Not sludged , just really black water.
Systems only 18 months old , grant vortex pro external boiler , new delonghi radiators and a mix of new copper and new poly pipe.
Fitted with sentinel inhibitor – double dose. All rads heat evenly and work well
Last time i dropped it and cleaned it at the 12 month mark( which coincided nicely to me needing a radiator off for decorating) the fluid came out like straw.
Tbf i only bled the expansion tank last night via the safety valve but it wasnt pretty …. My patios now stained black.
Dont worry ill be getting some cleaner in the system and then flushing it through but it was suggested i fit a magnaclean device at the same time to minimise it happening again.