Master Thermostat MASTER 4240.290 (automatic reset) 4240.290

The Master Thermostat MASTER 4240.290 (automatic reset) is a mechanical thermostat with automatic reset, designed to open or close a circuit when the sensed temperature reaches a set point. It functions as a limit device, cutting the burner or pump if temperature exceeds a safety threshold. The probe can be immersion, clamp-on, or sleeve type. This thermostat features a drifting operating point and a broken cable as potential failure modes.

When selecting a replacement thermostat of this kind, key parameters include the switching point, reset differential, probe form (immersion, clamp-on, or sleeve), electrical rating (voltage and current), cable length, and connector type. The reset characteristic must be automatic, and mechanical dimensions must allow proper fitting into the existing pocket or clip.

Common failures include a drifting operating point, where the switching temperature shifts over time, leading to erratic burner behavior. A broken cable or poor thermal contact between a clamp-on probe and the pipe can cause the device to sense the wrong temperature, resulting in nuisance trips or lack of protection.