel flow and return: Compare temperatures of pipes at the boiler and at radiators. Large differences indicate flow restrictions.
• Check zone valves: On systems with motorized valves, feel if the valve body is warm and the lever moves freely when calling for heat.
• Professional flushing: A technician may need to power-flush the system, clean or replace blocked radiators, and adjust balancing.