The Essential Guide for Choosing a new Variable Speed Pump

It’s surprising, but some people don't seem to spend much thought when selecting a variable speed pool pump. In the old days of single-speed pumps, you may have just selected a pump because it had a certain horsepower and then you were done. A variable speed pump's benefits are so attractive, and their operation is so flexible, that it might be the case that many people get a pump that is in the ballpark for the specs that they need and then figure its setup as they go. For some variable-speed pump users, things might work out in the end with this approach, but with a little forethought you can seriously maximize the energy-saving benefits and pool system operation improvements that a variable speed pump provides.

Put simply, it is crucial to accurately size your pool pump though, since virtually all other parts of the pool system depend on its operation. The appropriate pump size ensures that your whole pool system will be able to keep the pool water chlorinated, filtered, and safe for swimmers. Since the pump is the heart of a swimming pool system, not having enough flow or "turnover" (moving all of the water in your pool through the filtration system) will shortchange your pool system. Your chlorinator won't be able to sanitize the whole pool, the filter won't have enough opportunity to take out the dirty stuff, and other pool equipment might not work properly without enough flow, so it is critical to choose a pump with enough power for your pool.

With variable speed pumps, you essentially want to select a model size that will meet the minimum and maximum flow needs of your pool. How you determine that is where the trick is. This guide will help illustrate how a variable speed pump is intended to operate (and how that is a little different than a traditional pump setup), the general principle for determining a pool system's needs when sizing the pump, and then a "streamlined" way to find out more precisely what your particular pool system's needs are without getting into complex measurements and calculations. We’ll also help you understand how to compare pump efficiency, overall value, and features. This guide will overall be fairly simple as it takes you through all the steps, but there can be a lot of details so be sure to give it a full read-through, and don't be afraid to reach out to the experts to discuss the details your pool's unique needs.

1) What a Variable Speed Pool Pump Needs to Be Able Do

Let's start with a reminder of the overall goal of any swimming pool pump: to "turnover" all of the water in your pool through the filter 1-2 times a day. A properly-sized pump will allow you to achieve the precise amount of flow needed (no more, no less), to minimize your electrical usage and equipment expenses, and to maximize the other benefits your pool system performance (like improved filtration, quieter operation, etc...).

Selecting a Variable Speed Pump to Achieve Proper Operation

This means that the primary focus of selecting your pool pump is to make sure it can achieve more than the minimum amount of flow performance (water movement) required to achieve these turnovers.

Since a variable speed pump (VS pump) runs on low speeds for longer durations to achieve the pool system turnovers, you'll be choosing a VS pump with a higher total horsepower than would otherwise likely be installed on your pool if using a comparably sized single-speed pump. With the higher total horsepower of a variable speed pump and with the ability of its controls to be programmed to run at lower speeds, this allows a lot of flexibility for many variable speed pumps to be able to cover a wide range of pool sizes. However, even though you can turn down the settings of a VS pump as needed, you also want to just double check that the pool pump is not wildly over-sized, as a pool system does technically have a maximum flow rate, and of course you wouldn't want to spend more money than needed when it comes buying a pump with unnecessarily-high performance .

How a Variable Speed Pump Needs to Run

If the goal of a pump generally is to provide 1-2 pool turnovers a day, the goal of a VS pump is to do this with maximum efficiency, which means running the pump on lower speeds for an extended period of time. Unlike a single speed pump, the goal is no longer to run the pump a certain number of hours; many variable speed pump models come with programmed with 16-24 hour run times, and you’ve got the flexibility to adjust this if needed to move enough volume of water Running a VS pump for an extended period improves your filtration performance, allows the chlorinator to work at a lower rate, and keeps the water moving so its harder for dirty stuff to collect on the walls or floor. From the variable speed motor's perspective, running on low speeds uses an exponentially lower amount of energy, causes less wear, and hardly creates any motor noise. Running a variable speed pump like this will save you substantial electrical costs and helps create a better pool environment.

