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My building has a set of magnetic door locks that seem permanently warm to the touch: whenever I press the exit button to release the lock, if I touch the magnets, they feel warm even if the building is cool.

I couldn't find much information about whether this is normal or not. The door has been fixed a few months ago (they stopped working less than a year after construction), so I worry that this could be a sign they might stop working again in the near future.

Are such magnets supposed to be warm at all times? I never noticed this in this kind of doors before.

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In a word, yes.

Different models have different holding forces, but all maglocks rely on passing a current through a coil. That coil has a resistance, which means it dissipates heat. And the way they work means that whenever they're holding the door shut, they're energised - other mechanisms exist but maglocks fail safe (not secure) if power is lost, and easily allow emergency exit buttons that interrupt the current.

Typically (Wikipedia) this heat is in the range of 6W. A big lump of metal will take some time to warm up with that amount of power, but it will warm up. Even in a very warm room it's unlikely to be hot to the touch, but will always be warmer than other items made of the same materials.

The failure modes I've seen as a user are more often in the controller/power supply than the magnet unit, though movement between the door and magnet (e.g. loose screws) can cause them not to hold properly until readjusted.

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    minor nitpick: always be warmer than anything else, and feel warmer than other items made of the same material. I believe you allude to thermal conductivity - same reason you can stick (briefly) your hand in a hot oven, but not touch the pan - air and pan are at the same temperature though.
    – Vladimir Cravero
    Commented Aug 6 at 8:10
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    @VladimirCravero yes, I didn't want to go into too much detail here (and heat capacity comes into it as well), but a steel (realistic example) door handle will conduct heat away from your hand at room temp. A steel lock body may conduct heat into your hand, or just away much more slowly, depending on how warm it and your skin are
    – Chris H
    Commented Aug 6 at 8:14
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Yes, it's normal for door-holding by electromagnets to feel slightly warm.

All electromagnets, other than superconducting magnets, dissipate energy, and due to eddy currents, those powered by alternating current are worse offenders than those powered by direct current (DC).

AC electromagnets may be used because they're less expensive to make, not requiring a rectifier, but DC devices are more efficient.

Fire doors are often held open by electromagnets, so even if power fails during a fire, the doors still close.

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  • @Kromster no, he's correct with "still close" -- fire doors can't do anything to stop a fire if they're simply flapping around in the smoky breeze!
    – ThreePhaseEel
    Commented Aug 6 at 11:45
  • Ah, I missed the first part - "Fire doors are often held open"
    – Kromster
    Commented Aug 6 at 11:55

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