I was on vacation a few months ago and the cabin we stayed at had a standalone hot tub. That tub had a thermal sensor and would turn the pump and heater on whenever the water temperature got below the preset level. It's possible you've got a similar system, Timmie.

Make sure your cover is on the tub securely, consider reducing the temperature it's set to when not in use quite a bit.

Why is this thread in the china shop?