A humming recirculation pump does not prove that hot water is circulating. Check the system type, intended operating period and safely visible valve arrangement, then have actual circulation assessed. A closed path, bypass installation issue or pump fault needs a different repair. Do not open a hot, pressurized pump or leave it running without the required water circulation.
Identify the loop and control
Find the pump model and the installation diagram. A dedicated return line and an under-sink crossover valve route water differently. Grundfos's cited Comfort System uses a valve at a distant fixture and the existing cold line as the return. Its separate COMFORT product booklet describes a broader pump range; those products are not all the same timer-equipped system.
Compare the selected control state with the relevant manual. A timer, demand button or temperature control can deliberately limit operation. For the timer-equipped system, setting a Grundfos Comfort System timer explains its operating positions.
Look for a changed water path
Report whether the symptom began after plumbing service, valve replacement or a heater change. Observe accessible isolation valves and hose routing without loosening fittings or forcing a valve. A closed flow path or an incorrectly installed crossover can stop the intended result even when the pump makes sound.
Grundfos's Comfort System fault chart separates a pump that does not run from no hot water at the faucet; the latter directs attention to the bypass-valve installation. That does not mean every noisy pump has a bad bypass. A dedicated return system needs its own flow and valve checks.
If hot water appears only while the pump runs, hot water depending on the recirculation pump addresses that different pattern. This page concerns an apparent running pump that fails to provide the expected circulation.
Avoid opening the pump to prove it runs
The cited Comfort System instructions say the pumped water cools and lubricates the pump and must be allowed to circulate during operation. Do not leave it running against a known closed path or open an air-vent screw on a hot, pressurized appliance to experiment. Service that opens the water path requires the correct power isolation, cooling and pressure precautions.
Have a technician establish actual flow, control operation and installation before replacing the motor. Record whether every fixture or only one branch is affected. Whether a recirculation pump is worth installing covers its purpose; correct diagnosis keeps a failed circulation path from becoming an unnecessary pump replacement.