This article was found April 2018 at http://www.swimmingpoolsteve.com/pages/go-vs.html
How A Variable Speed Pump Will Save You Money
This article will help you to understand exactly how a variable speed pump will save you money. Additionally this article will highlight the limitations of variable speed pumps, or more accurately which pools will benefit the most (and the least) by switching from a single speed pump to a new variable speed pump. This is a very hot button topic and one that the industry itself is not yet unanimous about. Many older and established pool technicians are slow to adopt to the newer technology while others, myself included, tout loudly the benefits and advantages in switching to a VS pump. As we move forward it is clear that VS pump popularity is on the rise...and when you start to do the math on the cost savings most pool owners are quick to agree that there are significant savings to be had.
Most pools will very likely experience a cost savings so great that it will actually pay for the purchase price of your pump - possibly even a few times over! Does this mean that every single pool pump in North America should be variable speed? This is actually the direction that the industry is moving. In May 2017 the Department of Energy finalized a new federal regulation that establishes the minimum efficiency that pool pumps need to be in order to be sold. This will come into effect fully by 2021 and will require pumps over a certain horsepower to meet minimum efficiency thresholds. In between the lines almost all single speed pumps will be unable to meet these established guidelines. They simply are not efficient enough. The laws regarding this are not quite that black and white simple. There are occasions where a single speed pump may be used, and certain sizes (smaller ones) may squeak through the new established minimum efficiency standards. For the most part however the pump that runs the filtration system of your pool or spa will need to be very small and efficient, or upgraded to variable speed.
Please Note - The Department of Energy is mandating variable speed pumps. This should be a pretty clear indication that variable speed pumps really are much more energy efficient. So much so that it will soon be a law that you must have one on your pool. You should be happy about this as this means you will be spending less money on electricity...but not everyone is happy.
Reasons Why You Should NOT Get A Variable Speed Pump
Not every person is excited about variable speed pumps in general and the thought of being forced into a much more expensive purchase certainly rubs some people the wrong way. Some of the people who resist the variable speed pump movement do so simply because they have not done the math. There are however many legitimate reasons and concerns about variable speed pumps and why they are less-than-perfect on some pool installations.
Before I can convince you why you should get a VS pump we should look at the exceptions to the rule. Some pools will not benefit, or benefit only minimally, from a new variable speed pump. Some of the people voicing concern over the VS pump movement have valid concerns as well. Consider some of the counter arguments to variable speed pumps:
1) Variable speed pumps are complicated, have expensive parts, and are hard to work on.
2) Variable speed pumps cost too much to buy.
3) Variable speed pumps do not work well on every pool.
These three statements are the most common arguments that you will hear against getting a variable speed pump. In order to convince you why a variable speed pump might be a solid investment for you we need to first address the potential objections.
Variable speed pumps cost too much to fix when they break - This is a valid concern in the sense that variable speed pumps do have more expensive components than single speed pumps, and they are more difficult to work on. Just like new cars are filled with electronics and computers, so too are variable speed pool pumps much more complicated than their older single speed counterparts. So...what kind of car do you drive? Do you drive a 1973 Ford Mustang? A 1973 Ford Mustang is way easier to work on than a 2017 Ford Fusion Hybrid. If you need to buy a new computer for a 2017 Hybrid car it will probably cost more than a carb cleaning kit for your Mustang. Does that make the Mustang a more economical choice for your commuter car? Likely not. The concern that variable speed pool pumps have expensive computer controlled components is valid, but not a reason to over-pay for your electricity usage to keep your pool clean. All indications from pool equipment manufacturers is that VS pumps have a very low failure rate. In almost every case that I have encountered with early variable speed pump failure I am able to identify significant errors in the installation which at the very least contributed to the early failure. Most pool pumps (and pool equipment) are not installed correctly. If you doubt this for a second then take a look at this 50 part video series I made inspecting pool equipment installations. Assuming that your equipment is installed correctly would be a mistake.
