Wall furnaces require reliable internal temperature regulation to maintain efficient heating and avoid over-temperature conditions. A remote bulb capillary switch offers a mechanical, power-free solution that controls burner operation by sensing heat inside the unit. These switches are ideal for OEM wall furnace designs using millivolt gas valves or non-electronic control systems. To specify a remote bulb capillary switch for your wall furnace design, contact our team. We offer engineering support, datasheets, and product samples for OEM evaluation.
Mechanical Temperature Control for Wall Furnaces
The sensing bulb is mounted within the furnace enclosure, typically near the base where heated air accumulates. Inside the bulb, a temperature-sensitive fluid expands as air temperature rises. This expansion increases pressure within the capillary tube and actuates a mechanical switch in the control body. When the temperature reaches the user-defined setpoint, the switch opens the circuit to the gas valve, shutting off the burner. As the furnace cools, the pressure drops, the circuit closes, and burner operation resumes.
This on/off cycling provides consistent thermal performance without requiring external electrical input. The switch connects directly to standard millivolt gas valves using TH and TH/TP terminals and functions in the same manner as a wall-mounted thermostat—without the need for remote wiring.
Technical Specifications for Wall Furnaces
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Switch Function: Single-pole, double-throw (SPDT), mechanical
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Sensing Bulb Material: Copper, dull nickel plated, or stainless steel options available
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Bulb Diameter: Common sizes include 5/16″ and 3/8″
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Capillary Length: 12″ to 72″ standard; custom lengths supported
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Setpoint Range: 50°F to 200°F (10°C to 93°C) typical for wall furnaces
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Switch Differential: 5°F to 15°F, depending on configuration; custom options available
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Electrical Rating: Compatible with millivolt systems; low current switching
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Reset Options: Automatic reset standard; manual reset available for high-limit designs
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Mounting: Panel or bracket mount with external knob for user setpoint adjustment; additional brackets available
Common Installation and Integration
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Bulb Placement: Position the sensing bulb at the base of the unit where air temperature reflects heating operation. Use clips or brackets for secure thermal contact.
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Capillary Routing: Route the tube along the interior wall of the furnace casing using thermal-resistant clips. Avoid exposure to flame or obstruction.
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Knob Access: Install the control knob on the outer casing where it is easily accessible to the user.
High-Performance Remote Bulb Capillary Switch for Wall Furnace Control
At Stemco, we manufacture a range of single-stage bulb and capillary thermostats designed for high-load temperature control and safety shutoff applications. These devices are available in both fixed setpoint and field-adjustable models and can be configured with either manual or automatic reset depending on system requirements.
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Current Rating: Up to 25 amps resistive load
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Temperature Range: -40°F to 700°F setpoints
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Ambient Operating Limit: Up to 250°F
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Contact Types: SPST and DPST models
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Reset Configurations: Automatic or manual, based on safety and application needs
These thermostats offer durability and performance in demanding furnace environments, providing both control and safety switching functions.
Designed for OEM Wall Furnace Applications
Stemco devices are engineered specifically for OEMs manufacturing wall-mounted gas furnaces. Our mechanical switches and thermostats simplify system design, reduce component failure points, and provide precise thermal regulation compatible with millivolt control systems. Products can be customized to meet specific mounting, sensing, or control interface requirements.
To request technical documentation, CAD drawings, or product samples for evaluation, please contact us or visit the product pages.
