Temperature Control PENN A421 Digital - Electronic Thermostat for Lobster Tanks and Seafood Refrigeration (120V / 208–230V)

If your lobster tank temperature is running too warm or your seafood refrigeration system is struggling to hold a consistent temperature, a failed or outdated analog thermostat is often the cause. The PENN A421 Digital Electronic Temperature Control is a professional-grade replacement used in commercial lobster tanks, live seafood display cases, and walk-in coolers. Unlike basic snap-action thermostats, the A421 features a precision NTC sensor with ±1°C accuracy, a digital backlit display, and fully programmable setpoints — so you always know exactly what temperature your tank is holding. It works with standard 120V and 208–230V systems, making it compatible with nearly every commercial lobster tank setup. The robust construction and reliable output relay are why seafood distributors, grocery stores, and seafood restaurants trust this controller to keep lobsters alive and healthy.

The PENN A421 is a line-voltage electronic temperature control designed for commercial refrigeration applications including live lobster tanks, seafood display cases, walk-in coolers, and produce misting systems. It operates on both 120V and 208–230V electrical systems, making it a versatile replacement for older analog or electromechanical thermostats. Temperature accuracy is ±1°C with a backlit LCD display and a touchpad interface. The adjustable temperature differential prevents short-cycling, which is a common cause of compressor wear in seafood refrigeration. It includes an anti-short cycle delay (ASd) setting to protect refrigeration compressors. The remote probe cable allows you to monitor water temperature directly inside the lobster tank rather than measuring ambient air. Compatible with single-stage cooling or heating applications. Available in NEMA 1 (standard) and NEMA 4X (weatherproof) enclosures.s

Mount the A421 in a location protected from direct water spray and excessive humidity. Connect the load (compressor or refrigeration unit) to the output terminal — make sure total amperage does not exceed 16A. Plug the remote sensor probe into the sensor port and route the cable so the probe tip is fully submerged in the lobster tank water, away from tank walls, heaters, or return flow jets. Power on the unit. Press the UP or DOWN buttons to set your desired OFF temperature (the temperature at which refrigeration stops). For lobster tanks, the typical OFF setpoint is 34–36°F (1–2°C). Set the ON temperature slightly higher to create a differential — a 2–4°F differential prevents short-cycling. Use the ASd (anti-short cycle delay) setting to add a time buffer between compressor cycles, which extends compressor life. To verify the reading is accurate, cross-check with a calibrated thermometer. If the unit reads differently than expected, use the sensor offset feature (hold UP and DOWN simultaneously for 5 seconds to enter advanced menu) to calibrate. Replace the probe every 1–2 years in saltwater lobster tank environments to prevent corrosion-related inaccuracy.s

Use this thermostat only as an operating control — it is not a safety-limit device. If your lobster tank temperature is running too warm or fluctuating unpredictably, check that the sensor probe is fully submerged and not touching the tank wall or heater element. Never exceed the rated load amperage (16A at 120V or 208–230V); overloading the output relay is a leading cause of control failure. If the display shows "SF" (sensor failure), inspect the probe cable for damage or a loose connection — a failed or disconnected sensor will cause the unit to default to a failsafe mode, which may allow tank temperature to rise and endanger your lobsters. Do not install in a wet or splash-prone location unless you are using the NEMA 4X weatherproof model. Avoid mounting near heat sources such as compressor housings or direct sunlight, as ambient heat will cause inaccurate readings.