How to Size a Filter for a Commercial Pool

Commercial pool filters are rated by gallons per minute (gpm) which is exactly what it sounds like, how many gallons of water the filter is able to filter or "turn over" in one minute. So the first question when sizing a commercial pool filter is how many gallons of water are in the pool. There are several formulas for calculating this based on the shape, dimensions and average depth. For rectangular pools, you would multiply length x width x average depth (in feet) x 7.5. For round pools, multiply diameter x diameter x average depth (in feet) x 5.9. For oval pools, multiply long diameter x short diameter x average depth (in feet) x 5.9.

This calculation is trickier for free-form pools but you can first calculate the pool's square footage by drawing a diagram of the pool (to scale) on graph paper. Count all of the squares, also accounting for the partially filled ones (in other words, estimate how many full squares the partial ones would equal). This will give you a pretty accurate estimate of the square feet; multiply square feet x average depth x 7.48 to get the number of gallons.

Once you have calculated the gallons of water in the pool, the next step is determining the desired flow rate. You would divide the number of gallons by turnover time (hours) to get the gallons per hour (gph). Take that number and divide by 60 to get the gallons per minute (gpm). For example, if you have a 100,000 gallon pool and want to turn over all of the water in 8 hours, your formula would be 100,000/8 = 12500 gph and then 12500/60 = about 208 gpm. In this example, your pool pump would ideally be sized properly to output 208 gpm. You would then choose a filter rated to handle that flow and then some.

It is highly recommended that you oversize your pool filter to increase efficiency and prevent undue strain on the pool pump. An oversized filter provides greater filter area inside the tank which means water will pass through at a slower rate per square foot, allowing more time for the filter media to catch debris. This is especially important for commercial pools that accommodate heavy bather loads. The larger size will help with backwashing also but you do want to be sure the filter is not so oversized that you lose the pressure needed to backwash. Oversizing may also help you meet required filter flow rates if these are regulated for commercial pools in your area. The NSF sets these rates and you should see a NSF approval on commercial pool filters, for example the Pentair advertises that their THS Series commercial sand filters are NSF approved from 5-20 gpm/sq. ft.

Some commercial pools can be sufficiently filtered by a single tank system. The Pentair Triton C filters and Hayward Commercial filters would be examples of single tank filters designed for commercial pools. Depending on the particular filter model and it's filtration area, these commercial filters will work with pools approximately 35,000 - 70,000 gallons (based on a 8 hour turnover). The Triton C filters can also be manifolded together to create a two filter system; a tandem filter piping kit is required for this. We'll go into more detail about manifolding two filters together using the THS Series as an example.

To connect two commercial sand filters together, a kit containing valves, hardware and plumbing subassemblies is required. Pentair refers to this as a face piping kit; you may also see it referred to as a manifold kit. The Pentair THS Series Horizontal Sand Filters have kits available with either butterfly valves or diaphragm valves. Since butterfly valves are also used on the Pentair Triton C commercial sand filters, we'll be giving a general overview of installing a manifold kit using butterfly valves.

It is very important that the face piping kit is installed only after the fiter tanks are properly positioned in their permanent location. The butterfly valve face piping kits come with pre-glued subassemblies; some components will need to be glued by the installer. Dual tank systems will require 8 glue joints with the remaining connections being flanged or grooved coupling connections. Pentair recommends hand tightening only for these connections at first so adjustments can be made as necessary later on in the installation.

To start, the two tanks must be leveled so that the influent and effluent pipe connections are level. This may require shimming and saddle adjustment. Then the influent pipe assembly would be trimmed if necessary, glued, allowed time to cure, and installed onto the tanks.

Then an effluent subassembly will be installed on the effluent pipe of each tank, followed by a 90 degree elbow and another subassembly. Leveling and possibly some cutting of pipe may be required at that point; once that has been completed, the assemblies would be glued together, allowed to cure, and then connected to the effluent pipe connections.

Now butterfly valves would be installed on the two flanges on the influent piping the side of the tank, positioned so that the handles are pointing downward when valves are closed. Another subassembly is attached to influent piping side of the tank and then hardware is installed on the two flanges. This process is then repeated for the flanges on the effluent side of the tank.

After all of the piping is in place, make any necessary adjustments and then follow Pentair's specific tightening pattern instructions for all flange bolts. They suggest lubricating each bolt or nut to help relieve stress due to friction. Pentair also cautions that all piping should be fully supported with adequate bracing and hangers to prevent damage from weight and vibration.