Another common misnomer about hot tubs is that more jets is always better than less jets. This is not always true. Although a relatively high jet count (30-60 larger, higher volume jets) is therapeutic, there is a point at which the value of adding extra jets diminishes and the likelihood for problems can actually increase. This is because when the number of jets increases in a hot tub then the size of the jets are typically shrunk to compensate for space and plumbing. For example, a 7’ x 7’ square hot tub has only so much surface area for jet placement along its interior shell walls. A manufacturer could easily plumb 40 large therapy jets (4”-5” diameter) in a hot tub that size. If they wanted to add more than that then they must shrink the jets in order to make them fit properly. Similarly, a manufacturer might plumb 80 small jets (1”-3” diameter) in a hot tub that same size. The hot tub with the higher jet count is less therapeutic and in many cases less expensive to manufacture because the smaller jets costs less to produce than the larger jets. Also, the 80 jet model has twice as many holes drilled into the shell which increases the potential for leaks and failure. The funny thing is that the 80 jet model has a higher perceived value to most consumers who don’t know any better even though it is a cheaper hot tub to manufacture than a comparable size, 40 jet model with larger jets.
Moreover, smaller jets are less therapeutic than larger jets because they are higher pressure and lower volume which makes them feel itchy and irritating. This reduced therapeutic benefit and discomfort can make the bathing experience much less enjoyable. Larger jets typically have a much higher water to air ratio making them higher volume and lower pressure which is much more comfortable for bathers. Furthermore, jets have a certain gallons per minute (GPM) flow rate based on size. The larger the jet the higher the GPM flow rate. Also a larger jet works a bigger surface area of the body part it is focused on. The takeaway here is to pay close attention to jets sizes (and relative pump sizes) and avoid falling into the super high jet count trap.
Directly related to jet performance is pump size or motor size. First off, many manufacturers use the term “pump” and “motor” synonymously even though they reference different things. The pump is actually the wet end or plastic portion of the jet pump where the water comes out. It usually has an intake and a discharge where water comes in and out. The motor is the electronic portion of the jet pump that mechanically spins a shaft inside the wet end of the jet pump and forces the water out of the pump (which feeds the jets). In this section, we will refer to the sum of the pump and electric motor as the “jet pump” for simplicity sake.
The hot tub industry uses two standard electric motor sizes, 48 frame and 56 frame. 48 frame electric motors are typically smaller and have lower horsepower ratings. 56 frame electric motors are slightly larger and have a higher horsepower rating. A simple analogy in understanding motor size might be to think about a vehicle engine. A 48 frame would be similar to a six cylinder engine and a 56 frame might be more like an eight cylinder engine. Additionally, a 48 frame electric motor would have a smaller wet end on the pump side of the jet pump and similarly the 56 frame electric motor would have a larger wet end on the pump side of the jet pump. Also another way to measure jet pump performance is to look at the gallons per minute flow rating (GPM). Larger jet pumps have higher GPM flow ratings.
Reputable hot tub manufacturers use the principles of fluid dynamics or more specifically hydrodynamics when designing hot tubs. Variables such as jet pump size, length and diameter of plumbing, fittings, and jet size are all appropriately balanced to achieve optimal pressure and flow. For instance, if a 5hp jet pump is capable of producing 200 GPM (at 20 feet of head pressure) and that pump is powering 15 jets rated for 13 GPM each then with the proper plumbing sizes and distance calculations that design would be considered optimally balanced. This example further reinforces the point made earlier about jet sizes in that you could have a 3hp motor that might only produce 140 GPM (at 20 feet of head pressure) powering 25 jets rated for 4-5 GPM each. In effect a smaller jet pump powering more, small jets can be less therapeutic than a large jet pump powering fewer, larger jets.
Another important consideration is the design of the jet pump itself. Most of the jet pumps used by hot tub manufacturers come from only a few suppliers and are fairly similar withstanding performance characteristic like frame size and horsepower, however, some of the more leading edge hot tub manufacturers have gone outside of the industry and have borrowed technologies from other industries that use better jet pumps which are more energy efficient, longer lasting, and higher performance than traditional jet pumps.
HTI would advise purchasing a hot tub with 30-60 large therapeutic jets (mostly 4-5” in diameter each).