How Variable Speed Pumps Affect Solar Pool Heaters

Pentair IntelliFlo Variable Speed Pool Pump
Pentair IntelliFlo Variable Speed Pool Pump

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.
Pentair SolarTouch Solar Pool Heating Controller
A basic solar pool heating controller balances energy savings and heater performance.

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!

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  • Craig K says:
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    i’m learning from your answers. But here’s my question. I just installed a propane heater for my pool. I already have a good solar system. Can I use them both at the same time?
  • Lorie says:

    What is the pest 10 hour RPM cycle? Is 1550 okay as the lowest RPM and 2000 as the highest? Our Pentair is over 16 years old. Our Pentair solar is at 2500.

    Thank you .

    • Jason Szumlanski says:
      Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
      Every pool is different. It sounds like you need a pool professional that is familiar with solar to help you on-site.
  • Lorie says:

    Hello,
    We have Pentair pump with the pool and just installed Pentair SolarTouch. It was working but now the water is not pushing through the PVC pipes where we see the water flow through the pvc pipes like we did. The RPMs was turned down to 2500 from 3000 during the program cycle. Would this make a difference?

    • Jason Szumlanski says:
      Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
      Yes, that would matter. External control programs 2 and 4 need to be set on the Intelliflo VSF (or VS) pump to run at a sufficient speed for solar.
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  • James says:

    We want to get a home solar system installed but the solar pool heater is taking up the prime space for that. Can we ditch the pool solar then put up a home solar system to cover our electric bills. Then put in a VS pool pump and a VS pool heat pump? Also If I use a VS pool heat pump do I need the other pump?

    • Jason Szumlanski says:

      Hi James,

      Yes, if you prefer to have solar electricity and you don’t have room for both, your strategy would work well. You will want a variable speed pool pump for circulation and an electric heat pump. There is no such thing as a “variable speed heat pump.”

      A “heat pump” is not really a pump at all in the traditional sense – it does not circulate any water. A “heat pump” is a misnomer or at least a confusing description. The “pump” part refers to the “moving” of heat from one place to another.

      Think of a heat pump as an air conditioner working in reverse. It is the job of a heat pump to move heat from one place (the air) to another place (the pool water). It does this by compressing a refrigerant with a compressor. It relies on your variable speed pool circulation pump to move pool water through a coil inside the heat pump. The flow rate needs to be sufficient to promote good transfer of heat. The heat pump unit itself runs at a single speed – the fastest it can. It will heat your pool at a rate that is based on a variety of factors, most importantly the relative and absolute temperatures or air and water and ambient humidity. The electricity use will also vary based on the conditions. But it is not referred to as “variable speed.”

  • Karen Taylor says:

    We are about to upgrade our pool equipment to an IntelliPro VSF Filter Pump:
    013004

    And a Clean and Clear Plus Filter: 160332

    If we have been considering having solar panels installed to heat the pool, do we need to be doing it all at the same time?

    • Jason Szumlanski says:

      You do not need to do it at the same time. One VERY important thing is to keep the cable that comes with the pump. It is proprietary and required to connect the pump to an automation system in the future, including solar controllers. It’s expensive to replace if you lose it.

  • EDWIN EVANGELISTA says:

    Bill Lathan, what sensors? I just replaced a single speed to a varíable pump and when I tied it before the closing season here in NY, the heater was not heating the pool.

    • Jason Szumlanski says:

      Edwin, Bill is probably talking about the solar valve not turning on at all due to a failed sensor on an automated controller.

  • Robert leon says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hi i have the same trouble like john laplante. I have a Pool 11,000 gallons here in arizona and i have 10 panels (two more panels for the size). And doesnt rise the temp, i have a variable pump speed program to 2000 RPM working 9 hours at day.
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  • Peter Sorvino Please advise says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hello Jason, I have a 21 foot round aboveground pool running on a two speed 1 1/2 hp motor. I would like to add solar heating on my roof approximately 12 feet above the pump. I was wondering if I can simply add a Y connection, run my pump on high and feed both the Solar heating system Infiltration at the same time. Of course I will aired a cutoff valve on the heating system, in case I do not want to use the heating system. Then I can run the pump on low. I have an above ground pool and really don’t want to spend a lot of money on electronic valves and thermostats, etc.. What I’m suggesting is totally passive. I run my pump approximately seven hours a day and it would be nice to run the heating system at the same time.
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  • john laplante says:

    solar panel not producing heat. have tried changing rpm settings, changed thermostat temp settings. all valves working properly. pipe going to panels feel warmer than pipe coming from panels, water is circulating and electric valve opens/closes when needed. am in fl and water temp has not changed in 2 weeks. pool temp stays at 68 degrees

    • Jason Szumlanski says:

      Hi John,

      It sounds like you need a pro to come out and see what’s happening. In all likelihood, a valve is in the wrong position. You should not try to gauge effectiveness on the “feel” of the pipes going up the wall. If it is working properly, the difference in temperature in the water going up and down will be so small that it will be imperceptible to your touch.

      • Bill Latham says:

        Hi John- since this is an old thread you probably have this fixed. I had the same thing (i’m in FL also) and had to replace the sensors.