Variable speed pumps cost too much to buy - Variable speed pumps certainly cost more than single speed pumps to buy, but when you calculate cost you really should be looking at the total equation and not simply the purchase price of the product. You are paying for a new technology in order to experience energy cost savings. If you are not counting these cost savings from your purchase then you are not accurately able to calculate your return on investment. The ROI on a variable speed pool pump most likely will be the greatest cost saving purchase for your pool you can make with the only other possible exception being a solar blanket cover since they cost so little and do so much to improve the energy efficiency of your pool by limiting heat loss at night.
Variable speed pumps do not work on every pool - This is the real meat and potatoes of the argument against VS pumps. The reality is that a variable speed pump is not ideal for every pool. In order to save on electricity you need to have the ability to run your pool pump on a very low RPM. If your pool system is heavy on demands and requires a pump that runs at high RPM's then your rate of return on an expensive variable speed pump will be minimal if anything. Various pieces of your pool equipment have flow requirements. For example a salt chlorinator will need more than minimal RPM's in order to close the mechanical flow switch and generate chlorine. If you have a salt system and a pool with a high chlorine demand then you may struggle to generate the chlorine you need with a reduced RPM schedule. Pool heaters have a flow pressure switch which must be activated in order for the heater to fire up. Since a pool heater is so large of an appliance the pressure switch requires a generous flow in order to close. If you operate your pool in colder climates and you like the swimming temperature on the warm side, then you will need to run your pool pump at a higher RPM more often so the heater can fire. This can be partially alleviated by using an automation system to allow your pump and heater to "talk" to each other which can allow the heater to tell the pump to ramp up RPM when there is a call for heat. There are a lot of other pieces of pool equipment which require specific flow rates. An in-floor cleaning system requires a huge amount of flow, as do any waterfalls or water features. If your pool needs flow for heat, a salt chlorinator, an in-floor cleaning system or to simply meet large pool turnover rates then your ROI buying a variable speed pump will be lower and slower than smaller, less flow demanding pool systems.
The return you get from an investment in a variable speed pump will depend on how often you can get away with running at low RPM's. If you have anything other than a huge residential pool, and you do not have a bunch of waterfalls or an in-floor cleaning system then right away you are probably a good candidate to buy a variable speed pump. One of the biggest concerns for me is the fact that salt chlorine generators are so common so many of you reading this will have a salt chlorinator. You will need to run your pump above the minimum value to close the flow switch in order to let your salt system generate chlorine.
Since electronic chlorine generators make chlorine slowly over a long period of time this is a concern for anyone calculating their potential ROI on a variable speed pump. If you live in an area with variable electricity rates then this will help quite a bit as you can program to run the pump at a higher RPM at night to let your salt chlorinator make a bunch of chlorine when electricity rates are cheap. During the day when rates are much higher you can run your pump on minimal RPM's.
Calculating Savings With A Variable Speed Pool Pump
One of the biggest false arguments against variable speed pumps versus single speed pumps is that you can just use a single speed pump and turn it off part of the day. This is how people used to save money with their pool pumps and the argument exists today that this is a viable option. In my expert opinion this is a fallacy and one that is argued mostly by people who do not understand the math behind exactly how (and how much) energy you save by using a variable speed pump. Let's look at this equation of a variable speed pump versus a single speed pump that only runs part of the day.
First, a single speed pump of even moderate size will usually have the ability to meet the turnover rate for filtration of the average pool many times over per day. This part of the logic is sound. Single speed pumps move a lot of water and most pools can easily turn off the pump for part of the day and still meet one, two or three turnovers of the pool water per day. For the purposes of illustration we can use the calculated flow rates for a generic 1.5 horsepower pump.
