Bi-metal & Magnetic Switches - Short #252

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 12 August 2025

⏱️ 12 minutes

🔗️ Recording | iTunes | RSS

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Summary

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 (though failing open is relatively common as well).

Magnetic switches are usually more external to the equipment or parts they're protecting (such as in the contactor or starter assembly instead of inside a motor itself). These switches are also more instantaneous and are better for mission-critical applications. These also respond to amperage and aren't affected by ambient temperature. They're more likely to fail open than closed.

Compared to bi-metallic technology, magnetic switches trip faster and are better for sudden issues rather than long-term operational overheating. In many cases, we use both of them to get the benefits of each.

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Transcript

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0:00.0	Hey there, Sonny.
0:05.1	We're going to talk about some old school technology today, buy metal switches.
0:09.6	Oh, by the way, this is the HVAC school podcast, the podcast that helps you remember some things you might have forgotten along the way as well as talks about some old, tried, and true tech and how it's different than some of the modern equivalents.
0:21.6	But before we do that, we want to thank our great sponsors.
0:24.9	Carrier and Carrier.com.
0:27.2	Navak at Navakglobal.com.
0:30.7	Santa Fe dehumidifiers in their new five-year complete replacement coverage.
0:36.6	Find out more at HVACR school.com slash Santa Fe.
0:41.0	Refrigeration technologies and specifically Viper wipes, the all-purpose wipe that is really amazing
0:48.5	what it can cut through. Everything from mastic to oil grease tar, grime, and it's nice on the hands.
0:55.7	Make some nice and soft when you're done.
0:57.5	So that way, you can go home and head out a dancing when you get back to the house.
1:04.4	Find out more at refrigetect.com slash viper dash wipes.
1:08.5	Refraggetech.com slash viper dash wipes.
1:14.2	All right, so what are we talking about here?
1:15.7	We were talking about bimetal switches or overloads.
1:20.1	They don't always have to be overloads.
1:21.4	They can be switches.
1:22.2	We'll talk about that difference quickly.
1:25.2	But what is a bimetal?
1:26.9	A bimetal is just two dissimilar metals. That's the
1:30.5	fancy talk for two different types of metals that are bonded together. And so when they are heated
	...

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