Honeywell Aquastat L4064B2210/U

This is an aquastat, a mechanical thermostat for boiler systems that senses water temperature and directly controls the burner circuit. Its switching element is a snap-action switch, and its sensing element is either a bimetallic element or a liquid-filled capillary. The device is rated at 8 A at 120 VAC, 4 A at 120 VAC, and 24 A at 240 VAC, with a switching current of 14 A at 120 VAC and 7 A at 240 VAC. It features a switching differential of 25 bar and a maximum temperature of 121.11 °C, with an operating temperature of 40 °C. The reset type is automatic or manual lockout, and installation is via a universal surface-mount bracket with an immersion probe.

Aquastats of this type are selected by the required switching temperature, the differential between cut-in and cut-out points, and the reset type (automatic or manual lockout). The probe form must match the boiler tapping—immersion, clamp-on, or sleeve—and the mounting thread or flange must be compatible. When replacing, the new unit must have the same temperature range, differential, and contact configuration as the original. Wiring must match the burner control circuit, and the probe must be correctly seated in the thermal well for accurate sensing.

Failures of this kind typically involve a drifting switching characteristic, causing the boiler to cycle incorrectly or fail to modulate. Mechanical wear and capillary damage are also common. Symptoms include the burner not cutting off at the set limit, or short-cycling. Checking the switching points and the condition of the sensing element can help diagnose the issue.