T678A1437 | Honeywell Remote Bulb 2-Stage Temperature Controller

Description

Product Overview

T678A1437 is a Honeywell mechanical remote bulb two-stage temperature controller (also known as aquastat). It uses a copper sensing bulb and capillary tube to measure fluid or air temperature remotely, providing dual SPDT relay outputs for two-stage heating/cooling control of boilers, hot water tanks, ducts and process piping. This model is discontinued by Honeywell and only available as surplus or refurbished spare parts for legacy HVAC and industrial thermal control systems.

Technical Specifications

Key Features

1. Two Independent Control Stages: Dual SPDT contacts for staged heating or cooling, enabling sequential load activation for energy management.
2. Remote Sensing Bulb Design: The copper bulb can be installed in pipes, tanks or air ducts, while the controller body mounts in a convenient accessible location away from high-temperature or wet areas.
3. Adjustable Differential: User adjustable temperature gap between stage 1 and stage 2 switch points to prevent rapid cycling of pumps, valves or burners.
4. Ambient Temperature Compensation: Compensates for cabinet ambient temperature variation to maintain accurate setpoint readings.
5. Mechanical Capillary Technology: No electronics; long service life in high vibration, humid industrial environments, simple field calibration.
6. Bulb Holder Included: Comes with bulb mounting bracket for easy insertion into boiler wells or duct probes.
7. Legacy HVAC Spare: Original production ended; used for maintenance replacement of existing boiler and district heating control panels.

Applications

• Commercial hot water boiler aquastat control
• Heating and chilled water tank temperature monitoring
• Air duct temperature control for large HVAC systems
• Industrial process fluid temperature two-stage regulation
• Retrofit of old Honeywell T678 series thermal control panels

Installation & Maintenance Notes

• Protect capillary tube from sharp bends, crushing or puncture; damaged capillary causes complete loss of temperature sensing function.
• Insert copper bulb fully into thermowell or direct contact with process medium; poor thermal contact leads to inaccurate readings.
• Keep controller housing away from direct heat radiation, moisture and vibration.
• Calibrate setpoint and stage differential after installation or bulb replacement.
• Do not splice or repair broken capillary tube; entire unit must be replaced if capillary leaks.
• Verify full part number match during spare replacement, confirm temperature range and capillary length.

FAQ

Q: What does two-stage mean for T678A1437? A: It has two separate SPDT switches that trigger at different temperature setpoints, used to turn on primary and secondary heating/cooling equipment sequentially.

Q: Can the capillary tube be shortened or extended? A: No. Capillary length is factory sealed and calibrated; cutting or extending will destroy the sensor.