Calculating your Optimum Variable Pool Pump Speed
New smart pool pumps come with variable speed motors that you allow you to optimize your flow rate through speed control in order to keep your pool clean, run water features, and/or pump water to solar collectors on your roof while using less electricity to do so compared to a pool pump with a single speed motor.
You can do this by adjusting the pump speed, through RPM settings, which increases the flow, measured in gallons per minute or GPM.
Some pool pumps have built in flow meters that will tell you your gpm flow rate, but most only provide performance charts that let you determine your gpm at specific RPMs. However, to use these charts you need to know your Total Dynamic Head (TDH).
This article will help you calculate your total dynamic head allowing you to determine your gpm flow rate at various RPM speeds letting you run your smart variable speed pool pump at the slowest, most energy efficient, settings to save money and run your pool pump quietly.
Calculating Total Dynamic Head with Pressure and Vacuum Gauges
You can calculate the total dynamic head (TDH) in your pool plumbing by placing a vacuum gauge on the suction side of the pool pump and a pressure gauge at the pressure side. If your pool filter is clean and has a pressure gauge you can use that as well.
After the gauges are installed, run your pool pump at the various RPM speeds that match the performance curves in your pool pump manual and record the vacuum gauge reading in Hg and the pressure gauge reading in psi.
To calculate the TDH, total dynamic head, multiply the Hg reading by 1.13, the psi reading by 2.31 and add the two for the TDH.
- Multiply Hg reading by 1.13
- Multiply Psi reading by 2.31
- Add products together for TDH
Using Total Dynamic Head to find GPM flow rate
In your pool pump manual there should be a performance chart like this one from the Pentair SuperFlo below. Using the total dynamic head value you calculated paired with your RPM speed you can use this chart to determine the GPM flow rate.
For example, let's pretend our TDH calculation came out to 55 when we were at 3000rpm. Start on the left axis of this chart labeled total dynamic head, draw a line to where it intersects the curve for 3000rpm, and then draw a line down to the bottom GPM axis.
If you follow the red line in the chart above for our example you'd see our GPM is 45. At 3,000 RPM our pump would move 45 gallons of minute per water under that amount of dynamic head.
The TDH values change at different speeds so be sure to rerun the calculations for the different RPM curves to determine GPM at higher and lower speeds.
Using GPM to Optimize Runtime and Speed
A variable speed pump will use the least amount of electricity at low speed, even if you need to run it longer, to filter the same volume of water in your pool than running it faster for less time.
Some people run theirs for 24 hours a day at an extremely low speed. It saves money and keeping the water circulating keeps it more sanitary than if it were stagnant.
However, there are several factors to consider when determining your run time.
- Total gallons of water in your pool
- GPM requirements of pool features
- GPM requirements of suction-side pool cleaners
- Solar collector flow rates
A simple calculation without accounting for 2 through 4 would have you divide your pool volume in gallons by your GPM to determine how long you should run your pool for. In most residential settings a 1x turnover of the pool volume is recommended
Example for a 20,000 gallon pool w/ 50 GPM flow rate
- 50gpm * 60min = 3000gph
- 20,000 gallons / 3000gph = 6.67 hours of runtime needed
Considerations for Solar
In my case, I have a suction side pool cleaner and solar heat collectors on my roof. When my solar controller goes on there is more pressure so my TDH changes. In addition, the solar heater works best at a 32gpm flow rate.
This means I have to run at a higher RPM to make up for it when heating the pool.
My Pentair SuperFlo VS has 3 different speed programs it can automate per day so I paired the runtime and RPM to run at the slowest speeds that will filter through the total volume of water in my pool per day (19,000 gallon pool) during daylight hours while staying at flow rates good for solar and my pool cleaner.
- 1900rpm @ 350 watts / 31gpm / 5 hours
- 2600rpm @ 775 watts / 47gpm / 2 hours
- 2200rpm @ 500 watts / 37gpm / 2 hours
That gives me a 9 hour runtime and filters 19,380 gallons.
I posted my Pentair SuperFlo VS flow rate calculation spreadsheet on drive if you want to use it for your own project.
I found this method very helpful for calculating my pool pump's flow rate. Alternative calculations either require a dedicated flow meter, that I didn't have the plumbing space to add, or error prone estimates based on length of pipe, diameters, and height.
I hope you also found this useful if you were in a similar situation and trying to figure out how to best take advantage of your variable speed pump controls.
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