Note: The information contained in this periodical weblog may be outdated. This was my personal weblog published before January 1, 2015. Since then I have been the co-owner and Principal Solar Designer at Florida Solar Design Group.

My Solar Pool Heat is Not Working!

This is the time of year that I hear quite a few people saying that their solar pool heat is not working. In most cases, the system is working well, but they may not be comfortable swimming at the current pool temperature. The easy solution would be to ask them to turn off their solar pool heater and report back with the temperature after a few days. I bet it would be much colder!

Generally speaking, an unheated pool will be approximately the same temperature as the Gulf of Mexico. The average historical Gulf temperature in Naples, FL in December is 68°F. Today, December 20, 2011, the current Gulf temperature is 69.6°F. A typical well-sized and oriented solar pool heating system will raise the pool temperature about 10°F above the Gulf temperature, which would imply a temperature of almost 80 degrees.

Gulf Temperature Naples Florida

To get the current Gulf temperature in Naples, FL, go here:

A pool cover will add a few extra degrees to the pool temperature, especially when nighttime air temperatures dip below 65°F. In fact, when temperatures get below this level at night, all or most of the heat generated by the solar panels (or heat pump, or gas heater) during the day simply escapes into the cold night air.

Why do people come to the conclusion that the solar pool heat is not working when their pool temperatures are well above the Gulf temperature? I think there are a few factors at play.

  1. The average Gulf temperatures in Southwest Florida in October, November, and December are 81°F, 73°F, and 68°F. This is a precipitous temperature drop in a short period of time, and people recall swimming comfortably “just a few weeks ago.”
  2. “The water coming out of the jets feel like the same temperature as the pool water.” There is a common misconception that the water returned to the pool should be and needs to be significantly warmer than the pool water. The fact is, solar pool heating works on a high-volume, low temperature rise principal. The temperature differential can vary significantly. As long as the water returned to the pool is warmer than the pool, you pool is being heated. Do you think you can perceive the difference between 80 and 81°F water by holding your hand underwater? The water may not spend enough time on the roof to rise in temperature significantly, and that is okay! Over time, the pool will warm up. Keep in mind that a cover will help retain much of this heat gained during the day, and the cumulative effect over days will make your pool much more comfortable.
  3. Temperatures and weather patterns vary from year to year. Seasonal residents may remember swimming on Christmas one year, while the next year may be unbearably cold.

Our service department is observing covered solar pool heated pools this time of year in the 80-84°F temperature range. Depending on the size of your system, size of your pool, orientation, micro-climates, shading, and many other factors, your system’s performance may vary.

Don’t hesitate to call your dealer if you think something is wrong, but consider whether your system is working properly or not using the information above!

If your pool is not reaching temperature levels described above, there may be something you can do:

  1. If you do not have an automatic controller, Make sure that your pump is running or your solar is on only when the sun is out and the roof temperature is likely higher than your pool temperature. Otherwise, you may be cooling your pool! A solar pool heating controller will automate your pool heater and ensure you are not unintentionally cooling your pool.
  2. Your system may be undersized to reach your heating expectations. Adding panels will increase performance. You can also use panels with a higher BTU rating.
  3. There may be shading on the panels from trees that have grown.
  4. If you are seeing NO heating above the Gulf temperature, there may be a legitimate problem with your system. Give your dealer a call!


  1. I’m willing to bet that the majority of the people claiming that their solar isn’t heating are not using a pool cover religiously, or don’t use one at all.
    By allowing the pool to cool during the majority of the hours of the day when the pool pump is Not running, you’ll Never get the pool to 80 degrees by solar alone unless you’re using a cover.

  2. my system has air in it can you help.I have checked all conections and does not have air if panels are turned off…thank you

  3. @Pool Water:

    “…you’ll Never get the pool to 80 degrees by solar alone unless you are using a cover” is not true. In Southwest Florida this time of year, a solar heated pool can be in the mid-to-upper 80’s each day. We are seeing covered pools with properly sized solar pool heaters in the upper 80’s and lower 90’s right now.

  4. I have a 10k gallon pool. I have 280 sq/ft of solar panels and they face south west. Gulf is 76. My pool is 76. It was 76 at 8 am it was 76 at 4 pm. Air temperature reached 91 today. Solar heater did not warm the pool 1 degree. Would you say my heater is not working? 7 panels at 4’X 10’= 280 sq ft

    1. Clearly not working. It could be any number of issues, but usually it’s not related the the solar pool heater specifically. You probably have a pump, controller, or valve issue. If you are in SW Florida, give me a call in the morning and hopefully I can help. 239-574-1500.

    2. I’m guessing you are further north than my territory. Unheated pools are in the upper 70’s or low 80s around here now.

      1. I hope I was able to help you on the phone. I definitely think high back-pressure on your pump caused by the in-floor cleaning system is causing very low flow in your system, resulting in poor solar pool heating. You should have a pool professional take a look. It could be a simple fix, but it could also be a major repair. In-floor cleaners should be installed with bypasses in pools (my opinion), especially if heating systems will be included. Even better would be if pool builders installed dedicated solar return lines to efficient return heated water to the pool.

  5. I live in Upstate NY near Saratoga Springs, NY and I have an 18×52 pool and I use solar heater panels. The total of the panels are 2 feet x 20 feet. Is that enough to heat my pool?

    1. Hi Mark,

      It isn’t clear how many panels you have, but I’m guessing two. These are sold online to do-it-yourselfers in pairs and are generally marketed to the above-ground pool market. Many people are misled about the expected performance by unscrupulous online dealers.

