Solar Pool Heater: Best Flow Rate

Experts say that the optimum temperature for drinking tea is 60ºC (140ºF). We have all experienced the joy of a hot cup of tea in cold hands and the pleasant and quick warming effect it has. Our hands warm up quickly because the temperature between the cup and our hands is significant. As our hands warm up, the rate at which our hands warm slows down, and eventually reach equilibrium with the cup. At this point, no additional heat transfer occurs. Once the tea cools off, the warming capability still exists, but the warmth transferred to your hands occurs much slower.

This effect highlights the science of solar thermal technology and heat transfer. People in the biz refer to this a “Delta T,” or the difference between the ambient temperature and the temperature of the fluid to be heated. The larger the temperature difference, the more heat is transferred in a given time. I wrote previously about slowing water down to improve solar pool heating. It doesn’t work — for the same reason a tepid cup of tea doesn’t warm your hand much.

What you want to do is move a lot of water through solar pool heating panels, heating a lot of water a little bit as it circulates. Another way of looking at this is that you want to keep the water in the panels cool relative to the surface temperature of the solar collectors. Maintaining a high flow rate continually introduces cool pool water into the panels, allowing them to work more efficiently.

It may sound counter-intuitive at first, but once you think about relative temperatures and how heat is transferred, it becomes common sense — a higher flow rate results in more pool heating overall.

The graph below shows the efficiency of a typical solar pool heating panel based on test conditions at various flow rates. The recommended flow rate for most panels is 3-5 gallons per minute. Once you reach these flow rates, additional flow results in small marginal increases in performance, and the additional cost of water pumping becomes uneconomical. A variable speed pump is the ideal solution to achieve proper flow, but valves can also be used to control flow rates. In most pools, allowing full flow through the panels is desirable unless you have a very powerful pump.

If you are looking for the total flow rate, just multiply the number of panels by the desired flow rate. For example, if you have 8 panels and want 4 gallons per minute, the total system flow should be at least 32 GPM. It is advisable to go a little over to compensate for filters getting dirty and flow rates decreasing. Always start with a clean, preferably new, filter cartridge.

Typical solar pool heating panel flow rate vs. efficiency. Courtesy: FAFCO, Inc., Sunsaver Solar Panel
Typical solar pool heating panel flow rate vs. efficiency.
Courtesy: FAFCO, Inc., Sunsaver Solar Panel
  • Comment (4)
  • Mark S says:

    Could you share where this data comes from and the conditions under which it was obtained including the specific test setup. I am an engineer and when I do a complete heat transfer analysis, I get lower efficiencies so I would like to understand your assumption sets a little bit better. Thanks

    • Jason Szumlanski says:

      The efficiency curve shown here comes from FAFCO, Inc. Other manufacturers have similar efficiency curves. The assumptions and test conditions are not publicized to my knowledge. If you are looking for independent test data, I would direct you to the Florida Solar Energy Center who has done extensive testing on solar pool heating panels. Their test data certificates list the test conditions. SRCC also has test data for solar pool heating collectors.

  • Bruce Roberts says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Hola – I’m in Ajijic, Mexico. We are thinking about adding roof top passive solar heaters for our pool. Problem is that the roof top is situated about 15 meters above the pool equipment. I’m think that the cost of pumping the water up to the roof top at a rate of 10 litres per minute will require about 25 kW of power and at $US 0.20 per kWh will be prohibitively expensive. No one talks about this cost when expounding the virtues of passive solar heating. What am I missing??
  • Marc says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    I have a mixed panel combination right now: (5) 4×12 and (4) 4×8. Each with different flow rate. Is that an issue?
  • Randy Dorman says:
    Your comment is awaiting moderation. This is a preview; your comment will be visible after it has been approved.
    Jason, The pool/spa combo is approximately 6400 gals serviced by one Pentair variable speed/flow pump. Does this additional info help? Any additional info I can provide? Thanks.
  • I purchased a house with solar panels for heating the pool and also a water heater. There are 5 solar panels and it appears 4 are for the pool and 1 for the water heater. Sizes appear to be 3’X12′. I have what appears to be appears to be the original purchase documents with purchase info.

    I’m trying to determine the optimal/recommended flow rate for the pool to take advantage of a new variable speed and flow pool pump. There is not a separate pump for the panels.

    Regards,

    • Jason Szumlanski says:

      Randall,

      They are probably 4×12 solar pool heating panels. That’s a pretty small system by local standards in Southwest Florida, but I don’t know your location. Without having more information I would say you should aim for at least 20-30 gallons per minute. That’s a big range, but for most pool ls and pool pumps, this would be quite easy to achieve and the difference in energy costs would not be substantial in absolute terms.

      Jason

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