If the unit can’t push the supply temperature above 25°C and then drops back to 10°C, something is preventing proper heat transfer. Even with correct settings and defrost working, the loop may not be flowing as expected. A missing buffer tank can also cause fast cycling, because the system has too little water volume to work against. Air in the system, a circulation issue, or a sensor reading out of range can all lead the heat pump to shut down early. Checking flow rates, purging air, and confirming that the external pump is controlled correctly are good first steps. A technician may need to verify that the heat pump isn’t limiting itself because of a protection warning.