  • rob says:

    Hey Jason do yo know of anyone using a solar panel to power an independent pump on the solar panels. The water only flows when there is enough sun. Any recommendations of the pump to be used would be great. I understand the whole heat transfer stuff, what water speed to you recommend? do you have a thermal model for the systems it must rely somewhat on the thermal diffusivity of the plastic in the panels.

    • Jason Szumlanski says:

      Rob: That is not a practical (read: cost effective) use of solar energy. AC powered variable speed pumps are now so efficient that there is no case for a DC direct pump. You would need a DC pump of a substantial horsepower and a lot of solar electric panels to make something like this work effectively. Since you need to circulate pool water anyway, it costs nothing, or next to nothing, to send water through the solar panels with a simple bypass valve. Years ago the local solar industry dabbled in solar direct pumping for pools, but they proved to be an unreliable solution to pool sanitation needs, so a backup AC powered pump was needed anyway.

      You are better off getting an efficient AC variable speed pump and installing a small photvoltaic system to offset its net electricity use.

  • Stephen Meece says:

    I’m currently using a solar touch with an intelliflo pump. I have solar heating installed but find I have to run the pump at a higher speed just to move the water up through the panels in order to use the pool sweep. What is the best way to tackle this and keep my power usage to a minimum? Is there a product that can control the automatic valve to the solar to help with efficiency?

    • Jason Szumlanski says:

      The Solartouch does exactly what you are asking. It turns the solar valve on and off as solar is needed and adjusts your pump speed to the appropriate speed for effective operation balanced with energy savings. It sounds like your pump or solar installer did not program your pump speeds properly. If you are using a suction side vacuum, you can just set a schedule on the pump for an hour or two per day to ensure it runs at a speed that is suitable for good suction.

  • Chucklechan says:

    Is there a way to avoid automation electronics? Controlling my filter and salt water generator is easy and can be handles by the motor programming controls.

    BUT, turning the solar system on and off according to heat demand is not addressed. I don’t want to have to buy an electronics panel to put between them just to run a VS pump.

    Suggestions?

    • Jason Szumlanski says:

      You really need a pool automation system or basic solar controller if you want to run a variable speed pump with solar pool heating. Otherwise you will need to run the pump at the solar speed all the time, or at least when solar is most likely to be needed. The best way to balance solar performance with energy savings is to use a controller. There is really no way around that.

  • Frank Tuccori says:

    Does it stress the variable speed pump to push to solar panels at lower speeds? Wouldn’t want it to burn out.

    • Jason Szumlanski says:

      This situation will not stress the pump, but it will also not heat your pool or circulate water adequately for proper sanitization. When you are running water through solar panels, you should shoot for an adequate flow rate that balances energy use and solar panel performance. Your solar professional should be able to advise you on the proper flow rate.

  • Chris says:

    Hi Jason. This is a very informative article, but one thing doesn’t make sense to me. You stress the importance of reducing head loss. Since head loss is converted to heat, I don’t see that as a real loss of efficiency when heat is the objective of the solar system. What am I missing? – Thanks

    • Jason Szumlanski says:

      I think we’re talking apples and oranges here. Head loss is converted to heat due to friction. That does happen when a variable speed pump operates at faster speeds. But the amount of heat generated from head loss and transferred to pool water is minuscule. The increased performance at higher speeds comes from better heat transfer in the solar collectors (higher delta T, or difference in temperature). Heat exchange efficiency is related to fluid flow rate. This has nothing to do with heat conversion from additional friction.

  • Christina says:

    What should the flow rate be and what should the rpm’s set at?

    • Jason Szumlanski says:

      Hi Christina,

      That depends on a lot of factors. A good rule of thumb for 4×12 collectors is 5 gallons per minute per panel. However, the speed will depend on how much total dynamic head (pressure) is in your system at that flow rate. It is impossible to determine the speed without evaluating your specific pool equipment and measuring the flow rate. The cleanliness of your filter will also be a factor. You want to set the speed slightly higher with a clean filter to account for your filter getting dirty.

      I wish there was a simple answer for you, but unfortunately, it takes a bit more work to figure this out. If you have a properly located vacuum relief valve, there are some tricks we use to approximate a good flow rate, but that is a big assumption because I have seen some strange installations out there.

  • Jeff P says:

    I live in Central Florida and have 7 solar panels installed a few years after my pool was built which came with a 1.5 HP single stage pump. Do i need to increase the HP if I go to variable speed pump or stay the same. Can I use a 2 stage pump.

    • Jason Szumlanski says:

      You do not need to upgrade the horsepower, but I suggest you do. Higher horsepower means you can run it at a lower speed to achieve the same flow rate. That results in cooler operating temperatures and far less noise. Learn more about why you should oversize a variable speed pump.

  • Dan Passmore says:

    That is a great idea! Move the speed of the pump to a higher speed when the heating is going to be impacted and slow it back down at night. I bet you save a ton of money this way.

    • Randy says:
      Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
      Keep your single speed pump for heating and only run it when heating. No running the pump other wise. What a concept!

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