Example 1.5HP Pump - This example pump is rated to move 65 GPM assuming 40 ft pounds of head. This calculates to 3900 gallons per hour and a potential for 93,600 gallons per 24 hours. Even if you apply the semi-commercial standard of three turnovers of the pool water per day through the filtration system this means this 1.5 HP Pump could effectively filter a 31,000 gallon pool. That would be a 20'x40' rectangle pool with an 8' deep end. Again, this is based on three turnovers per day which is great, but more than what the average residential pool owner aims for. The average residential pool gets by with a single turnover per day which is enough to keep all but the heaviest bather load pools clean and clear. This is not ideal but it is very common.
Looking at some common numbers a 16'x32' pool with a six foot deep end might be around 17,000 gallons. Using a minimal approach to filtration this means you need to pump 17,000 gallons per day through your filter. A single speed 1.5 HP Pump might be able to do this in 4.36 hours of run time. Since one horsepower is equal to 746 Watts, this means the 1.5 HP pump might use 1119 Watts which is approximately 5 Amps at 230 volts. In the real world this pump will actually draw more than this amount with just over 7 amps at 230 volts being a more realistic number (more on this later in the article), but for this example we will use the extremely conservative (unrealistic) calculation of 5 amps. When you pay for electricity you are paying based on kWh (kilo-watt hours). To determine the kWh rating for this pump simply multiply the Watts times the hours that the pump will run. At 4.63 hours and 1119 Watts, this gives you a total of 5181 Watt-hours or 5.181 kWh. To meet the bare minimum turnover you would need to pay for 5.181 kWh of power consumption. Now let us try this equation again but this time using a comparably sized variable speed pump.
Example 1.5HP Variable Speed Pump - For simplicity an easy comparison can be made using an established energy law for centrifugal pump motor energy consumption versus RPM speed. The pump affinity law states that for a reduction in RPM's by half there is a reduction in energy consumption of eight times. This means if you reduce your pump motor RPM by half then you reduce the electricity it uses by eight times. It is the nature of this non-linear reduction in energy consumption that is the backbone of the efficient technology that variable speed pumps are based on. Turning a single speed pump off for part of the day can simply not compete with the exponential decrease in electricity you use as you dial down the motor RPM of your pump.
A pool pump that uses 1119 Watts per hour at 3450 RPM (single speed pump RPM's) will use only 140 Watts at 1725 RPM. At 862.5 RPM's the motor is using only a scant 17.5 Watts. Most variable speed pool pumps go as low as 500 to 600 RPM's so you can really see how this can reduce on the electricity that you use. It is important to note that these are all theoretical numbers which most likely will not be reproduced on your system due to pump and plumbing system inefficiencies. This also helps to illustrate how horsepower ratings mean nothing since a 1.5 HP pump should run at 1119 Watts - but double this number is actually much more accurate once you take into account energy losses and service factor for the motor.
For this example a single speed pump uses 5.181 kWh of energy to pump 17,000 gallons of water (3450 RPM for 4.36 hours). You could run a similar variable speed pump at 862.5 RPM for over 63 hours for the same amount of energy it takes to run a single speed pump for 4.63 hours. Do you see how drastically these numbers do not line up? Every time you reduce your motor RPM by half the energy used goes down by eight times. Most variable speed pumps go as low as 600 RPM or so, and the electrical cost savings from this are unmistakable. This is purely a math exercise as well and does not even begin to factor in that filtering your pool 24 hours per day is vastly superior to having your pool sit stagnant for almost 20 hours every day.
To put this calculation another way, for every 24 hours that you run a single speed pool pump you can run a similar size variable speed pool pump at 1/4 the speed for 1536 hours. 24 hours of inefficient operation versus over 1500 hours of low RPM, more energy efficient operation. If you have a pool where you can get away with a lot of low RPM running then you are a very strong candidate to get a good return on your investment in a variable speed pump.
