located in the footwell of every spa I've looked at (which isn't many) is a small inlet about 3/4" in diameter. What is it? Is it a temperature sensor, or maybe a pressure sensor?

I asked 2 salesmen and neither knew what it was!

thanks,

found a salesman at another store who says its a jet and that it comes from the ozone filter. my old 1991 tub doesn't have ozone but perhaps it was built ozone ready back in 1991. This apparent jet has a 1/4" rubber hose running up to the top of the canister style filter. I don't know what that would be for, perhaps overflow drainage.

That filter you are referring to is actually a check valve. It's there to prevent water backfeeding into the ozone generator in the event the ozone pump fails. The valve is about an inch by inch, cylinder, i'd imagine.

I took on an old 1991 tub and rebuilt it over the summer. Knowing nothing about tubs at all, I had a great time figuring out the mechanics, electronics, and chemisty of the thing. I was thinking of rebuilding another just for the fun of it.
I understand when you say you're addicted and want to rebuild more, it's a good time.