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Swimming Pool & Spa Pump Sizing
Regardless of the type of pool you own or are considering, accurate pool pump sizing is extremely important if you are trying to electrical costs to a minimum. It’s important that you choose the right size pump for your pool to ensure that enough water is circulated to keep the water chlorinated, filtered and healthy for anyone enjoying the pool. We highly recommend a variable speed pool pump given the reasons that follow.
A common misconception is to purchase the largest pool pump you can find so that you know you have enough circulation for your pool. Although this is a good idea in theory it can lead to unnecessarily high electrical usage and can be very costly. The ideal pump will turnover your entire pool in an eight (8) hours cycle and since all pools are different it’s important to get your pool pump sizing correct.
The larger the pump, the more flow is generated, and subsequently the faster turnover rate. Turnover rate is the period of time required to circulate a volume of water equal to the pool or spa capacity. If you want to ensure you are using the best pump possible for your pool follow the sizing guidelines below.
Pump Sizing: Step-by-Step
Following is a simple step-by-step guide to help you calculate what size pump is ideal for your pool. This will ensure that the pump is the most efficient size for your specific pool system. Working through these steps will help ensure you choose the correct size pump to circulate your pool water, while saving you money in electrical savings.
Calculate Pool Volume
The first thing you need to do is determine how many gallons of water are in your pool. Doing so determines how much water needs to be circulated in an 8 hour period and will give you an idea of the required turnover rate.
Rectangular pool = length (ft) x width x average depth x 7.48
Oval pool = ½ length (ft) x ½ width x average depth x 3.14 x 7.48
Round pool = radius (ft) x radius x average depth x 3.14 x 7.48
Pentair provides a handy Pool Volume Calculator that can help make this step a bit easier.
Minimum Flow Rate
After you have determined approximately how many gallons of water are in your pool, you need to convert this into a rating of Gallons per Minute (GPM). It’s assumed that you want to achieve two (2) complete turnovers per day. If you use or are planning on using a saltwater chlorine system and want to run the pump less, you will need to increase chlorine output on the chlorine generator.
Let’s plug in some sample values based on the results we got in the previous step (Calculate Pool Volume):
16,200 gallons x 2 turnovers a day = 32,400 gallons per day
32,400 ÷ 24 hours = 1,350 gallons per hour
1,350 ÷ 60 minutes = 22.5 gallons per minute (GPM)
So, you can see that based on our sample 16,200 gallon pool running 24 hours a day, a 22.5 GPM output is required.
If you want to achieve a complete turnover twice a day, but want to run a 6, 8, or 10 hour cycle twice a day, you can use the following calculations:
6 hour cycles x 2 = 45 GPM
8 hour cycles x 2 = 33.8 GPM
10 hour cycles x 2 = 27 GPM
Your goal is to achieve the highest efficiency. To do so, your options include running a higher horsepower pump for shorter durations, or a lower horsepower pump for longer durations. If your planning includes saltwater chlorine, the advantage of running a low horsepower pump for longer periods is that your water chemistry will be more balanced, creating a more reliable, steady amount of chlorine from the salt cell; and why using a variable speed swimming pool pump in conjunction with a saltwater chlorine system is beneficial. A variable speed pump can be run at low speed when your chlorine levels are appropriate, and you can increase the speed when you require more chlorine output, or should you need extra horsepower to operate a solar powered pool heater. You will not only notice substantial electrical savings, but your pool will be healthier due to the constant circulation.
Maximum Flow Rate
Now that you have determined the minimum flow rate required to complete two (2) turnovers a day, you need to figure out the maximum flow rate. The maximum flow rate is limited by the length and diameter of the plumbing, pool equipment, and additional features or accessories in your plumbing system. It is important not to exceed these limitations as it could cause damage to equipment, use excess electricity, and pose a safety hazard.