Variables To Consider When Calculating Cost Savings From VS Pumps
The above calculation is very simplified. It makes a lot of assumptions and there are some technical flaws. For example, what is the flow rate for a variable speed Hayward Super Pump running at 600 RPM with 40 feet of head? Zero, according to the Hayward published pump curves since this pump does not operate (well) at 600 RPM on a system that has 40 foot-pounds of head resistance. Fortunately the same pump affinity laws for motor RPM and electrical usage also apply to head pressure. If you reduce motor RPM's by half, you reduce the head pressure experienced by the system by four times. This non-linear drop in head pressure certainly helps your system to operate more efficiently at lower RPM's however the sliding scale of head pressure means that your pump will not always be operating in its most efficient range. Depending on your system you may not be able to operate your equipment effectively at 500 or 600 RPM. Sure you are using less power but the pump still needs to be able to overcome the resistance to flow inherent in the system.
By the time you actually install a VS pump into your system and find the lowest RPM you can run your system on (for the most part) you will be nowhere near to the theoretical numbers calculated here. In the real world you can however still count on a substantial cost savings on energy consumption for your pool pump versus a single speed pool pump. This is mostly due to the fact that single speed pumps are very inefficient, and are often run more than they need to be to meet the minimum filtration needs of the pool.
Important note on system efficiency - By this point you are probably getting over run by the number of variables involved with actually calculating how much you can or will save. Giving an exact number is very difficult due to the unique and dynamic nature of these calculations. The most I can hope to do with an article like this is point you towards some of the calculations, energy laws and physics behind variable speed pump technology which all point towards VS pumps providing dramatic reductions in energy consumption.
Head Pressure - Head pressure is the resistance to flow that exists within your plumbing system. The more head pressure that you have, the harder the pump needs to work to overcome it and the more energy it will consume doing so. As you reduce this resistance to flow you also reduce your efficiency losses. When you reduce a given pump motor by half the RPM's you reduce the head resistance by four times.
Friction Loss - One of the greatest energy losses in a pool plumbing system is to friction within the plumbing lines. Friction, as well as turbulence, reduce drastically as you reduce the velocity of the water. This is why it is important to have larger pool pipes as opposed to smaller ones. With larger plumbing lines you can allow the water to travel more slowly, which results in much less friction and turbulence within the system.
Variable Speed Pumps Versus Two Speed Pumps
When you compare variable speed pool pumps against single speed pumps the advantages are obvious. When you look at comparing a variable speed pump with a two speed pump this becomes a much more valid comparison. Since a two speed pump can reduce the motor speed by half this means that it only uses 1/8 of the power as it does on full speed...in theory. In reality a two speed pump is a much better option than a single speed pump but still falls short of the benefits of a variable speed pump. New variable speed pump technology is just more efficient to begin with so for RPM to RPM the variable speed pump will still use less power. Also the ability to drop the RPM's in half again, and even more, give you an even greater ability to reduce your electrical costs.
From a technical perspective I am on board with the idea that a two speed pump is a valid alternative option to a VS pump. Where you lose me is with the pricing. Already a two speed pump is more expensive than a single speed pump which means the gap between the VS and the two speed purchase price is even smaller. Can you justify the additional cost over a two speed pump to make the jump to variable speed? In my opinion you will most certainly be able to recover the difference between these purchase prices in electrical savings when you calculate it over the service life of the pump. A two speed pump is certainly a better option than a single speed pump, but I would say that a variable speed pump is the same mount better than a two speed. A dual speed pump can operate at half the RPM of a single speed pump...but a VS pump can operate at half the RPM of the dual speed...plus all points in between.
A two speed pump will save you money versus a single speed pump but you will still be overpaying to keep your pool water filtered. Without question variable speed pumps provide the lowest possible operating costs for your filtration system. As an average real worldnumber most variable speed pool pump owners claim anywhere from 40% to 80% decrease in the electricity they are using. Unless you own a solar farm then buying a VS pump is a smart long term decision. Just be sure that you install your pump using best practices to make sure you get the longest service life possible from it.
This is a Pentair 1.5 HP SuperFlo two speed pump and when you compare it to the variable speed SuperFlo pump they are, at least as of writing this article, $220 apart in price.
