Central Heating Thermistor. This thermistor serves as both the flow and return thermistor on an FEB24ED3*. For earlier FEB24 models, this only serves as the central heating thermistor and an FCB1080 is required for the hot water FEB24EFEB24EDFEB24ED3*FEB24E Combustion CircuitFEB24ED Combustion CircuitFEB24ED3* Combustion CircuitFEB24ED3* Water Circuit