I finally figured out why those old OTIS relay controllers kept failing in the heat

I was working an HVAC emergency call at a 12-story office building in Dallas back in July and noticed the elevator room was hitting 110 degrees. The OTIS relay panels in there were dropping out every afternoon like clockwork. Turns out the thermal overloads were just barely rated for the ambient temp plus the heat from the coils themselves. Now I always check the room temp before I start troubleshooting those old setups. Any of you guys run into heat related failures on those older controllers?

3 comments

3 Comments

rosebarnes1mo ago

@bennett.nora you hit the nail on the head with the sun beating on that cabinet door. That reminds me of something nobody talks about with these old OTIS relay panels in Texas specifically. Its not just the ambient heat from the room or the sun on the cabinet, its the fact that those old panels used a specific type of wax based thermal overload that actually changes state slower in high humidity environments. We had a building near a lake where they kept failing even when the room was only 95 degrees. It took us forever to figure out the humidity was making the overloads sluggish and they'd stick closed then blow on startup. So yeah, placement matters, but so does the air moisture level in those older controllers.

bennett.nora1mo ago

Not too long ago I had a similar headache with a Schindler Miconic V controller in a parking garage stairwell that had zero ventilation. The power supply kept tripping until I realized the sun was beating right on the metal cabinet door all afternoon. Makes you wonder how many of those "it just failed" calls are really just bad placement and no one checking ambient temps first.

finley_price241mo ago

Actually that wax based overload thing is more about the specific grease used in Texas during the 70s and 80s, not the wax itself. The manufacturers back then cut costs by blending in a lower grade paraffin to save a few bucks, and that blend absorbs moisture way faster than the standard stuff they used elsewhere. So you're right about the humidity being the real culprit, but the material variance between regions is probably what made those lake side buildings such a nightmare.