This has been our experience, too. On the hot days where the pool is getting warmed by the panels, the interior of the house seems to stay cooler. In fact, it is usually the difference between the A/C running or not as most of the sunny part of the roof is covered in panels. The roof is a pitched tar-and-gravel with about 10" of rigid foam. Inside, it's open-beam and tongue-and-groove, so there is no attic to buffer us from an excessively hot roof. On a couple of really hot days this summer, I did up the thermostat on the pool just to keep water flowing through the panels. I would rather the pool climb an extra few degrees than have to run the A/C. I think we peaked the pool to around 93-95 degrees on a couple of days.

I had seen the attic pool heaters, but had also heard about the potential of a leak should something fail in the system.

Why can't you use both systems, Solar Cells for electric, solar heat for the water?

You don't have to have roof-mounted panels for both. As many of you know, my deck around my pool is my solar system--and it keeps the deck cool, too.

There is also that fascinating attic-heat transfer solar system--this too would not intrude on roof-top solar cells. It's expensive but certainly not as expensive as would be sufficient solar cells to provide resistance heating for the pool.

Usually, as a rule of thumb, the fewest number of energy conversions is the simplest and most efficient. In solar panels, infrared light (heat energy) is transferred directly to the water to heat it. But in a solar cell system, you must first convert light into electricity, then electricity into heat via resistance (pretty inefficient) before it finally heats the water.

I'm confused about this. I thought you needed less square footage of glazed collectors than Fafco-style collectors. This is complete opposite of what you say. Maybe you are talking about glazed systems with pool water running through them and the smaller systems are closed-loop system with a glycol solution running through them. In that case, the pool water and/or domestic hot water would get heated via heat exchanger.

Look at the chart in the first post in this thread and you can see that flat black mat solar panels are MORE efficient than glazed panels when the temperature difference between the water being heated and the air temperature is low, there is little wind and the solar intensity is high. The glazed panels do better when you are trying to heat your pool to be quite a bit warmer than your daytime air temperature or when there is wind or the sky isn't clear (hazy or cloudy) or the sun is low in the sky (spring or fall). Note that the glazed panel efficiencies in the table are panel efficiencies so do not account for any losses if using a heat exchanger (i.e. not using pool water in the panel).

There is no single simple answer as to whether one panel is better than the other because it depends on the situation. In areas of the country where the days are warm and the sun is intense and the skies are generally clear, then flat black mat panels would be the only way to go, especially given the 3x price difference. So this would be most desert-like areas and much of the southern U.S. In northern climates where skies are not always clear and the air temperature is considerably colder, then glazed panels make more sense. So this would be areas in Canada and the cold areas in the northeast or some high mountain areas that don't get hot summers. Areas in between such as northern California aren't so clear cut since it then depends more on how the pool is used: Is the pool going to be kept quite a bit warmer than the air temperature? Is the pool going to be used in the spring and fall when the air temps are cooler?

The reason I think that I would use glazed panels in my pool if I were to do it over is that we keep the pool at 88F and use it as much as possible, starting in May or late April and going through November. Our use of gas heat to supplement heating is VERY expensive and uses a non-renewable energy source (natural gas) and adds CO₂ to the atmosphere.

Note that I don't agree with Ecodad who implies that it never makes sense to use glazed panels. As noted above, the answer really depends on the climate and intended use of the pool.

Richard

Unless we use our gas heater, we really have to shut down the pool in late September now. The days are still so hot, but the sun angle doesn't provide as intense energy transfer to the panels at that point. Unfortunately, the gas heater also worked better with our previous pump. We use it for our spa and before parties now because the solar no longer has the pool warm enough. Maybe it is time to spring for a cover, but it definitely makes the pool more work and less fun. Also, we will have to put up the fence again to keep the dog out.

Though not cheap, you might consider an electric automatic pool cover -- that's what we have. You can add a top-track model to an existing pool as shown here. It's convenient and will help retain heat at night, though not as well as a bubble-type more insulating solar cover. Also, if the cover is opaque (as most automatic covers are), then you lose the heating during the day unless you open the cover. In low humidity climates, evaporation could cool the water even during the day with the sun trying to heat it though where you (and I) live in the Bay Area in northern California, our 40-60% typical humidity and low wind usually means that keeping the cover off during the day results in a small net heating.

We have considered the automated covers. We love them on the pools we see them on. Unfortunately, I don't think it would work on our pool that has a spillover spa along one side.

Several posters in this thread have referred to Fafco panels. Which type of Fafco are you using? There is the Solar Bear for above ground pools, which are the cheapest. (http://www.fafco.com/SolarPoolHeater/FA ... ters.html#) and a couple different models of "in-ground" heaters, such as Sunsaver, Revolution, and Sunsaver ST (http://www.fafco.com/SolarPoolHeater/FA ... aters.html). There were several mentions of the efficiency of Fafco panels, but is there a particular version that you are calling more efficient?
I am in the pricing stage of our project, and am trying to narrow down to most heat for the money. Our pool is 20K gallons, above ground, we have high winds but the sun is out and hot most of the time. We are primarily looking at increasing the temperature in the spring and fall and increasing the length of the swim season. We will not be roof mounting our panels as the pool is too far from the house. We use a solar cover constantly during the spring and fall to reduce heat loss at night.
Thanks for an interesting post to follow.

When I've referred to Fafco and to their specs, it's about their SunSaver and Revolution models that usually get mounted on the roof (I have mostly SunSaver and one Revolution). Nevertheless, the SolarBear specification here gives data and charts similar to the Fafco Sunsaver Fafco panel. The SolarBear is designed for in-ground pools and is less expensive (it doesn't have the draining H/W since it's expected to be at ground level and easily accessible)..

Most black mat panels have roughly the same efficiency. You can see a list of solar collectors and their ratings from the Florida Solar Energy Center (FSEC) here. The BTU/sq.ft. is the relevant number for comparison as panels can vary in size. You can see that the Fafco Sunsaver is 1014 (SolarBear is as well, from the Fafco spec I lined to above) and Fafco Revolution is 1063 which is the highest (tied with Performance Solar's Performance Plus). Note that the difference between these is only around 5% so isn't very much.

Richard

Well that Florida chart is very helpful. It does look like I will be going with the Fafco Solarbear- surprisingly good efficiency. Any thoughts on how they manage to get such good efficiency out of a "low-cost" system?

You'll notice that most of these solar panels are black plastic of various sorts. If you make the plastic black enough, it will absorb most of the sun's energy. The only trick left is to have that heat transferred to the water and that means having the plastic tubes thin enough for efficient heat transfer but thick enough to prevent bursting and also having them close enough together to have most of the panel be water surface area. A technically more efficient panel would have a thin film of water, but as you might imagine that creates a lot of stress on the places the "sheet" gets attached. So the compromise is to use tubes and smaller tubes are "flatter" so more like a sheet.

The main efficiency advantage of the Revolution panels is that the tubes are dimpled in a way that causes the water to rotate within the tube which causes the water to mix more and expose all of the water to the "top" hot surface. Without such dimpling, the water flowing at the top of the tube gets hotter than the water at the bottom and that makes things a little less efficient.

Richard