How Variable Speed Pumps Affect Solar Pool Heaters

Solar pool heating systems use the existing pool filtration pump to circulate water, so the selection of a pump can have a profound impact on solar pool heating performance. The concept of a variable speed pump is pretty simple — when a centrifugal pool pump runs at a slower speed, the resulting energy use reduction is the cube of the speed reduction, resulting in an exponential savings of energy, but the pressure reduction is only the square of the speed reduction.

Plain English?

You can get the same water filtration turnover in your pool at a fraction of the energy cost. Given that the pool pump is often the second biggest energy hog in a Southwest Florida home (after air conditioning), a variable speed pool pump is the single most affordable upgrade with the best return on investment when making energy efficiency upgrades to a home.

Variable speed pumps have been available for several years, and have proven reliability and energy savings. Many people are finding that they are required to upgrade their single speed pumps when they go bad, and that changes the dynamics of pool operation and flow. Since older pools were often built without energy efficiency in mind, the speed and horsepower required of a pump is typically higher than new pools. Nonetheless, big energy savings can be achieved by circulating pool water longer and slower.

Pool service companies are still getting on board with this technology. Some claim that variable speed pumps cause poor sanitation and algae growth. The big problem here is that people don’t set their pumps up correctly from the start. You can’t just select a slower circulation speed and leave your daily pump run time the same. The idea is to run your pump for a much longer duration at a lower speed. I recommend setting a schedule to run the pump at a relatively high speed for a couple hours per day to get effective skimmer action and boost the turnover a bit. You should have no problems with sanitation of algae growth if your pool is filtered and turned over enough.

Solar pool heating requires a suitable flow rate for peak performance. Running a variable speed pump at a very low speed will not heat the water up more by slowing the water flow in the panels. The result is quite the opposite. Heat transfer is best when there is a large differential between the pool water and the solar collector surface. A small temperature rise with a high flow rate is the most efficient way to heat a pool with solar panels. For that reason, one of two strategies must be employed to make solar pool heating work best with a variable speed pool pump:

1. Use a low circulation speed for a long period of time (as long as 24 hours per day) and set a schedule to run the pump at a higher speed during the hours most likely to result in effective solar heating performance. This option will allow you to guarantee your pumping cost reduction, but you may run your pump at a higher speed unnecessarily when solar heating is not needed or possible, and you may actually cool your pool at times when solar pool heating is needed but not available.
2. Use an automated solar controller to control the pump speed, increasing the pump RPM when solar heating is needed and available. This will drastically improve your solar pool heating performance overall, but will make your energy savings variable, because you lose a bit of control over when your pump speed increases.

No matter what, you will still save a tremendous amount of energy with a variable speed pump with solar pool heating. We recommend a solar pool heating controller to balance energy savings and heating performance.

All Florida pools built under the new building code are required to have multi-speed or variable speed pumps if the pump required is over 3/4 horsepower. We are seeing two major trends in pool building:

1. Pool builders are building super-efficient pools with large plumbing size, very low restriction, and no pool features like waterfalls and spas that require higher pump horsepower to achieve desirable flow rates.
2. Builders of larger pools and pools with spas or features are opting for variable speed pumps (over 2-speed) for their incredible energy savings, flexibility, and quiet operation.

In either case, integrating a solar pool heater with a new pool has some important considerations.

In the case of a single speed 3/4 HP pump, you are walking a fine line between energy savings and having enough flow to achieve good solar pool heating performance. Any increase in pressure, like from a dirty filter, will reduce the flow rate substantially, and may make solar pool heating performance suffer. To combat this, you want a solar pool heating panel and system that results in very little added head pressure to the pump. That is precisely why we do two things: we use the iSwim solar pool heating collector with the lowest flow restriction in the industry, and use all 2 inch PVC plumbing and valves. This reduces the restriction in ways our competitors cannot, and improves performance when a low horsepower pump is used to save energy.

When a variable speed pump is selected for a new pool, we often find that the pool builder just leaves the pump at the factory default, or just sets a single speed and a single schedule each day. (Side note: the building code naively requires that a pump over 3/4 HP be capable of multiple speed control, not that it is actually set up for multiple speeds). This is like buying a more efficient air conditioner, and then leaving the windows open during operation. The beauty of a variable seed pump with solar pool heating is that we can control the pump speed (and consequently the flow rate) using automated controls, and balance your energy savings and pool heating performance. We can set your pump for a long run time at a slow speed, and ramp up the speed when solar pool heating is needed. This has the added benefit of increasing your circulation for periods of each day, reducing the chance of pool sanitation and algae growth.

One big sales feature the solar pool heating industry has used over the years is that solar pool heating systems will not cost you a penny once you incur the initial cost. The new reality is that we need to run new variable speed pumps at a higher speed than would be used without a heater to get proper flow during solar pool heating, but this cost is very low — pennies per day typically. To combat this potential for higher energy costs, it is critical that you select a panel and system that will result in the lowest added back pressure. Ask you solar pool heating dealer about the back-pressure added by their panels and compare it to others. Do not settle for 1-1/2″ plumbing and valves. Two inch PVC pipe carries about 65% more volume of water and has only 30% of the friction loss of 1-1/2″ pipe.

Side note: Regardless of your heat source, you will need to run a variable speed pump at a high enough speed for heater operation. Both electric heat pumps and has heaters require adequate flow for safety and performance. Gas heaters can require very high flow rates, increasing pumping costs substantially. Solar pool heaters will operate at very low flow rates, albeit with lower performance, where other heaters may not operate at all.

The best thing about the combination of a variable speed pump and solar pool heating is the ability to both save energy when you can, and optimize the flow rate for solar pool heating performance when needed. Single speed pumps don’t give us the option to “dial in” the best flow rate for solar pool heating, so a panel and system with low back pressure is important. If you are considering a variable speed pump to retrofit your pool or for your new pool, it is important that you make an efficient selection for your solar pool heater, and critical that your pump is set up properly to balance energy savings and heating performance. If you have questions abut how to make this happen, we are here to help!