Why does nobody talk about the old magnetic reed switches on steel doors?

I was on a call in Portland last month to swap out a failing panel, and the homeowner wanted to keep his original 1980s door sensors for the look. I figured, sure, they still read closed, so why not? I wired them into the new system and did a full walk test. Everything seemed fine until the first windy night, when the alarm kept going off for no clear reason. After three trips back, I finally caught it: the steel door frame was flexing just enough in the wind to break the magnetic field on those old, wide-gap reeds. The fix was simple (I put in a modern recessed sensor), but it cost me half a day of extra work. It taught me that old tech can work until the conditions change, even if it checks out on a calm afternoon. Has anyone else run into a problem like that with vintage parts on a new install?

3 comments

3 Comments

holly_henderson861mo ago

That part about the steel frame flexing in the wind is so true. I read a forum post a while back where a guy had the same issue, but with an old garage door. The whole metal track would shift just a tiny bit when it got really cold, and his old reed switch would lose connection. He spent weeks thinking it was a wiring fault before he caught it. It's wild how something that worked fine for decades can fail just because you put it on a newer, more rigid system that shows up the old weak points.

alice_palmer201mo ago

Honestly that "worked fine for decades" line is so spot on. Tbh it's always the tiny flex or shift that gets you. Makes you feel crazy until you finally see it.

uma_johnson1mo ago

My uncle's 1970s cabin in Vermont had the same reed switches on the exterior basement hatches. The real killer wasn't wind, it was heavy rain making the old wood swell. The door would sit just a hair higher and break that magnetic field. We chased ghosts for a weekend before I saw water marks on the frame.