How Does
Pump
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Water must pass through the pump, or pumps, to power its journey through the internal systems. Hot tub pumps use either suction or pressure to circulate the water. Each method requires its own type of filter.
Filter
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Suction filters are connected to the intake, cleaning the water as it enters the internal systems. These filters are easy to replace because they're mounted near the surface. By contrast, pressure filters are mounted internally, inside a sealed canister not connected to the intake. This makes them harder to access than suction filters; however, top-mounted pressure filters can be conveniently replaced too.
Heater
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The water enters the heating element, remains there until it reaches the desired temperature, then continues to circulate. Heaters are equipped with safety features including a high-limit switch and flow switch, so if the water stops flowing as it should, the heater will shut itself off.
Jets
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After completing its path through the internal systems, the water re-enters the shell through the jets, creating forceful, massaging streams that bathers enjoy. It achieves this force by being pushed in high volumes through relatively small jet openings---a phenomenon known as the Venturi effect. Meanwhile, the tub introduces air to the water in the shell using either an air blower or air induction.
An air blower sends air into the shell much like a vacuum cleaner in reverse. Air induction mixes air with the streams of water entering the shell through the jets, infusing them with tiny bubbles that brush the body and make the streams more robust. Because air induction is a more efficient system, it has largely taken the place of air blowers.
eHow Article: How Hot Tubs Work
