Honeywell, L4006A1678/U, High and Low Limit Aquastat, 5 to 30 deg F

Honeywell L4006A1678/U – Aquastat® Controller with Immersion Bulb, 100–240°F Range

The Honeywell L4006A1678/U is a single-function Aquastat® control designed for regulating water temperature in hydronic heating applications. It features an immersion-type bulb sensor for direct insertion into boilers, tanks, or piping systems, and is used to break or make electrical contacts based on fluid temperature rise.

This line-voltage temperature control includes a visible setpoint dial, snap-acting micro switch, and adjustable differential for application-specific calibration. Suitable for use as a high limit, low limit, or circulator control depending on wiring configuration.

Product Features

• Setpoint Range: Adjustable from 100°F to 240°F
• Differential: Adjustable from 5°F to 30°F
• Electrical Rating: 5.1 A to 48 A resistive, depending on voltage and load type
• Switch Type: SPDT snap-acting switch; breaks on temperature rise
• Mounting Options: Suitable for horizontal or vertical immersion wells (well not included)
• Capillary and Bulb Assembly: Fixed bulb, 3/8 in x 2-7/8 in copper
• Setpoint Scale: External visible dial for on-site adjustment
• Control Voltage: Compatible with 120–240 VAC line-voltage systems

Technical Specifications

• Model: L4006A1678/U
• Capillary Length: N/A – fixed bulb configuration
• Temperature Sensing Element: Immersion bulb (copper)
• Application: Temperature regulation in hydronic systems
• Mounting: Direct to well (requires appropriate 3/4" NPT immersion well)
• Standards Compliance: UL Listed, CSA Certified

Installation Notes

This controller is intended for use by trained HVAC or plumbing professionals. Proper well sizing and insertion depth are required for accurate temperature sensing. Wiring must comply with applicable electrical codes and match system voltage and load ratings. The L4006A1678 is not designed for remote sensing or low-voltage control systems.

Use Case Examples

• High-limit control for boiler protection
• Operating control for water heating systems
• Circulator control based on water temperature thresholds