The two factors that will help you determine your pool’s maximum flow rate follow:
Plumbing Maximum Flow Rates
All plumbing is rated for maximum flow rate, which is essentially the maximum pressure it can withstand. Typical swimming pool plumbing maximum flow rates follow:
Consider the following example: let’s say you run two (2x) 2” lines from your pool’s skimmer and drain which, on their own, would allow a 146 GPM (2×73) maximum flow rate. But you want to figure in a spa that would be connected to the same filtration system as well, one that adds one (1) 2” line, which would drop your system’s maximum flow rate to 73 GPM. You must use the lower maximum flow rate (73 GPM) in this case to prevent pipe bursting if your spa and pool are plumbed using the same pool filtration system.
Always use the lower of multiple maximum flow rates when determining what your maximum flow rate requirements are.
Filter Maximum Flow Rates
The second limiting factor is the type of filter in use with your pool system. A filter works optimally when the flow rate is within the recommended rate for the model of pool filter in use. If the flow rate is too low, your filter won’t provide adequate filtration; if it is too high, you could damage the filter or it won’t filter properly.
1.8 sq ft Surface area = 40 GPM Max Flow Rate
2.3 sq ft Surface area = 50 GPM Max Flow Rate
3.1 sq ft Surface area = 60 GPM Max Flow Rate
4.9 sq ft Surface area = 100 GPM Max Flow Rate
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
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
Manufacturers may specify a maximum flow rate or ideal flow rate, but it’s best to stay near the low end of the recommended rate for your pool filter. If you are using a sand filter, it will work effectively at any rate under the maximum.
Flow Rate Range
Now that you know how to calculate the minimum and maximum flow rates for your pool system you can determine the flow rate range. If we use the example above we can determine the following:
A salt water pool with 16,200 gallons of water on a 24 hour cycle that completes 2 turnovers per day will require a minimum flow rate of 22.5 GPM.
If we assume we have 2″ plumbing supplying our spa this limits us to 73 GPM. If we also assume we are using a 3.1 sq ft sand filter that has a maximum of 60 GPM we take the lower of the two rates and determine that our maximum flow rate is 60 GPM.
The flow rate range for our example pool system is 22.5-60 GPM.
Total Dynamic Head (TDH)
Total dynamic head is the total resistance to the flow of water in your pool system and is caused by the pump, filter, plumbing and any other hardware or accessories you have installed. If you would like to determine the TDH for your pool you should visit our total dynamic head page that will help you figure this out.
In most residential in-ground pools you can expect between 50-60 TDH. In most above ground pools you can expect about 30 TDH. If you don’t have any additional features or unique plumbing you can assume these numbers will be close to your pool’s TDH.
Final Range and TDH
You have now completed your pool pump sizing and will be able to purchase a pool pump that will work efficiently and safely with your pool system. We can see that for our example pool we can determine the following:
22.5-60 GPM or Flow Rate Range
50-60 TDH or Feet of Head
When purchasing a pool pump, each model will have a chart to determine the best Horse Power (HP) for your required Flow and TDH or Feet of Head.
Additional Pool Pump Sizing Recommendations
If you are operating a salt water system and want the best efficiency while providing adequate chlorine production and circulation, we have a few recommendations. The goal is to use the smallest and most energy efficient pump for your unique pool and system. A pool pump timer is an affordable device that will also help reduce operating costs while adding simplicity to your pool system.
If you use a low powered high efficiency pump and run it 24 hours it will cost less than running a larger pump. The pool will be under a constant low level of filtration and stay in perfect condition around the clock. Variable speed pool pumps or multi-speed pumps are becoming more popular because they can operate at a low setting for longer periods of time while consuming less energy, keeping your pool sparkling clean and healthy.
If you operate a shared spa, in-floor cleaning system, solar heating system, or pressure side cleaning system, you may have to use a larger pump. For most pool owners though, we recommend a variable speed pump. That said, a two speed pump may cover the increase in flow rate at different settings, or perhaps even a booster pump might suffice.
Following is a link to a pump reference guide (courtesy of Jandy Products) which you can also use for sizing purposes. You may find it useful when making your choice to purchase new equipment: