Two-speed Pool Pumps vs. Variable Speed Pool Pumps

Right before I published this article I was contacted by a nice lady in Georgia who felt her pool service company was steering her wrong with a decision to replace her ailing single-speed pump. There is a lot of misinformation out there regarding two-speed pool pump and variable speed pumps, and many dealers are not up to speed (no pun intended) on the advantages of these swimming pool pump choices.

This is becoming more of a subject because the Florida Residential Energy Efficiency Code requires that all pool pumps over 3/4 horsepower be either a two-speed or a variable speed model, and that they are capable of operating in the low setting if a two-speed model. When you have your pump replaced or if you install a new pool, contractors are required by law to install a pump that meets the new code requirements. Incidentally, I was at a home yesterday where a new pool had a 1 HP single speed pump installed — a clear code violation. The building inspector actually noted the deficiency on the inspection, but somehow it passed a second inspection. Worse yet — the homeowner was told by the builder that they were installing a two-speed pump, which it clearly was not.

This article will allow you to compare the two options available for pools in Florida and see the advantages over single speed pool pumps.

The Pentair IntelliFlo is a versatile variable speed pool pump with good automation options.
The Pentair IntelliFlo is a versatile variable speed pool pump with good automation options.

Variable Speed Pool Pumps

The common reasons or myths cited for not installing a variable speed pump are:

Variable Speed Pumps are expensive

When compared to single speed, or even two-speed pumps, variable speed pumps are definitely more expensive. The shock of the initial price goes away quickly when you compare the operating costs of the various options. A variable speed pump will easily pay for itself in electricity savings over its lifetime.

Variable speed pumps don’t save energy

Variable speed pumps save energy in two ways. The first, which even some pool builders don’t understand, is that the pump motor is inherently more efficient. Single and two-speed pumps use traditional induction motors. These are just about the least efficient motors you can use, but they are cheap, so they are popular. Quality variable speed pumps use permanent magnet brushless motors. At the same flow rate (if we are comparing apples-to-apples), a variable speed pump will use about 30% less energy as a result of the inherently more efficient motor. Where variable speed pumps really shine is when you reduce the flow rate (and motor speed). Running your pool pump for a longer time at a lower flow rate will result in the same overall pool water turnover, but reduce the energy required tremendously. For centrifugal pumps, this is what’s called the pump Affinity Law in physics.

The pump Affinity Law involves a lot of math, but the concept is actually pretty simple. The flow rate is proportional to the motor speed. The pressure is proportional to the square of the shaft speed. The power (energy) required is proportional to the cube of the shaft speed. As an example, if you reduce the pump speed by 1/2, you will get 1/2 the flow rate, 1/4 the pressure, and 1/8 the power. Think about that – to get twice the flow you need eight times the power!

Even if you run a variable speed pump at half speed for three times longer, you will still save 62.5% on energy! (1/8 x 3 = 3/8 = 37.5% the energy required)

The combination of the inherently efficient motor and the Affinity Law combine to reduce pump operating costs dramatically when properly programmed.

Variable speed pumps don’t keep the pool clean

This myth is spread by pool service companies that have had bad experiences with variable speed pumps. If you reduce the pump speed and don’t increase the time it runs, then you will surely have problems with water chemistry because your pool will not get adequate turnover, filtration, and chemical dispersion.

Variable speed pumps don’t work with pool features

When you reduce the flow rate in your pool, you will get less flow over spa spillovers, waterfalls, and deck jets. That is to be expected. However, since these are all ambiance features, it is usually not necessary to operate these features at all times. You can operate these features on-demand or on a schedule using valves and automation systems. You may also be able to adjust existing valves to send more water to features, resulting in no change to features. Note: water features also cool pools, which is often not desirable. Less flow reduces this issue.

Variable speed pumps cause vacuums not to work

Suction vacuums, whether manual or automatic, require a certain level of suction to operate properly. Whether you vacuum your pool on a schedule or leave your pool cleaner in at all times, you can overcome this issue easily. You can ramp up the speed on your pump when manually vacuuming. If you use a suction cleaner that usually stays in the pool, you can operate the pump at a higher speed for a period of time each day on a schedule. One or two hours at a higher speed is plenty of time for a cleaner to be effective, and the lower flow rate for the remainder of time will make your suction cleaner last longer.

Variable speed pump drive electronics fail

This was an issue with very early model variable speed pumps. Because the drive electronics are mounted directly to the motor, the electronics on early generation models suffered from early failure. These issues have been resolved by major manufacturers, and variable speed pumps have been out for several years, proving that this is largely an issue of the past.

Variable speed pump don’t work with pool heaters

All pool heaters need a certain flow rate to be effective, and electric heat pumps and gas heaters will shut down if the flow rate is not adequate. The solution is simple – when operating a heater, increase the speed of the pump to the required flow rate. Because you can select a high enough flow rate, and no higher, you can strike a perfect balance between heating performance and energy savings. The changes in pump speed can happen automatically with an automated pool controller.

Variable speed pump don’t work with salt chlorine generators

Chlorine generators need a minimum flow rate that is very low. Variable speed pumps require very little energy to achieve the needed flow rate, even at normal low-speed filtration rates. Unless you have very inefficient pool suction and return plumbing, this will not be an issue.

