Boiler runs but radiators do not heat up or only get lukewarm.

Safety first: Allow radiators to cool before bleeding. Protect carpets and furnishings from water spills.

• Check system pressure: Ensure pressure is 1.0–1.5 bar when cold. Top up via the filling loop if needed.
• Bleed radiators: Use a radiator key to release trapped air from the top of each radiator, starting with the highest. Repressurise as required.
• Check TRVs and valves: Make sure radiator valves and TRVs are open and not stuck in the closed position.
• Engineer tasks: If performance remains poor, an engineer should check pump operation and speed, inspect for sludge, clean filters, and balance the system.