How does a variable speed pump need to run? The volume of water moved every day is the primary goal; run times are secondary to this, and the pump’s programming will have it change speed and flow rate one or more times throughout the day based on the needs at the time (ex: priming the pump, achieving the needed flow rate for a water feature or certain another of pool equipment like heat pump, salt cell, cleaning system, etc. To say it simply: it needs to run long enough every day to move all pool water through the filter 1-2 times per day, at the minimum necessary flow rates required at the time for any other components of the pool system which are also running at the time.

Don’t let that sound too complex. If the variable speed pool pump model you’re considering is able to achieve the overall minimum flow that the pool requires when set at low speeds, and if its high speed maximum flow is also able to be set within range, then the other aspects of the variable speed pump’s operation are what the pump is designed to be adjustable for. So the next step is to determine what the overall minimum flow rate might be for your pool.

2) Determining Your Pool’s Target Average Flow Rate

In order to achieve 1-2 turnovers a day with maximum efficiency, you need to know what the lowest average flow rate is that the pump needs to provide every day. First, you must determine how many gallons of water are in your pool (see chart below). Based on the pool size, you can easily figure out the minimum required average flow rate you need to achieve your target number of pool turnovers. For our example, we'll use a 20,000 gallon pool and target 2 turnover per day, and for our example we'll assume an 18 hour run time for your VS pump (this would be fairly typical, though you can decide your own run time or use the default run time in your pump’s programming). Once you have calculated your pool's approximate water volume, convert it into a rating of gallons per minute. So for our example, the average flow rate you would need to achieve 20000*2/18/60 = 37 gpm. In other words, over the course of its daily operation, a 20,000 gallon pool with an 18 hour runtime would want to make sure its VS pump can achieve an average of 37 gpm. Remember that a variable speed pump is intended to spend most of the day on its low-speed settings, so this average flow rate that you calculated is going to be what you want the pump that you choose to able to achieve when using the lower end of its capabilities.

This is a very important target to look at when choosing a properly-sized pump: if the pump can achieve this target flow rate on its low-speed settings, the pump will likely be a good choice.

3) Which Pump Model Size Provides the Flow Rate I Need?

Now that you know the average flow rate that is ideal for your pool, its time to look into the specifics of what pump models will be able to achieve this. Keep in mind that the flow rate a that the any particular pump is going to be able to achieve will be based on the combination of its power (how effective it is at moving the water) and your pool's Total Dynamic Head (how much resistance to flow the water will encounter). So, what pump model will provide the target flow rate that your pool needs? The answer depends on how hard it is to move the water through your particular pool system, so you need an idea of what your pool’s Total Dynamic Head is.

4) What is my Pool’s Total Dynamic Head?

The Total Dynamic Head of your pool system is really the key that unlocks the question of which size and model pump is a good fit. In reality though, determining the Total Dynamic Head of your pool system is complex and can be very difficult. We'll start with an easy generalized approach to the question, and then offer a way to get more into the specific details of your own pool system without getting too crazy with math or measurement.

The overall idea is that if you know your pool system's Total Dynamic Head (TDH), you can look at look at a pump's "performance graph" to figure out what flow rate it will provide you at a particular RPM. On a variable speed pump’s performance graph, you’ll cross-reference the pool system's Total Dynamic Head (TDH) on the vertical axis with the curve of the variable speed pump's charted RPM's, and where the line for the TDH crosses over the RPM curve, you can trace it down to the horizontal axis to see the resulting flow rate. So, if you have an idea of the pool's TDH, you check to see what flow rates its low-speed RPM settings can provide. If a pump’s low-speed RPMs can provide your target average flow rate (which you determined above), then the pump is likely a good match.

The General Approach to the Pool’s Total Dynamic Head

So if you're asking, "what is my pool's total dynamic head (TDH)?" - that's a great question. You can do some very complex calculations (https://www.discountsaltpool.com/assets/doc/Simplified-Total-Dynamic-Head-Calculation-Worksheet.pdf) to find this out based on things like the total length of all of your pool's plumbing, its diameter, the number of elbows, and much more. For the general approach if you are just beginning your research, you might instead start with a ballpark number, in order to narrow down your options. To that end, for the general approach you could use as average example numbers 40-60 ft TDH if you have an inground pool, and 20-30 ft TDH if you have an aboveground pool. (If you have a built-in spa, in-floor cleaning system, solar heating system, pressure-side cleaner, or any other flow intensive equipment, you'll likely want to assume that your TDH is on the high side.)

