Radiators only partially warm or remain cold while boiler runs, flow and return pipes at boiler are hot.

Safety first: Allow system to cool before removing filters or bleeding radiators. Have containers ready for dirty water.

Step-by-step checks:

• 1. Check radiator valves: Ensure all radiator valves (including thermostatic radiator valves) are open and not stuck.
• 2. Clean boiler filter: If your Viadrus boiler has an integrated return filter, isolate the boiler, remove the filter cap, and clean the mesh thoroughly. Refit and check for leaks.
• 3. Bleed radiators: Bleed all radiators starting from the lowest floor. Re-pressurise the system as needed.
• 4. Assess sludge: If radiators are hot at the top and cold at the bottom, or water is very dark when bleeding, a power flush or chemical clean is recommended by a professional.

When to stop: If circulation remains poor after basic cleaning and bleeding, the pump may be undersized or failing, or the system may require professional flushing.