Hot Tub Pump Horsepower
Hot tub pumps are typically specified with horespower or HP. (in Europe PS or other designation).
Spas are designed by the manufacturer to have certain plumbing sizes, number of jets, manifolds and other manufacturer based decisions that will lead to calculating the right Horsepower for the job. If the horsepower of the pump is too low for the number of jets, length of the plumbing run and plumbing thickness among other factors, then the jets will seem weak. If the horsepower is too strong for the plumbing line, suction etc., the pump may cavitate or become damaged.
Some believe you can put a larger impeller in a pump and get more power. But anytime adjustments like this are made to the pump's components, the risk is undue strain on the pump motor. Many times it is acceptable to go one level up on the specified horsepower when replacing a pump such as from 3-4HP
STEP 3: Identify Pump Horsepower 
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| 1 HP and lower Pumps | 1-3 HP Pumps | |
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| 3.5 - 4.5 HP Pumps | 5 HP Pumps + | |
Insiders in the hot tub industry have correctly stated that horsepower alone is irrelevant. Many times horsepower ratings are liberally sprinkled through advertising and marketing departments to increase the perceived value of the hot tub.
The real factor is GPM or gallons per minute (or LPM - liters per minute for metric denizons). Btw. when converting from gallons to liters, divide by 3.8 (or multiply x 0.38) as there are about 3.8 liters to a U.S. gallon.
Gallons per minute that flow out of a given jet will truly give us the water flow and power of that jet. Most manufacturers do not publish GPM ratings for their spa jets, but this would technically be a better way of publishing maximum massage power.
In any case, many jets today have a ring around the outside of the jet. These are called adjustable jets, and the amount of water coming out of the jet can be reduced as desired in case the massage is too strong.
When replacing a spa pump, the best way to compare pump strengths since GPM numbers are not publicized and also depend on the actual plumbing scheme of the spa, we can use the published HP or horsepower rating as the way to find a similar replacement pump.
If the spa had a 3HP pump and a 3.5HP is available with all of the other characteristics such as frame size, voltage, mounting pipe size etc. being the same, then it is acceptable to move up a little in horsepower.
Of course if everytime you moved up in horsepower a little, you would eventually be at an unacceptable horsepower level in relation to your particular plumbing setup, so use upgrade with caution depending on if you are upgrading an original pump, or if it is already a replacement pump where yo may not know the specifications of the original pump.
Many manufacturers could also not tell you the exact specifications of your original pump especially if the spa was purchased many years ago, because hot tub manufacturers change suppliers over time and also pump models.
