There is no guarantee that your pool builder sized the pump properly to begin with. Lots of builders put in an oversized pump because their competitor does. no one wants to lose a sale from sizing the pump smaller.
Properly sizing the pump helps avoid wasting electricity and extra wear on the other equipment.
Step 1: Determine the MINIMUM FLOW RATE
This factor calculates the minimum flow rate that must be maintained in order to circulate all the water in the pool through the filter within a certain number of hours, otherwise known as the “turnover time”.
A good turnover time for a home pool is usually 8-10 hours. A heavily used pool will need to have a shorter turnover rate. Once you know the desired turnover rate, you can determine your desired flow rate.
Use the following equation to determine the desired flow rate:
Pool Size – number of gallons of water in the pool
Turnover Time – time to move entire capacity of the pool through filter
Flow Rate – desired flow rate in gallons per minute
Pool Size divided by Turnover Time divided by 60 Minimum Flow Rate
30,000 divided by 8 hours divided by 60 equals 63 GPM
30,000 divided by 10 hours divided by 60 equals 50 GPM
25,000 divided by 8 hours divided by 60 equals 52 GPM
25,000 divided by 10 hours divided by 60 equals 42 GPM
20,000 divided by 8 hours divided by 60 equals 41 GPM
20,000 divided by 10 hours divided by 60 equals 34 GPM
Step 2: Determine the MAXIMUM flow rate
The flow rate through any given pool system is limited by the size of the piping and the equipment. This determines the maximum flow rate through the system.
The first thing to check is the pool system plumbing. For instance, the maximum flow rate through a 1.5 inch PVC pipe is 44 Gallons per minute. Even if you have a pump that can move 100 gallons per minute, it will do very little good if the piping will only allow 44 gallons per minute through it. An oversized pump will actually strain against the piping and can be damaged in the process.
Check the PLUMBING FLOW RATE
As a general rule, the velocity of the water through the pipe should not exceed 7 feet per second.
Count the intake lines and you will see what the maximum flow into the pump will be.
For each 2.0 inch intake line, the maximum flow into the pump will be 73 gallons per minute (GPM).
For each 1.5 inch intake line, the maximum flow into the pump will be 42 gallons per minute (GPM).
In the diagram below, the system is plumbed in 2.0 inch pipe. Since the system can be run in either spa or pool mode, you have to calculate the flow rate each way.
Spa Mode – 73 GPM (One 2.0″ pipe)
Pool Mode – 146 GPM (Two 2.0″ pipes)
The maximum flow is the lower of the two, which would mean a MAXIMUM PLUMBING FLOW RATE of 73 gallons per minute (GPM).
Check the FILTER FLOW RATE
Another limiting factor is the pool filter. Each filter has a maximum flow rate. If you exceed that maximum flow rate, the filter will not operate properly and can be damaged as a result. It is very important to
The proper filter flow rates are as follows:
Recommended flow rates for different sizes of D.E. FILTERS
Surface Area 24 sq. ft. 36 sq. ft. 48 sq. ft. 60 sq. ft. 72 sq. ft.
Max Flow Rate 48 GPM 72 GPM 96 GPM 120 GPM 144 GPM
Best Flow Rate 36 GPM 54 GPM 72 GPM 90 GPM 108 GPM
While the manufacturers will specify the Max flow rate, we strongly recommend staying with the Best flow rate to avoid crushing grids due to high pressure.
Recommended flow rates for different sizes of CARTRIDGE FILTERS
Surface Area 100 sq. ft. 200 sq. ft. 300 sq. ft. 400 sq. ft. 500 sq. ft.
Max Flow Rate 38 GPM 75 GPM 112 GPM 150 GPM 150 GPM
Best Flow Rate 30 GPM 50 GPM 75 GPM 100 GPM 125 GPM
While the manufacturers will specify the Max flow rate, we recommend choosing one within the Best flow rate for best filtration and better dirt loading capacity.
In this particular system, if you have a 48 square foot DE filter, your MAXIMUM FILTER FLOW RATE would be 96 gallons per minute. In a perfect world, we would try to keep the flow rate down near 72 gallons per minute, but for now we are dealing with the maximum flow rate.
Maximum plumbing flow rate – 73 Gallons per minute
Maximum filter flow rate – 96 Gallons per minute
The overall MAXIMUM FLOW RATE is the lesser of the two – 73 gallons per minute
Step 3: Choose an IDEAL FLOW RATING for your new pump
After you have calculated the MINIMUM FLOW RATING required to meet the turnover requirements of the pool, and after you have calculated the MAXIMUM FLOW RATING based on the limitations of the filter and piping, you are ready to select the proper size pump.
Draw a simple chart like the one below. Mark the MINIMUM FLOW RATING and the MAXIMUM FLOW RATING. The space between the minimum and maximum flow rate is the ideal range.
If you have a pool with no attached spa, you can choose a pump on the lower side of the ideal range. It will be sufficient to properly filter the pool without wasting energy in the process. In that case, we would look for a pump in the 50 – 60 GPM range.
If you have a pool/spa combination, you need to consider the flow requirements of the spa jets. This will often push you towards a pump on the higher end of the acceptable scale. In that case, we would look for a pump in the 75 GPM range.
If you have a pool/spa combination, you might want to consider a 2 speed pump. It can run on low-speed while it filters the pool, and then switch to high-speed when using the spa.
Step 4: Look at the POOL PUMP RATINGS to find the right pump
Once you have decided to buy a new pump, take some time to research your options..
Don’t just assume that you need the same horsepower pump as you now have. Newer pumps such as this Hayward Tristar are way more efficient and usually produce more flow per horse power than older pumps do. As we discussed in the beginning there is no guarantee that you had the right sized pump to start with.
The sample pump chart below shows the pump curves for different sized pumps.
Suppose your system has 60 feet of head and you want a pump that will deliver 50 gallons per minute.
From this chart, you can see that a 3/4 horsepower pump will give you 58 GPM flow on a system with 60 feet of head.
Note: Figuring out feet of head (below) is difficult. Typically a in-ground pool will have 50-60 feet of head pressure and an above ground pool will be somewhere around 30 feet of head. These are only approximations.
What about ‘FEET OF HEAD’ ? What does that mean?
The total resistance to flow is measured in “feet of head”. As you can tell from looking at the pump charts, the greater the resistance to flow (expressed in feet of head), the lower the flow rate. The greater the resistance to flow, the more powerful the pump needs to be to overcome it.
The resistance is measured in “feet of head“. The best way is to approximate the resistance is as follows:
1. Measure the vacuum pressure at the pump and multiply by 1.13.
A reading of 12 inches mercury times 1.13 equals 13.5 feet of head.
2. Measure the clean filter pressure and multiply by 2.31
A reading of 22 times 2.31 equals 51 feet of head.
3. Add the two together to get total friction loss in feet of head.
13.5 plus 51 equals roughly 64 feet of head.