Nibe 518721 with connecters is a temperature sensor used across many older and current Nibe heat pump systems. 

Key Features
•     Sensor type: Copper NTC
•     Resistance: 3.3 kΩ at 25°C
•     Function: Measures temperature for system regulation (flow, return, brine, outdoor, buffer tank depending on model)

Compatible Nibe Models
This sensor is used in many Nibe heat pumps, including:
•     F370 series
•     F470 series
•     Fighter 1120, 1110, 1140, 1240, 360P, 640P, 410P
It also fits a wide range of Fighter and VVM/ACVM units

Sensor?
If your Nibe system is older or part of the Fighter families, this is likely the correct replacement. But because Nibe has many variants, it’s always smart to confirm compatibility with your exact model number.