Solar Pool Heating Panel Ideal Flow Rate

Can You Run Water Through Solar Pool Heating Panels Too Fast Causing it Not to Heat Long Enough

In Education, Maintenance 8 Comments

This is always a good question. How fast should I run water through my solar pool heating system? Most think that running water fast through solar panels will not let them heat the water adequately. In fact, the opposite is generally true.

Solar pool heating panels work most efficiently when the difference between the panel surface temperature and the pool water is greater. As the pool water temperature approaches the solar panel temperature, the rate of heating decreases, making the panel less efficient. Most solar pool heating panel systems require an absolute minimum of 2 gallons per minute (GPM) per panel. Three to five GPM is a better target, depending on panel size. You won’t get much additional heating performance from higher flow rates.

Do not attempt to make your solar pool heater work better by slowing down water flowing through the panels. On the other hand, it is unnecessary to put more water through the panels than needed, which would just waste energy. The ideal scenario is to use a variable speed pump to “dial in” the optimum flow rate depending on the number of panels in your system. For example, if you have 8 panels, 32 gallons per minute might be ideal depending on the panel size, leaving a little extra flow to account for your filter getting dirty. Flow rates up to about 7 gallons per minute are fine for larger panels in the 4’x12′ range, but you get greatly diminishing returns from higher flow rates.

As you can see in the chart below for a FAFCO brand Sunsaver ™ solar pool heating panel, the panel reaches 80% efficiency with 4 GPM flow rate, and the curve really flattens out for flow rates above that.


Solar Pool Heating Panel Ideal Flow Rate


  1. Hi Jason. I was chatting with Susan and she said to ask you. Thanks so much for this article. I don’t understand ” if you have 8 panels, 32 gallons per minute would be ideal” Doesn’t the water flow from one panel to the next so that it would go about 30 gallons a minute through all? Or do you have a header that drops 4 gallons into each panel? Do you have a sketch or layout for how to use 5 of your glazed collectors?

    Also, I have my pool pump, but also a small solar pump – 2 Laing D5’s in series that can run all day long. Is there a way to have both running through the same panels so that when the stronger pump goes on it doesn’t force the others to run backwards into the pool? Perhaps a floating ball one way valve?

    1. Author


      Solar pool heating panels are arranged in a parallel configuration. It is essentially a z-configuration manifold. 32 gallons per minute of total system flow means you will have approximately 4 GPM flowing through each panel if properly balanced. Yes, each panel has an intake and discharge header. Solar pool heating uses unglazed collectors, not glazed collectors.

      Your DC solar pumps have nowhere near the flow rate required for effective solar pool heating performance. If run in parallel with your filtration pump they would be deadheaded with no flow. You could install a check valve to stop reverse flow, but again, the pumps would be deadheaded. There is no application for pumps this small in a solar pool heating environment. You need a lot more horsepower to generate the flow required for effective heating and the pressure required to close the vacuum relief valve. Even a higher horsepower solar pool pump (filtration pump) is not suitable due to the varying flow generated from such pumps.

  2. Thanks to your advice I upgraded to a Solar Powered Swimming Pool Pump, High-Flow 17000L/H 500W, Brushless Motor, with MPPT Solar Water Pump Controller
    Since solar energy is free Should I let it flow through my solar panels full speed? Or try to divert more directly to the pool to slow down what goes to the collectors?

    1. Author

      Hi Arthur,

      We do not recommend solar pool pumps (whether you have solar pool heating panels or not). You are probably using a solar direct pump with no battery. While the rated flow rate of the pump may be 17,000 liters per hour, you will never see that kind of performance, especially with 500W of power. The math just doesn’t work with typical pool plumbing. The flow rate will be highly variable, so you may get adequate flow at peak sun, the flow rate will be inadequate at other times. With a solar pool heating system, you will not be able to close the vacuum relief valve reliably and the flow will be unbalanced in the system.

      Even when using a solar pool pump without a heat source, we have found that the total pool turnover and flow rate is inadequate for sanitization in Florida. A few days of cloudy weather and you are left with a pool that has not been circulated sufficiently to maintain good chemical balance. And the filtration performance is very poor.

      The only way to alleviate this is with batteries that have adequate capacity to keep the pump running at maximum performance. In that case, you also need to oversize the solar array to keep the batteries charged. The whole system becomes very expensive and the performance still lags traditional AC-powered pumps.

      With the advent of efficient variable speed pumps with brushless motors, it is far more cost-effective to use a small grid-tied solar array to offset an AC pump’s electricity usage. You get the performance of a high-powered (up to 3 HP) pump with the energy savings of an efficient motor running at a speed just high enough to balance heating performance, sanitization, and energy savings. Since grid-tied solar has come down in price so much, offsetting the small amount of energy used by a variable speed pump makes the most sense.

      If someone insists on trying one of these solar pool pumps, I highly recommend that you leave your AC pump installed in parallel so you can run it selectively when you need more power. I haven’t heard many success stories with solar pool pumps, but wish you the best with your purchase.

  3. Hello, I’m confused by this article and your responses. I am by no means an expert and have no gauges to test water flow rates. I have a 15×30 52 inch pool supposedly having 15K gallons of water. I have a 1 HP Hayward super pump and it runs at generally 18 lbs of pressure and I have a thermostatic diverter valve by Hayward. My pool wasn’t getting warm at all this year getting about 7hrs of sunlight on panels. I just readjusted the valve and started getting warm water back through the jet but after 30 seconds the water went back to pool temperature. If I shut the pump off and wait a few minutes I be get warm water again for about 30 seconds then back to pool temperature. The only conclusion I can come to is that the panels are being overpowered by the pump. Would you have any other solution other than a smaller pump which I am not going to change. My only solution would be to put a diverter valve in the feed line to slow the water going to the panels instead of slowing down the return thereby building excessive pressure in the panels. Your thoughts please and thank you

    1. Author

      Hi Patrick,

      The water coming back from the solar panels is not the same temperature as the pool. You may think that, but it is probably just a couple of degrees warmer and that will be imperceptible to the touch underwater. Solar pool heaters heat the pool slowly by turning the water over quickly with a small heat rise and a high flow rate. If you attempt to slow the water down to get a larger heat rise with less flow rate, you will reduce the overall amount of heat generated. It sounds counterintuitive, but that is the physics behind heat transfer.

  4. We are accustomed to having really warm water coming from a water heater, while the solar heating approach is more gradual, as if comparing a tortoise and hare. A better way to tell if the solar heater assembly is functioning or not is to test the pipe temperature of the water going into the array and returning from it. If the outlet temperature is warmer, then the solar array is working. The key question, then, is how much to expect that temperature change to be?

    1. Author

      Hi Joe,

      You will not be able to measure the difference without very sensitive and expensive instruments. Typical pool thermometer probes do not have the accuracy required. The temperature rise might be just a couple of degrees, which is within the margin of error for such devices. The temperature rise will also vary based on the temperature differential between the collector surface and the pool water, the irradiance, and several other factors, so there is no “right” amount of temperature rise.

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