When it comes to pool filter replacement, most people tend to purchase the exact pool filter model that they are replacing. This is either because they are happy with it's performance, happy with the brand or want to keep their filter replacement task as simple as possible. There is nothing wrong with these reasons if you are happy and the system works. But many are looking for a better or more efficient pool filter, and we have the solution.
Pool filters, which one is best? This is a common question among all seeking to purchase a filter for the first time, or replace an existing unit. The answer may depend on a few factors, but going bigger is always better when it comes to filter size. An over sized pool filter not only increases the length of time between cleanings, but improves water quality. By slowing down the rate in which the water passes over the filter media, the more contaminates we can remove from the water. This filter rate is regulated in many commercial applications across the country. For most pool owners filter rate has been calculated during the pool design and not a concern. But for those looking to improve their filtering ability the easy way, simply remember bigger is better.
For those looking to maximize their filter system, this can be accomplish with some easy calculations. You must first calculate the pool volume, capacity and flow rates, and type of pool pump used. The pool filter fits into the equipment sequence after the pool pump, so regardless of whether you’re replacing an old filter or installing a filter on a brand-new pool or spa, matching the filter and pump is important.
1. Determine Pool Volume
The first step in finding the correct filter is to determine how much water needs to be filtered. Below are some simple formulas to use when calculating the volume of a swimming pool in cubic feet.
- 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: 1/2length x 1/2width x 3.14 (pi) x average depth = pool volume
- Irregular shape pools: We have provide a way for those to calculate pool volume using the method below.
Make a scale drawing of the pool on a piece of square-grid graph paper, with each square representing one square foot.
Now count the number of complete squares. To that estimate how many complete squares the partially squares would amount to. This gives you a close estimate of the pool’s area in square feet.
Now multiply the area by the average depth of the pool to find the volume in cubic feet.
2. Calculating Pool Capacity
Capacity is defined as the number of gallons of water that the pool will hold. Volume was in cubic feet.
Capacity formula - Pool volume x 7.48 = pool capacity in gallons
Below is an example of a large rectangular pool in which we are going to determine the pool's capacity in gallons.
A rectangular pool that is 20 feet wide and 40 feet long with an average depth of 5 feet. First you need to figure out the volume of the swimming pool by using this simple formula. 20 x 40 x 5 = 4,000 cubic feet
Now plug the volume into the capacity equation and multiply.
4,000 x 7.48 = 29,920 gallons
3. Finding The Flow Rate
The flow rate is the volume of water flowing past a given point during a specific period of time Flow rate is measured in gallons per minute (gpm) or gallons per hour (gph).
- Flow rate formula
- Capacity ÷ turnover time (hrs) = GPH
- Flow rate per hour ÷ 60 = GPM
Flow rate example below.
Say you want your pool to have an 8-hour turnover rate. The equation to find the flow rate for a 29,920-gallon pool is 29,920 ÷ 8 = 3,740 GPH
Calculating the GPM flow rate would be 3,740 ÷ 60 = 62.3. That’s the rate at which you’d want the filter to work.
4. Determining Filter Flow Rate
Next, determine the filter needed to handle the flow rate you require. Filter flow rate is defined as the amount of water filtered over a given period of time or GPM. This information has been determined by the manufacture, and is posted on the filter tank.
The filter area is the surface area of the filter medium. It is measured in square feet. The filter rate is the gallons of water that flows through one square foot of filter medium per minute during the operation of the pools filter system.
Filter flow rate formula - Filter area x filter rate = filter flow rate in gpm.
5. Filter Model Selection
No matter the filter type, be it sand, cartridge or DE, all have similar flow rate based on our formulas above. We have included a basic suggested filter size chart below to make this easy.
6. Over-Size The Filter
Always go big if you can afford it. This is because a larger filter area will lower the flow rate per square foot of media and allow the filter to capture more debris in the water, increasing efficiency.