Overview
The Mercoid M-51-71 air temperature switch is a heavy-duty bi-metal control device manufactured by Mercoid (a division of Dwyer Instruments). This automatic-reset temperature switch delivers proven reliability in HVAC systems, furnaces, air conditioning ducts, ovens, and dryers across industrial and commercial facilities. The unit features a visible dial temperature indicator, internal moveable adjustment pointers, and a rugged screw-terminal design for straightforward installation and commissioning.
Key Features
- Bi-metal construction with double adjustable set and reset points for precise temperature control
- Automatic reset operation with switch action on temperature rise, 3-opens configuration
- Adjustable deadband from 5°F minimum to full range for flexibility in control strategy
- Visible temperature dial display with internal pointer adjustments for easy setup and diagnostics
- Screw-terminal electrical connections for reliable, maintenance-free contact
- Mounting flange reversible for both flat and curved duct surfaces
Technical Specifications
| Part Number | M-51-71 |
| Manufacturer | Mercoid (Dwyer Instruments Division) |
| Product Type | Bi-Metal Air Temperature Switch |
| Temperature Range | 25 to 125°F (-4 to 52°C) |
| Minimum Deadband | 5°F (adjustable to full range) |
| Switch Action | Closes on temperature rise; 3 opens |
| Reset Type | Automatic reset |
| Electrical Rating | 3/4 HP @ 120/240 VAC single phase; 1/3 HP @ 120/240 VDC |
| Electrical Connection | Screw terminals |
| Insertion Length | 12 inches (305 mm) |
| Conduit Hub | 1/2″ NPT (12.7 mm) |
| Mounting | Mounting flange; reversible for flat or curved surfaces |
| Weight | 5 lb (2.27 kg) |
| Certifications | UL listed, CSA certified |
| Display | Visible temperature dial with internal adjustment pointers |
Typical Applications
- HVAC Systems: Duct air temperature monitoring and limit control in commercial and industrial heating/cooling systems
- Furnaces: Overheat protection and fan/burner control in residential and commercial heating equipment
- Industrial Ovens & Dryers: Temperature-based switch operation in bake ovens, drying ovens, and similar thermal equipment
- Fire Damper Control: Duct temperature monitoring to prevent spread of fire through air conditioning and ventilation systems
- Alarm & Limit Switches: Temperature-triggered alarms and motor shutdown in process control applications
Compatibility & Replacements
Compatible With
- Furnace blower motor control circuits (120/240 VAC single phase)
- HVAC system fan relays and damper control circuits
- Industrial oven and dryer control panels
- Damper control systems (1/2″ conduit hub connection)
- Alarm and limit-switch circuits in heating/cooling systems
Replaces / Drop-In Replacement For
- Earlier Mercoid M-51-71 units (exact model match)
- Generic bi-metal duct temperature switches in the 25–125°F range with equivalent SPDT or 3-opens switch configuration
Works With
- 120 VAC single-phase motor control circuits
- 240 VAC single-phase circuits
- 120/240 VDC control systems
- Manual and automatic heating/cooling control loops
- Standard HVAC relay panels and contactor circuits
