Hvac School - For Techs, By Techs

In this short podcast episode, Bryan covers some basics on bi-metal and magnetic switches. Bi-metal switches are made of two dissimilar metals that are bonded together and are often integrated into motors. When the temperature changes, the metals bend. In an overload circuit, they're designed to bend and open the circuit in an overheat condition. Mercury thermostats also use these; they have a bulb attached to a bi-metallic coil, which causes it to wind or unwind when heated or cooled. Fan limit switches also use bi-metallic coils. Bi-metallic discs also exist, and they snap open to make or break a circuit. Bi-metallic switches have a time delay and often auto-reset (with exceptions like rollout switches on furnaces), which is advantageous in some applications. However, they're also affected by the ambient temperature and are more likely to trip in warm weather and less likely to trip in cold weather. Nuisance trips are more common than in magnetic switches, and they may weld themselves shut and fail closed (