This is a Hayward 1.5 HP 2 speed Super Pump but when you compare it to the similar Hayward Super Pump VS you will see that the dual speed pump is actually more expensive than the variable speed version. As of this writing the VS version of the Super Pump was available for $83 less than the dual speed version.
This is a Hayward TriStar 1.5 HP 2 speed pump. Now, compare the price from this two speed pump to the similar model Hayward TriStar variable speed pump. At the time of writing the difference in price between these two pumps is $102.
This is a Jandy 1.5 HP FloPro two speed pump which when you compare to the Jandy FloPro variable speed pump you will find a difference in price of around $233.
The price of pool equipment, especially pumps, changes all the time. Some places will sell certain pumps as a "loss leader" to get you into their store where they make little to no profit on the sale. The prices above are based on online pricing and is only to illustrate the relatively small jump in price between dual speed pumps and similar variable speed pumps from the same manufacturers. The jump between single speed and variable speed in price is much more noteworthy but so are the potential for electrical savings. For my money a two speed pump is antiquated technology. It was good at the time, and certainly better than single speed pumps, but the time of two speed pumps has passed. Manufacturers do not sell a lot of two speed pumps compared to both single speed as well as newer variable speed units. This drives the per unit cost up for two speed pumps. New variable speed pumps are more efficient, even when matched RPM for RPM, and that alone could justify the newer technology in areas where electricity is at a premium. When you factor in the additional electrical savings when you are running your pump below the 1725 RPM low speed setting on two speed pumps, to me at least, the answer is obvious. Variable speed pumps are a better purchase decision than two speed pumps for most pools.
Cost Savings Breakdown For Variable Speed Pumps
When trying to level the playing field and figure out how much you can save with a variable speed pump you will likely get lost in calculations and much of what you "calculate" anyway will be an estimation at best. What you are able to compare more readily is what you would pay for electricity to run the same pump, on the same system, with only a reduction in RPM being different. Earlier in this article I used the example of 1119 Watts, or 1.119 kW to run a 1.5 HP pool pump. As mentioned earlier this is not a realistic real-world number. A more realistic number for a 1.5 horsepower pool pump is 2.2 kW per hour. If you want to know what your pump is simply check the motor label to find the voltage and the amperage. Simply multiply the volts times the amps to find out the Watts. This is the amount of power your pump uses (maximum) per hour.
"Average" 1.5 HP pump 230 volts 9.5 amps 230 x 9.5 = 2181 Watts (2.181 kW)
When you pay your electricity bill you are paying for kWh (kilowatt hours). In order to calculate how much your pool pump costs to run you would multiply the kW rating for your pump by the number of hours that you run your pump per day. You would then take this number and multiply it by your electricity rate. The national average in the USA is 12 cents per kWh. In my area in Ontario Canada I have variable rates depending on the peak and off-peak hours. In the past year my rates have been as high as 18 cents per kWh but on average 13 cents per kW is what I pay other than the middle of the night when rates are at their lowest. The cost of electricity where you live will have a TON to do with how much you can save with a VS pump and how quickly you will earn a return on your investment.
Detroit Michigan - 10.72 cents per kWh
San Francisco California - 21 cents per kWh
Houston Texas - 10.98 cents per kWh
Seattle Washington - 7.75 cents per kWh
Miami Florida - 10.4 cents per kWh
Chicago Illinois - 10.44 cents per kWh
Phoenix Arizona - 11.96 cents per kWh
Boston Massachusetts - 14.91 cents per kWh
New York New York - 23.21 cents per kWh
This short list will give you a small idea about the varying electricity rates across the USA. In order for you to make an informed decision about the validity of investing in a variable speed pump you really need to know how much you pay for your electricity. The higher your kWh electricity rates, the more important a variable speed pump will be for you to keep your running costs down for your pool. The following pool pump electrical costs are based on the national average of 12 cents per kWh.
