The Honeywell T675A is an analog remote bulb temperature controller designed for HVAC and industrial process applications that need simple, reliable on-off switching without external power. A fluid-filled capillary sensor and copper bulb mechanically actuate a single-pole, double-throw (SPDT) relay at a user-adjustable setpoint. The controller mounts in any position—on walls, ducts, or pipes—and compensates for ambient temperature at the housing location. Note: The T675A series has been discontinued; existing installations are supported with spare parts while Honeywell recommends upgrading to the T775 electronic controller for new projects requiring more precise control and remote setpoint management.

How It Works—Remote Bulb, No Wires Needed

• Fluid-Filled Capillary System: The copper sensing bulb is connected to the controller by a capillary tube. Temperature changes in the bulb cause the fill fluid to expand or contract, which drives a bellows mechanism that trips the SPDT switch at the calibrated setpoint.
• No External Power for Switching: The switch actuation is entirely mechanical—the 120/208/240/277 V AC rating applies to the load circuit, not to power the controller itself. This makes the device ideal for fail-safe applications where control must continue during a power interruption.
• Ambient Temperature Compensation: A built-in compensator adjusts the setpoint reading to account for temperature at the controller housing. This helps maintain accuracy when the controller is mounted in an unconditioned space while the sensing bulb is in a duct or tank.

SPDT Switching for Direct Actuator Control

• Single-Pole, Double-Throw: The switch breaks R-B and makes R-W on temperature rise. When the sensed temperature drops back below the setpoint, the switch reverses—breaking R-W and making R-B. This allows one controller to both open a cooling valve and close a heating damper with the same signal.
• Adjustable & Fixed Differential: Select models have an adjustable differential (3–10 °F), allowing field tuning to match system inertia. Fixed-differential models (typically 1 °F) are available for applications where tight switching is needed.
• Load Rating: 125 VA pilot duty at 120/208/240/277 V AC, sufficient to drive most HVAC dampers and solenoid valves directly without an intermediate relay.

Mounting & Accessories

• Mounts in Any Position: The rectangular enclosure (approx. 5-5/8" × 2" × 2-5/8") can be screwed to walls, strapped to pipes, or affixed to duct surfaces.
• Installation Hardware: Honeywell-supplied mounting brackets, clamps, and sun shields (p/n 34886A/U) are available for external installations and conduit mounting.
• Capillary Lengths: Stock models come with 5 ft or 20 ft capillaries, but many lengths can be ordered or worked through a dealer compatible with Honeywell OEM software.

Replacement & Retrofit Options

• T675A Direct Replacements: Look for T675A models with the same capillary length and differential type (adjustable or fixed). Cross-reference Honeywell part numbers such as T675A1029 (20 ft capillary, adjustable 3–10 °F diff.), T675A1508 (5 ft capillary, adjustable diff.), or T675A1136 (20 ft capillary, fixed 1 °F diff.).
• Electronic Upgrade Path: Honeywell recommends the T775 electronic remote temperature controller for new installations. T775 provides digital accuracy, programmable setpoints, and remote sensing without capillary—while retaining the same mounting footprint and 125 VA output.
• Spare or Repair: Discontinued status means limited stock availability at some distributors. Buyers should verify lead times with their Honeywell BMS distributor before finalizing a replacement plan.

Applications

• Heating, cooling, and heating-cooling damper control
• Boiler and tank temperature regulation
• Pipe, duct, and stack temperature monitoring
• Industrial process cooling and free-cooling changeover