Pentair 2-Speed Whisperflo Pump
The Pentair 2-Speed Whisperflo Pump looks a lot like a regular single speed pump.

Two Speed Pool Pumps

Two-speed pool pumps can save money if installed properly, but they are still standard induction motors, so there is nothing inherently efficient about them. However, because of the Affinity Laws explained above, the low speed of a two-speed pump can result in excellent savings.

Two speed pumps save energy

The high speed of a two-speed pump does not save any energy compared to a single speed pump of the same rating. The low speed will save energy according to the Affinity Law, but the savings are not compounded by having an inherently more efficient motor.

Two speed pumps require proper controls

I can’t tell you how many two-speed pumps I see connected to single speed timer. This completely defeats the purpose. You need a two-speed mechanical timer or a more advanced automation system with a two-speed relay to control these pumps properly.

Two speed pumps don’t work with pool heaters

Like variable speed pumps, two-speed pumps will likely have to run at a higher speed to make heating options work properly. Because there is only one “higher” speed on a two-speed pump, you are limited to that high speed, even if it is unnecessarily high for your heating option.

Two speed pumps don’t work with pool vacuums

Quite often the low speed of a two-speed pump is not adequate for suction pool vacuums. As a result, the high speed must be used, even if it is unnecessarily high.

Two speed pumps don’t work with salt chlorine generators

This one is a maybe. Sometimes the low speed of a two-speed pump will be enough to produce the flow needed for effective chlorine production. Variable speed pumps let you “dial in” a high enough speed without wasting energy at higher speeds.

Two speed pumps save energy

This is true, but only if set up correctly with proper timers and automation. Still, they do not save nearly as much as a variable speed pump, which is inherently more efficient at any flow rate.

Conclusion

There is a lot of resistance to variable speed pumps, mostly caused by lack of knowledge and myths. It’s true that variable speed pumps are more expensive than two-speed pumps, and two-speed pumps are more expensive than single speed pumps. However, the energy savings will easily pay for a properly installed variable speed pump over it’s life, among many other advantages.

In my opinion, it does not make sense to go with a two-speed pump over a variable speed pump. Given that a variable speed pump will pay for itself in energy savings, that alone should be enough reason to lean toward the variable speed choice. Since two-speed pump require additional automation options, either a multi-speed timer or a full automation system, and have no on-board controls, the cost of a two-speed pump starts to approach that of a variable speed pump anyway. Installation labor is essentially the same for both pump types. The versatility of a variable speed pump far outweighs the limits of a two-speed pump.

It should be noted that all variable speed pumps are not equal.

Don’t make a penny-wise, pound-foolish decision. Spend a little extra money and get a pump you will love and that will love you back. And now that you know about permanent magnet motors and the affinity law, don’t let an uninformed dealer steer you wrong!

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  • blank Elizabeth Guion Salter Wood says:

    I have a 3 speed pump for my solar heated in ground pool. Recently, the tech came out and and reset the speeds. At the running speed, my solar thermostat showed “LF”, low flow. I went out to the pool and did not hear any water going through the solar collectors, obviously not heating the pool. Today, I went out and tinkered with the pump. Now, it runs fast, heating the pool, but the control says “Pr000”. I assume it is saying I need to program this. I left it alone, as it is seemingly, now working to heat the pool. Should I leave it this way. or figure out how to program it? Every time the tech comes out, it is another big bill, which we cannot afford, as seniors. Suggestions?

    • blank Jason Szumlanski says:

      I am not aware of any 3-speed pumps on the market. What you probably have is a variable speed pump with three preset buttons. I can’t help much without knowing what brand and model of controller and pump you have. In all likelihood, you should get a professional there to program it so you get the energy saving benefits of your pump while optimizing the performance of your solar panels. But if you let me know what pump and controller you have, I may be able to provide some pointers.

  • blank Dennis says:

    I have a single speed timer. Does that mean I would have to replace that timer and the box to accommodate a variable speed pump?

    • blank Jason Szumlanski says:

      Absolutely not. Most variable speed pumps have an onboard timer and schedules. You simply bypass your existing timer, or leave it inline as a supplemental disconnect switch. If you have advanced pool functions you may want to consider a pool automation system, but that is entirely optional.

  • blank A Piller says:

    Great read about pumps! Helps me explain it better to our customers here in Central Florida.

  • blank Arthur Thornton says:

    If I have an existing pool w/ chlorine generator & want to replace my single speed motor w a variable ,is there any way to set or program the chlorine generator to work properly after the change over ?

    • blank Jason Szumlanski says:

      Most chlorine generators will work at pretty low flow rates. We suggest that you select a slow run speed that is sufficient for your brand of chlorinator. If you want to run at a slower speed, just program the pump to run at a higher speed for a sufficient time to get adequate chlorination. You may also wish to boost the percentage output of your chlorinator if your model supports it.

  • blank Jason Szumlanski says:

    Before some rocket scientist comes by to correct me, I should mention that variable speed pumps’ energy usage doesn’t really follow the affinity law perfectly because there are other factors involved, including the power required to run the drive electronics. For purposes of this discussion, the basic concept and savings are close enough and valid.

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