Hayward Thermistor, FD, FDXLTER1930

The Hayward FDXLTER1930 is the thermistor for the Universal H-Series Low NOx heaters, covering models H150FD, H200FD, H250FD, H300FD, H350FD and H400FD. It is the genuine Hayward replacement.

What it does

The thermistor is the heater temperature sensor. Its resistance changes with water temperature, and the control board reads that to decide when to fire the burner and when to shut it down. Every temperature decision the heater makes rests on this one small part.

Why it is worth checking first

When a heater reads a temperature that is obviously wrong, short cycles, or throws temperature related faults and lockouts, the thermistor is a common cause and one of the cheaper things in the heater to replace. It is worth ruling out before you start looking at boards and gas valves.