An Unique Idea to Find your Pool’s Total Dynamic Head

If you want to find your pool’s specific TDH, but doing the complicated math and measurements are a deal-breaker, here’s a unique idea. It’s a streamlined approach that should put you in a good position to finalize your research now when selecting a new pump, as well as put you in a good position to optimize your new pump after receiving it.

Rather than measuring all of the various aspects of your pool system that the complex TDH calculations require, you can work the problem backwards. If you have an existing pool pump, look up the performance graph for its brand/model. Then, you can then cross reference your pool’s flow rate on its performance curve to get a very close estimation of your pool’s unique TDH.

But what if you don’t you know your pool’s flow rate? Knowing your pool’s flow rate is the key that will get you your answer. Its also the key to perfectly setting up your new variable speed pump. Fortunately, there is a very easy and inexpensive way to measure your pool’s flow rate. A “Flow Meter” is a bolt-on device that measures the flow rate of the water moving through your pool plumbing. Able to be easily installed in minutes with basic skills, a flow meter is one of the most underrated and under-appreciated parts of a pool system, and its essential to pool system optimization. Being able to know your pool’s flow rate (gallons per minute) is about as important as having a gauge in your car to know your speed (miles per hour)! Putting a flow meter on your pool now will be immediately valuable for you (yes, even in the research stage before buying a new pump), but will be even more valuable for you later on when you install your pump (allowing you to perfectly optimize your new variable speed pump so you can ensure you are moving just the right amount of water through your pool system and achieving near-zero wasted energy)

5) Checking Maximum Flow Rates

In addition to determining the average target flow rate is that would be required to complete two turnovers a day and looking at which pump models are able to achieve this on their low speed operation, you should also take the time to double check what your pool system’s maximum flow rate might be. The maximum flow rate is limited by the length and diameter of the plumbing, the type of pool equipment, and any additional features or accessories in your plumbing system. When setting up your pump it is important not to exceed these limitations, as it could cause equipment damage, excess electricity usage, and pose a safety hazard. Variable speed pumps can always be “turned down”, but if you can find a pump that doesn’t have to be turned down very much, its operation will be a closer match to your pool’s needs, you might be able to save on its initial cost since you’re not just buying to most powerful pump model, and you’ll likely be able to select a pump model that has higher energy efficiency.

Maximum Flow Rate of the Plumbing

One of the primary factors in determining the maximum flow rate for your pool system is its plumbing size. A pool system’s plumbing usually has the following maximum recommended flow rates:

• 1.5” Plumbing: 42 GPM
• 2.0” Plumbing: 73 GPM
• 2.5” Plumbing: 120 GPM

You may or may not have parallel plumbing lines from the skimmer and drain (which would add to the maximum flow rate for each plumbing line), but you may also have single plumbing lines in situations like running a pool system during its spa-only mode, so its best to consider the lowest possible maximum flow rate that the pool could have.

Maximum Flow Rate of the Filter

Another factor that limits your flow rate is the size and type of filter that you have. The following chart provides typical maximum flow rates for common filter types and sizes:

Sand Filters

• 18” Diameter / 1.8 sq ft Surface area = 40 GPM Max Flow Rate
• 21” Diameter / 2.3 sq ft Surface area = 50 GPM Max Flow Rate
• 24” Diameter / 3.1 sq ft Surface area = 60 GPM Max Flow Rate
• 30” Diameter / 4.9 sq ft Surface area = 100 GPM Max Flow Rate

DE Filters

• 24 sq ft Surface area = 36 – 48 GPM
• 36 sq ft Surface area = 54 – 72 GPM
• 48 sq ft Surface area = 72 – 96 GPM
• 60 sq ft Surface area = 90 – 120 GPM

Cartridge Filters

• 100 sq ft Surface Area = 32 – 38 GPM
• 200 sq ft Surface Area = 55 – 75 GPM
• 300 sq ft Surface Area = 80 – 112 GPM
• 400 sq ft Surface Area = 100 – 150 GPM

Compare your maximum flow rate of the filter with the plumbing’s maximum flow rate, and then use the lowest of these two numbers. This is the target maximum flow rate that you’ll want to keep the pump’s operation within. You would especially want to avoid choosing a variable speed pump that was so oversized that the majority of its operational range is over this number, and you also want to keep this maximum in mind when setting your variable speed pump up later, to ensure you’re not inadvertently trying to run your pump harder than it needs to.

