Swimming Pool Blog
by Rob Cox July 5, 2013
Sizing a Pool Filter Pump
Sizing a pool filter pump refers to matching the pump size to the filter size, pool size and pipe size.
An easy way to size a pool pump is to replace with the exact same model that you had before - unless your state requires a Variable Speed pump.
With Variable Speed Pumps, you don't need to size the pump to your pool - it's a one size fits all pump for all inground pools - no sizing needed!
However, and VS pumps aside, there are times when a pool owner may want to make a calculated pump sizing; during one or more of these situations:
- Complete pool renovation and system replacement
- Pump was never very powerful, and filter pressure was low <5 psi.
- Pump was too powerful, and filter pressure was high >20 psi.
- Energy bills are steep, can a lower hp pump be used?
CALCULATE DESIRED PUMP FLOW RATE
The Turnover Rate is the time it takes for all of the water in the pool to pass through the filter. An 8-hour turnover rate is considered standard.
Pool Volume in Gallons ÷ Turnover Rate in Minutes = Flow Rate
The formula for Turnover requires that you know your pool size - in gallons. If you have paperwork from your pool builder, this may list the amount of gallons in the pool. If not, break out the calculator, and a measuring tape, if needed.
- Round Pools - Diameter (ft.) X Diameter (ft.) X Average Depth (ft.) X 5.9 = Total Pool Gallons
- Oval Pools - Length (ft.) X Width (ft.) X Average Depth (ft.) X 6.7 = Total Pool Gallons
- Rectangle Pools - Length (ft.) X Width (ft.) X Average Depth (ft.) X 7.5 = Total Pool Gallons
Average depth is calculated by adding the deepest part of the pool to the shallowest part of the pool, and dividing by 2. For instance, if the shallow end is 3' deep, and the deep end is 8', the average depth is 5.5. (3+8 / 2 = 5.5)
Sample Desired Flow Rate calculations, based on an 8 hour (480 minutes) turnover rate:
10,000 (gals) ÷ 480 (mins) = 21 GPM
25,000 (gals) ÷ 480 (mins) = 52 GPM
35,000 (gals) ÷ 480 (mins) = 73 GPM
OK - so, we've calculated the desired flow rate, in gallons per minute (GPM), based on our pool gallonage. This is a good start, but it's only half of the equation - we also need to calculate the resistance in the system that the pool pump must overcome.
CALCULATE SYSTEM RESISTANCE
Total Resistance is made up of all the pipe lengths, turns of the pipe, and all of the equipment that the pump must pull and push water through. Resistance is measured in Feet of Head. To calculate the system resistance, add together these two sums:
- Multiply the clean filter pressure by 2.31 to measure resistance after the pump
- Multiply the pump vacuum by 1.13 to measure resistance before the pump
- Add the two together to compute the total resistance in the system.
Now, most pool owners don't have a vacuum gauge installed before the pump. If you can get ahold of a vacuum gauge (not a pressure gauge), unscrew the front pump drain plug, and thread the vacuum gauge in its place. Turn the pump on and take your reading.
In the absence of a vacuum gauge, you can estimate the suction side vacuum to be something close to 1/3 of the pressure side reading. For example, if your filter pressure x 2.31 = 30, assume that the vacuum reading would equate to 10 ft. of head. Total resistance = 40 ft. of head.
Another way to calculate the suction side resistance, in the absence of a vacuum gauge, is to add up all the bits of pipe and valves before the pump.
- For each 1.5 in. skimmer or main drain, add 2 ft. of head, or 1 ft., if the piping is 2 in. diameter.
- For each 10 feet of 1.5 in. pipe used, add 1 ft. of head, or .5 ft. for each 10 foot run of 2 in. pipe.
- For each 90 degree fitting or valve between the pool and the pump, add 1 ft., or 0.5 ft. for 2 in. pipe.
- For a check valve installed before the pump, add 2 ft. of head.
FILTER DESIGN FLOW RATE
Another measurement to take note of, when sizing a pool pump, is the 'Design Flow Rate' for your filter. This is printed on the filter label, in GPM (gallons per minute). You can also find the information online, on our pool filters pages, or in the pool filter owner's manual. This should be considered the maximum flow rate, for optimum filtration and safe performance.
Just make sure that your Pump Desired Flow Rate is not more than the Filter Design Flow Rate, and for best performance, your actual flow rate should be at least 20% less..
PUTTING IT ALL TOGETHER
Now that we know the 'Desired Flow Rate', based on our pool size and an 8 hour turnover - and the measurement of 'Resistance' the pump must overcome - you're ready to shop for a new pump!
When comparing pool pump models and narrowing down your pool pump selection, look at each Pump Flow Curve or Flow Chart for each pump. On the vertical axis is the amount of resistance in your system, and on the horizontal axis is the Desired Flow Rate.
This means that a Whisperflo pump will produce more flow than the Max-Flo, as you can see in their respective flow charts, shown here.
At a Flow Rate of 80 GPM, and a Resistance of 40 feet of head, a 3/4 hp Whisperflo could be used. The 1.5 hp Max-Flo would be required for the same flow and head.
It may be wise to select a higher flow pump, in a lower horsepower size - for savings on your electrical consumption. A 1 hp pump can consume half as much electricity as a 1.5 hp or 2 hp pump.
To sum it all up, in sizing a new pool pump, you can either install a Variable Speed pump and be done with it, or do a little mathematics.
- Calculate your pool size in gallons
- Compute your desired flow rate
- Figure out the system resistance to the flow rate
- Consult the Flow vs. Head chart for each pool pump
Remember that a smaller horsepower pool pump that can do the job could cut your electrical bill in half, and that a pump that's too powerful may result in inefficient filtering and damage to the filter media.
Also, you don't need to match the manufacturer of pump and filter. You can use a Hayward filter with a Pentair pump, for instance - no problem there.