Half the Radiators Stay Cold and the Circulator Hums: A Four-Step Check

When some radiators stay cold while others overheat and the circulation pump hums, the pump is rarely undersized. Work through the system first: trapped air blocks flow and makes the pump noisy, so vent the high points and each radiator; uneven distribution between near and far circuits points to a hydraulic balancing problem rather than a lack of head; then check whether the pump speed or control mode matches the system and whether filters are clogged. Only after these steps should installation faults, resonance or worn bearings be suspected — dismantling the pump and any electrical work must be left to qualified personnel. You can compare control modes and official curves on our Wilo product pages, or send us your emitter type, which rooms stay cold and the pump model and setting, and we will help you narrow down the cause.