1.5 HP pump running cost for 1 hour: $0.26 (2.2 kW x 0.12 / kWh)
1.5 HP pump running for 6 hours: $1.56 ($46.80 monthly)
1.5 HP pump running for 8 hours: $2.08 ($62.40 monthly)
1.5 HP pump running for 12 hours: $3.12 ($93.60 monthly)
1.5 HP pump running for 24 hours: $6.24 ($187.20 monthly)
In an attempt to create a realistic comparison the following variable speed pump values are based on 2 hours per day of full RPM run time which accounts for vacuuming, and any other high flow demands that you pool might have. Of the remaining 22 hours 10 hours are based on 50% run speed (1725 RPM) followed by 12 hours on 862 RPM. This calculation gives a best estimate real-world program setting to give you a realistic cost comparison versus the single speed pump schedule shown above. For this example we will ONLY use the pump affinity laws to determine the electrical costs based on reducing the motor RPM. In reality the savings will be even greater than this due to a reduction in system efficiency losses from the slower moving water. This also does not account for the fact that variable speed pumps are a more efficient design. This can be considered a conservative real-world savings breakdown.
1.5 HP variable speed pump running at 3450 RPM: $0.26 per hour
1.5 HP variable speed pump running at 1725 RPM: $0.0325 per hour
1.5 HP variable speed pump running at 862 RPM: $0.004 per hour
(0.26 x 2) + (0.0325 x 10) + (0.004 x 12) = $0.89 per day ($26.70 monthly)
This calculation will obviously not apply to every pool however it does give a realistic schedule for what you might actually end up running your variable speed pump at based on "average" pool requirements. As you can see, running your pump around the clock is still vastly less expensive than even the most conservative estimate of running your single speed pump only 6 hours per day. You simply can not argue with the logic that a variable speed pump is both better for the pool, and cheaper to operate than even reduced single speed pump schedules.
With the average 1.5 horsepower (entry level) variable speed pumps costing around $700 as compared to the $350 that a singe speed pump of similar description would be, you only need to save around $350 for the pump to start to return on your investment. Part of the calculation for "how much you can save with a variable speed pump" is based on the length of time that you will own and operate the pump for. A very reasonable expectation of service life is 60 months of operation. With correct installation and proper care (and a little luck) you could potentially get twice this long from your pump...but for the purposes of this cost savings calculation let's just assume a 60 month service life to see what your potential for savings could be (based on how often you run your pump now).
1.5 HP single speed pump electricity costs after 60 months of run time
If you run your pump 6 hours per day: $2808
If you run your pump 8 hours per day: $3744
If you run your pump 12 hours per day: $5616
If you run your pump 24 hours per day: $11,232
By comparison, the cost to run a variable speed pump on the schedule described above for a total of 60 months: $1602
Again, these represent the bare minimum savings and if you get lucky and have your pump run for longer than 60 months then your savings and ROI are even greater. However this is only one example, using one set of assigned variables, and can only be used as a single benchmark for comparison - not a standard for what each pool owner will experience. What you should hope to gain from this information is a better understanding of how you might go about doing a comparison like this for your own pool while doing your best to substitute numbers that are accurate for your unique situation. If you live in an area with higher than 12 cents per kWh hour, or you have a pump larger than 1.5 HP, or you can get away with even less than 2 hours at 3450 and 10 hours at 1725 RPM - all of these will make your cost savings and ROI even more pronounced. Looking at these numbers it is easy to see why people are talking about variable speed pump technology. Even in the minimum sense you will easily save the cost difference between a single speed pump and a comparable variable speed pump. If you are able to get maximum returns on your investment then a variable speed pump will literally pay for itself, in full, multiple times over.
Once you really start crunching numbers and applying real world factors you simply can not argue against the cost effectiveness of variable speed pumps - and the Department of Energy agrees. By 2021 all single speed pumps over 0.5 HP will be banned from sales (and service) so there is little incentive for pool owners to take the cheap way out with a single speed pump...since in the long run it is going to cost you far more than you could have paid.
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