Hot Tub Pumps and Motors

Help Choosing the Right Hot Tub Pump and Motor

In most cases we can just assume the original designer or builder used the correct size pump and motor for the job and make our replacement accordingly. But what if the original equipment was too small or too large? What if the plumbing has been repaired (which may have added or deleted pipe and fittings) or equipment has been added or deleted, thus changing the system and requiring the pump to work more (or less)? What if the identifying rating plates have been removed or are so weather-worn that now we cannot tell what size the existing equipment might be? Finally, what if this is a brand new installation? How can we choose the right pump/motor for the job?

Well, I'm glad you asked all those intelligent questions. The answer is that we need to know a little about the hot tub system's needs and hydraulics. A detailed explanation of pump and plumbing hydraulics can be found in another of my books, The Ultimate Pool Maintenance Manual (2d ed., McGraw-Hill, 2001). Most hot tub owners will not need the information in such detail, but it is important to understand the turnover rate of your hot tub.

Turnover Rate

The turnover rate of the body of water reflects how long it takes to run all the water through the system. It is desirable for the water to completely circulate through the filter at least one to two times per day, but local codes will generally require a specific time period. In Los Angeles, for example, commercial hot tubs must turn over in half an hour, and pumps must be run for several hours each day to ensure clean water. That's a pretty good rule of thumb for a hot tub in your home; too, since the goal of any turnover rate is to keep the water clean and healthy.

Your pump will be labelled with a rating in gallons per minute (gpm) or litres per minute (1pm). This number will vary if you change the hydraulics significantly, by relocating the pump far away from the hot tub or by adding jets on the discharge side of the plumbing. Assuming that you haven't changed the hydraulics, you can consult the owner's manual for the pump and see the average rating for a typical hot tub. A little math will tell you if your pump, or the new one you intend to install, is adequate for the intended turnover rate.
Let's say the hot tub holds 1000 gallons (3785 litres) of water. The calculations look like this:

1000 gallons ± 30 (remember—half an hour to turn over all the water) = 33 gpm (125 1pm)

In other words, if you have a pump that can deliver at least 33 gpm (125 1pm), then you will have an adequate turnover of your hot tub water. Of course, as filters become clogged and dirty or debris in the skimmer or strainer baskets slows the flow, you may want a more powerful pump to keep up with the 30-minute turnover rate goal. You might look for a pump that can deliver up to 50% more gallons per minute (in this example, around 50 gpm or 189 1pm) to ensure that your circulation remains adequate under all conditions.

Another consideration in choosing the right pump for your hot tub is the number of jets. As a general rule of thumb, to operate efficiently, hot tub jets require 15 gpm (57 1pm) running through each one. Therefore, if you have a system that delivers 60 gpm (227 1pm), you can install up to four jets. As another generality, each jet requires 1A horsepower from its pump/motor, so that four-jet hot tub would need at least a 1-horsepower unit.

Remember, this assumes the pump is doing no other work. If it is pushing water through the filter and heater before it gets back to the jets, or if the equipment is more than 20 feet (6 meters) from the hot tub, then some power will be lost, so you will need to calculate more than V, horsepower per jet. If you have any doubts or find that the pump isn't moving the water as expected, consult a pool or hot tub technician to calculate the hydraulic pressures in your system and determine the correct pump for you.

Booster Pumps

All the information provided in this section applies equally to circulating pumps or booster pumps, but booster pumps do have some unique properties. As already mentioned, some hot tubs use the circulating pump to also power the jets, but other installations have a second pump that is not restricted by the other components.

The booster pump should be plumbed with its own suction drain and plumbing, so that it is not competing with the circulation pump, one starving the other for water flow. Some main drains are designed with two ports under one cover, so you can connect a suction line for each pump. This may be preferable to adding a second drain fitting in a small hot tub.

Similarly, don't try to force the return lines together into a few jets. The system can handle only so much water, so plumb some jets to one pump and others separately to the other. Finally, if a booster pump is likely to pull in debris, it will require a strainer basket just as the circulation pump does. In hot tubs, hair is the most common occupant of either pump strainer basket, but since the circulation pump is connected to the skimmer, it is more likely to contain leaves or other debris, while the booster pump strainer basket collects mostly hair and grit.

Portable hot tubs typically use two-speed motors on a single pump, instead of two separate pumps. The low speed is used for circulation and heating, the high speed for jet action.