A good filtration system for a pool or spa depends on your ability to properly size and select the filter.

To accomplish that, you must calculate the vessel’s volume, capacity and flow rates, and some other key factors — including the type of pump. After all, the filter fits into the equipment sequence after the pump, so regardless of whether you’re replacing an old filter or installing a filter on a brand-new pool or spa, you’ll need to match the filter to the pump and the size of the vessel.

Below is a step-by-step guide that explains how to correctly assess filtration needs.

1. CALCULATE THE VOLUME

In order to properly identify the correct filter model it’s important to determine how much water has to be filtered. There are smartphone apps that will perform these calculations for you. Otherwise, use these simple formulas when calculating the volume of a swimming pool:

Rectangular pool: length x width x average depth = pool volume

Circular pool: radius x radius x 3.14 (pi) x average depth = pool volume

Oval pool: ½ length x ½ width x 3.14 (pi) x average depth = pool volume

Irregular shapes: For pools that are not truly rectangular, circular or oval, calculate the volume using the following method:

Make a scale drawing of the pool on a piece of square-grid graph paper, with each square representing one square foot. (Note: The drawing can be made to any scale you want as long as you keep it uniform.)

Now count the number of squares. Don’t forget to estimate how many complete squares all of the partially filled squares would amount to since that provides a close estimate of the pool’s area.

Multiply the area by the approximate average depth of the pool, and you will find the volume in cubic feet.

2. CALCULATE THE CAPACITY

Capacity is defined as the number of gallons of water that the pool will hold (as opposed to volume, which is a spatial measurement).

Capacity formula: Pool volume x 7.48 = pool capacity in gallons

Example: Say you have a rectangular pool that is 40 feet long and 20 feet wide, with an average depth of 5 feet. First you’d need to figure out the pool volume, so simply plug these numbers into the volume equation and multiply: 40 x 20 x 5 = 4,000 cubic feet (pool volume).

Now plug the volume into the capacity equation and multiply: 4,000 x 7.48 = 29,920 gallons (the pool’s capacity).

3. FIND THE FLOW RATE

The flow rate is the volume of water flowing past a given point during a specific period of time, measured in gallons per minute (gpm) or gallons per hour (gph).

Flow rate formula:

Capacity ÷ turnover time (in hours) = flow rate per hour

Flow rate per hour ÷ 60 = flow rate per minute

*Example: Say you want to set up the pool to have an 8-hour turnover rate. The equation to find the flow rate for a 29,920-gallon pool (regardless of its shape) is: 29,920 ÷ 8 = 3,740 gph. *

To calculate the flow rate per minute in this case: 3,740 ÷ 60 = 62.3. That’s the rate at which you’d want the filter to work.

Stating this situation another way; the calculations for your 29,920-gallon pool show that you require a flow rate of 62.3 gpm to filter the pool’s capacity in eight hours. Therefore, your goal is to determine which model of filter can handle 62.3 gallons of water per minute — resulting in the desired complete turnover every eight hours.

4.DETERMINE THE FILTER FLOW RATE

Next, determine if the filter you want to use can handle the flow rate you require. The filter flow rate is defined as the amount of water filtered over a given period of time, expressed in gallons per minute.

Filter flow rate formula: Filter area x filter rate = filter flow rate in gpm.

The “filter area” is the surface area of the filter medium. It is measured in square feet. The “filter rate” is the number of gallons of water that flows through one square foot of effective filter medium per minute during the operation of the circulation system.

Example: Say you have a filter area of 5 square feet and a filter rate of 12.5 gpm. (These numbers are based on a model of sand filter.) To calculate the filter flow rate: 5 x 12.5 = 62.5 gpm. That’s awfully close to the desired flow rate of 62.3 in the example of Step 3.

Tip: If determining the filter flow rate seems a bit daunting, just get both figures — the filter area and rate — from the manufacturer. With their figures, you’ll be better able to select the correct filter to suit your needs.

5. MAKE YOUR FILTER CHOICE

Do you prefer sand, diatomaceous earth or cartridge filters? Pressure or vacuum filters? If you have narrowed your filter choice down to a preferred manufacturer, it will make it even easier to perform your calculations and select a filter that will do the job.

The example we’ve been using refers to characteristics of a sand filter. It’s important to note that both DE and cartridge filters have notably lower filter rates. (Typically, high-rate sand models filter water at a rate of 20 gpm per square foot, DE models work at 2 gpm/sq. ft. and cartridge units at 1 gpm/sq. ft.) Nevertheless, the mathematical relationships between filter area, filter rate and filter flow rate will remain the same.

6. OVERSIZE THE FILTER

That’s right. When matched with the proper pump, you can and should oversize the filter. This is because a larger filter area will lower the flow rate per square foot of media, enabling the filter to capture even more debris and thus increase its efficiency.

Filters that are larger than indicated by calculations are particularly necessary for pools with heavy bather loads and for backwashing (because the filter requires additional size for backwashing).

How oversizing works: Referring back to our earlier example, you could either select a model with a larger filter area or a model with an increased filter rate. To allow for debris buildup and backwashing, you might select a model with a filter area of 5 square feet and a filter rate of 20 gpm. That would yield a filter flow rate of 100 gpm — well beyond the desired rate of 62.3. Alternatively, you could choose a filter with 4 square feet of filter area and a filter rate of 25 gpm per square foot.

Note: If you size your sand or DE filter too large, there won’t be enough pressure to backwash it clean. So, be sure to maintain the necessary backwash flow.

7. LIMIT THE FILTER RATE AND ADJUST THE FILTER AREA

Filter rates on pools are regulated in many locations. The National Sanitation Foundation sets maximum filter rates to ensure effective filtration because the faster the water passes through the medium, the less effectively it is cleaned.

Filter rate ceilings are imposed most often for commercial facilities. If this is the case where you live, you may have to compensate by selecting a model with a larger filter area. By doing so, you can achieve the same flow rate without exceeding the maximum filter rate.

8. SELECT THE CORRECT FILTER

Taking all the above calculations and factors into consideration, you’re now ready to select the proper filter for your particular pool.

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