      At 40 sq. ft. each, you have 80 sq. ft. of total coverage. Here in Florida with highly rated panels sold by professional solar contractors, we recommend a 80-100% of collector to surface area ratio. This will generally result in about 10 degrees of temperature rise over an unheated pool when no cover is used. Panels of this type produce more heat and measure 4×8, 4×10, or 4×12. Pools are smaller here, averaging 14 x 28. A typical solar pool heating system for a pool this size would be about 384 square feet using eight 4×12 panels… and that’s in FLORIDA!

      You have a huge pool by our standards, which is not going to experience much noticeable increased heat with just 80 sq. ft. of coverage (under 10%).

  6. Your pools in Florida are only 14 round by 28 high?

    1. Sounds like some miscommunication. Our pools are often 14′ wide by 28′ long, inground. It sounds like you have an above ground pool, 18′ diameter and 52 inches high.

      Inground pools lose the majority of their heat at the surface, which is why we size pool heaters based on surface area.

      If you have an above ground pool, your surface area would be 250 sq. ft. However, you also have heat loss through the walls. A solar pool heater of 80 square feet would still have relatively poor performance. You didn’t indicate how many panels you have.

  7. My pool is 18 round by 52 inches high.

  8. Yes, I have an above ground pool 18′ diameter and 52 inches high and I have two panels that equal all together 2 feet by 20 feet.

    1. Unfortunately, I’m not an expert in your climate, especially for above ground pools. We don’t sell the 20′ long panels, as we consider these consumer grade. I would try to find a reputable solar dealer in your state.

      If I had to guess, about 320 square feet of coverage with eight 4×10 panels would provide good heating performance. Again, seek local advice.

  9. Thank you for your help. So basically I have 2×20 which is the same really as a 4×10!

  10. My surface area would be 250 sq. ft

  11. Yes, your surface area is 250 sq ft, but as I understand it, above ground pools can lose a substantial amount of heat through the walls, so you may need additional coverage. This is a question for an above-ground pool heating expert in your locale.

    To put things in perspective, our top rated 4×10 panel is rated at about 42,000 BTU/day. That means it can raise the temperature of 42,000 pounds of water by one degree. At test conditions, that is enough to raise your 8,000 gallon pool about 0.6 degrees Fahrenheit per day – basically imperceptible. I’m pretty certain that all of that heat will be lost every night, even if covered and walls are insulated.

  12. My cousin lives in NJ and only has one panel 2×10 and it heats their 15′diameter and 48 inches high above ground pool to 88 degrees and at night the pool is 86 degrees.

  13. I have no idea about the local conditions. You really need to contact a local expert with above-ground experience. I’m sorry I cannot help further.

  14. I live in central flordia & when the sun is hitting my panels good it only raises the water temp 1 degree. I have good flow & with a heat gun I measured the water temp going to the panels and coming back from the panels. Is this normal? What should be the dlta T be for a properly sized system.

    1. One degree rise is fine depending on the conditions. You don’t want a large increase in temperature. It takes time.

      I don’t know how many panels you have or any details about your system. I also don’t know what the conditions were when you took your measurements. However, let’s say the the panels have 40 pounds of water in them and you are running at 4 gallons per minute. You would be heating 160 pounds of water one degree every minute, which means you transfer 160 BTU of energy to the pool. If you do that for 4 hours on average, that’s 38,400 BTU/day – well within the expected range.

      That’s my basic math sitting on the couch on my iPhone. I could get more detailed, but I’m in the ballpark.

      What’s driving your question? Perception about how it is supposed to work, or overall performance?

  15. my question was based on the overall performance of my panels. 1 degree of delta t did not seem like much. I have a 6,000gal pool with 4 4’X 10′ panels facing north east on my roof. At this time of year I run the pump about 4hrs a day(not using the pool & the sun is not hitting the panels if I run it any longer. The water temp is average around 70 to 73 degrees. I could add more panels on the east south side of my roof but did not know if it was a waste of money. Could more panels ever get the pool to around 85 degrees this time of year? Also thank-You for your feed back

  16. More panels is always better, but only as part of an overall good design. A covered pool can reach or exceed 85 degrees this time of year depending on the weather.

    It’s hard to say whether 1ºF is a reasonable temperature rise. Again, it depends on many factors and the system design. If you have high flow rates relative to the system size (square feet of panel area), you will have a lower delta T. Four panels is a pretty small system, but it sounds like you have a pretty small pool (surface area is more important than gallons when considering heating performance because heat loss occurs mainly at the surface).

  17. We have an argument about the cost of the solar heating of our pool. My friend argues that by having heating on and the pump has to pump water to the roof to heat it, it cost more than when the heating is off. I argue that it does not matter or the extra cost is so minimal because the pump is running to filter the pool water anyway. Who is right?

    1. Teresa,

      You win. Sort of…

      If you have a single speed pump, it does not cost more to pump water to the roof. A single speed pump uses the same amount of energy at its given speed. Technically there may be a negligible difference if the pressure changes, but in the case of solar pool heat, pressure increases slightly, which means the flow rate decreases. That means the pump does LESS work and it takes LESS energy in a given amount of time. For all intents and purposes, this effect is negligible in a typical system.

      Now, I said sort of…

      If you have to run your pump for a longer period of time to get adequate heating, you will obviously need more electricity to do that. Also, if you have a variable speed pump, you need it run it at a speed that is adequate to both close the vacuum relief valve and provide good flow for optimum heating. That may mean increasing the speed and flow, but that also may mean you can run it for a shorter period to obtain adequate turnover. This relationship is complicated, but for practical purposes it usually increases costs (but it is still far less costly than using a single speed pump).

      I think you were referring to a single speed pump, so I have to take your side and declare you a winner!

  18. Thank you for your response. We do have a single speed pump. Regards.

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