Honeywell Immersion thermostat with manual reset L6189C2007U

This immersion thermostat is a mechanically operated high-temperature limit switch with manual reset. It uses a liquid-filled sensing element and a remote bulb to open its SPDT contacts when the water temperature reaches the set point. The operating range is 40–110 °C, and the device is rated for ambient temperatures between 0 and 70 °C. The electrical contact rating is 10 A resistive or 2.5 A inductive at 250 V. The immersion well is not included and must be ordered separately.

For a replacement of this type, first match the switching point (temperature at which the contacts change state) and the differential, which must correspond with the system's design. Confirm whether the original has automatic or manual reset; for a limit application the reset type must be identical. Also verify the contact rating and switching logic (normally open or normally closed) against the wiring diagram. The probe form must suit the installation: immersion types need a matching immersion well for the remote bulb, and the well itself must be selected separately if not already fitted. Finally, compare the mechanical dimensions and mounting arrangement with the original to ensure the new unit fits the well or pocket.

Typical failures of this type include drift of the switching point caused by ageing of the sensing element, corrosion inside the immersion well, and rupture of the capillary tube on remote-bulb units. A distorted switching point makes the boiler cut out at the wrong temperature, either too early for proper heating or too late for safety. A broken capillary tube usually causes the safety circuit to open, leaving the burner locked out. Poor thermal contact between the bulb and the well can lead to erratic on/off behaviour, so inspect the well and ensure the bulb is properly seated.