To filter the water and run the jets, spas have a mechanism called a circulating pump. It is typically comprised of a plastic casing with an impeller inside. An impeller is similar to a propeller, yet instead of being mounted on the outside, it is on the inside. Its purpose is to pull water through the pump. The pump has an inlet, where the water is drawn in, and an outlet where the water moves out. Some spas use one pump for both the filtering of the water and for providing water to the jets. Other spas have two pumps--one for the filter and another for the jets.
The more common spa pump design is the single pump, operating both the filter and jets. In this design, the filter is mounted on the intake side of the pump. The pump's impeller pulls the water from the spa, drawing in through the connecting pipes on the intake. The water is pulled through the filter, moves through the pump and out the other side of the pump. From the outtake, the water is pushed through the pipes leading to the jets and back into the tub of the spa. Hence the term "circulating," as the water has circulated from the spa tub and back again.
There are two ways to turn the spa pump on. One is a timer, which is set to automatically turn the pump on to filter the spa. The timing settings will depend on the size and type of spa being filtered. Some spa manufacturers may suggest filtering the spa twice a day for several hours, while others may require twice a day for 15 minutes each. The second way to turn the spa on is to push the pump's on and off button. This method would be used if the spa owner was sitting in the spa and wanted to turn the jets on, and the spa pump was not in filtering mode.
Spas typically have two types of on and off switches for the pump. One is an air switch. When pushed, the air switch forces air into the switch and turns on the pump. This does not require electricity, and can be mounted on the top of the tub, near the water line. The second type of switch is an electric switch. Instead of air, electricity is used to power the switch. An electric switch is located on the outside of the spa, away from the water.