Hot Tub Pumps and Motors

If the heart of a hot tub is the pump, which is driven by an electric motor and circulates the water in the same way as the human heart speeds blood throughout the body. The proper sizing and maintenance of your hot tubs "heart" are essential to the safe and healthy enjoyment of your spa or hot tub.

Overview

Hot tub pumps are typically centrifugal models. That is, they accomplish their task of moving water thanks to the principle of centrifugal force. The impeller in the pump spins, shooting water out of it. As the water escapes, a vacuum is created which demands more water to equalize this force. Thus, water is pulled from the hot tub and sent on its way through the plumbing and equipment.

There are two basic types of pumps used in hot tubs / spas circulating pumps and booster pumps. The only significant difference between the two is that circulating pumps include a strainer basket to filter out large debris which might otherwise clog the pump or other hot tub equipment. Booster pumps typically don't include this feature, because they take water that has already been filtered in some manner and are only intended to turbo charge the return flow for the hot tubs characteristic massage jet action. Jetted bathtubs also use this type of pump.

Regardless of function, most hot tub pumps are self-priming, that is, they expel the air inside upon start-up, creating a vacuum that starts suction. If the pump is located below the hot tub's overall water level, it may not need to be self-priming, since water will be in the pump at all times anyway. Basic understanding of the components of your pump/motor unit is essential to proper maintenance and performance.

Strainer Pot and Basket

The plumbing from the main drain and/or skimmer flows to the inlet port of the pump, usually female-threaded 11/2- or 2-inch (40- or 50-millimeter) for easy plumbing. Water flows into a chamber, called the strainer pot or hair and lint trap for obvious reasons, which holds a basket (generally 4 to 6 inches or 10 to 15 centimetres in diameter and 5 to 9 inches or 13 to 23 centimetres deep) of plastic mesh that permits passage of water but traps small debris. Some baskets simply rest in the pot; others "twist-lock" in place. Most have handles to make them easier to remove. The strainer basket is similar to the skimmer basket, which traps larger debris.

On some pump models, the strainer pot is a separate component that bolts to the volute with a gasket or 0-ring in between to prevent leaks. Sometimes the pot includes a male-threaded port which screws into a female-threaded port in the volute. In other models, the strainer pot is moulded together with the volute as one piece. As noted earlier, for bathtub hot tubs or booster pumps, where debris is not a problem, there will be no strainer pot and basket at all.

Access to the strainer basket is provided for cleaning out the debris. The strainer cover is often made of clear plastic so you can see whether the basket needs emptying. Various pump models use different methods of securing the cover, including a threaded version, a clamp, or T handles, which screw the cover in place.

In all styles of strainer pot, the strainer cover has an 0-ring that sits between it and the lip of the pot, preventing suction leaks. If this 0-ring fails, the pump will suck air through this leaking area, instead of pulling water from the hot tub.

The volute is the chamber in which the impeller spins, forcing the water out of the pump and into the filter (or directly back to the hot tub if the system is not filtered or heated). The outlet port is usually female-threaded 11/2 or 2 inches (40 or 50 millimetres) for easy plumbing. When the impeller moves water, it sucks the water from the strainer pot.

The impeller by itself will move the water, but it cannot create a strong vacuum to start the water flow. The area immediately around the impeller must be limited to eliminate air and help start that flow. A diffuser and/or closed-face impeller helps this process, but on many pump designs, the volute serves this purpose.
In some designs, the inside of the volute is ribbed to improve the flow efficiency. On other models, the volute itself closely encircles the impeller to create this diffuser effect without a separate component.

Impeller

The impeller is the ribbed disk (these curved ribs are called vanes, and the disk is called a shroud) that spins inside the volute. As water enters the centre or "eye" of the impeller, it is forced by the vanes to the outside edge of the disk. As the water is moved to the edge, there is a resulting drop in pressure at the eye, creating a vacuum which is the suction of the pump.

The amount of suction is determined by the design of the impeller and pump components and the strength of the motor that spins the impeller.