6) Pool Pump Sizing Recommendations Based on Flow Rates

Just like with the average minimum flow rate, once you know your pool’s maximum flow rate and you are equipped with a good estimation of your pool’s Total Dynamic Head, you can look at a pump’s performance graph to determine if the flow rates achieved its high-speed RPM’s that are reasonably within that maximum rate and be recommended for your particular pool’s application.

Remember that after it initially runs on its high speed for a short time after start up, you’ll set your variable speed pump to run at low speeds for longer times with the express goal for moving all of the water through your pool filter one to two times per day (1-2 turnovers).

If the minimum and maximum flow rates that the variable speed pump models that you are considering.

However long it takes to achieve this during the day is secondary, since the goal is to provide this precise amount of water turnover using the least amount of energy as possible. Any more water flow and you get little to no additional benefits and waste electricity, any less water flow and you are short-changing your pool system’s operation.

7) Comparing Efficiency

You want to a variable speed pump that has the capability to move the water exactly as needed, but once you’ve selected a number of pump models with the specs needed to achieve this, the next step is to compare the overall energy efficiency of the variable speed pump models that you are selecting. The pump needs the right amount of power and capacity, but not all models perform equally as efficient.

Fortunately, new Energy Star ratings provide an easy way to compare pump efficiency. The WEF (weighted energy factor) rating measures how much water the pump can move for every unit of energy that it consumes. This energy efficiency rating is measured in kgal/kw, which says how many thousands of gallons of water the pump can move for every 1000 watts that it uses up. This makes the WEF similar to the miles per gallon (mpg) of your car. The most energy efficient variable speed pool pump has the highest WEF rating, so a WEF of 8 would be better than a WEF of 4 since it means the pump can move twice as much water when using the same amount of energy.

8) Comparing Total Value

After selecting a few variable speed pump modelss that can achieve the water flow that your pool requires, and variable speed pump models that have the best energy efficiency, the next step is to select a pump that provides all of that with the best overall and long term value, which means considering the purchase price of the variable speed pump models you are considering.

You may have found a right-sized pump model with the best energy efficiency, but if its 50% more expensive than another right-size pump, but the less expensive pump is 90% as efficient, you have to weigh the overall value of the pump over the long run. Paying more but not getting significantly more efficiency means it might be a long time before you see actual cost savings in your bank account.

9) Comparing Features

Lastly, if you’ve found a right-sized pump model with an energy efficiency and price that make it stand out from the rest, take a look at the features that the variable speed pump and its motor have. These can affect its functionality and may make one model variable speed pump the right choice for you, or exclude it from your consideration.

Some common features to look at are the number of programmable speeds that the pump motor has, how much flexibility it has during programming (for example, what increments the RPM’s or run times can be set at), if it has a digital display or user friendly interface, whether or not it displays how many watts it is consuming, whether it has a “override” mode so that you can easily change its operation during its programmed operation, and other similar features.

Two features that can have a very big impact on your experience is whether or not the pump has a “freeze protection” mode (which can automatically run the pump at low speeds when temperatures drop to help make it harder for the water to freeze), as well as if the pump is designed to be a “drop-in replacement” for other brands and models. Especially if you have a very tight pool equipment pad, easily being able to install the pump in place of an existing model can add a lot of value and prevent the difficulty of replumbing.

Lastly, pool owners who use a third-party automation system or electronic controller should double check that the variable speed pump motor has the low-voltage connections or interface need so that the pump can be controlled by the automation system if desired.

This guide is for educational purposes only and is not intended to provide specific techinical advice or recommendations for your situation.