More than five years ago, I set out to solve one of the biggest grievances with my home: I had a very very cold pool. Even in the summer it was unbearable to bathe in for more than a few minutes; we even considered filling it up with dirt.
Here’s how i fixed it on a tight budget, and what I learned doing it.
The issue
Our house came with a very nice 32.6m3 freshwater pool; it was a big selling point and one of the main reasons we bought it. We imagined it would be great for the hot summers of central Chile, and a centerpiece of household social activities. It soon became clear that it would not be so.
That pool would, in the hottest of summer days, never really get past 21ºC. Getting into that might be refreshing for a while, but it soon chilled you to your bones. Most sources on the Internet indicate that a reasonable temperature for a freshwater pool is at least 24ºC, and those three degrees made a huge difference. Remember, 21º is the best case, in practice the actual temperature was quite a few degrees lower.
For one, the pool’s is lightly colored, and painting it anything short of pitch black wouldn’t have really made any difference, because there is a large tree that gives it shade most of the day. Fixing any of these problems was out of the question, as it would not pass aesthetic inspection (my mom). For a while, we even considered filling in the pool to get some extra garden space, but it always felt like a waste. The hunt was on then, a new way of heating the pool was needed.
Choosing the right way
The first question was which energy source was I going to use: It had to be cheap both upfront and over time, and already available at my house. This basically meant (at least in principle) either gas or electric. For gas-powered systems, you can install what is essentially an oversized boiler, while electric solutions involve resistive heating (like a hot water tank) or heat pumps. All of these systems quickly made no sense for my budget; both installation and running costs would have been massive, as energy is expensive here.
In comes solar heating. This boils down to circulating water through a black pipe placed in the sun; the pipe heats up and transfers its heat to the water. The advantages were clear: no energy costs and very basic infrastructure. Next to our pool filter lies our roofed driveway, which despite being on the south side (the worst side in the southern hemisphere) of a tall house, had enough space to clear its shadow for most of the day. This was the way to go.
Designing solar water heaters from scratch
You can buy ready-made solar pool heaters which are essentially a flat panel of small tubes (about 8mm in diameter) which can be laid on a roof and piped to and from the pool filter, but these are expensive and usually hard to get if you’re not an installer (at least over here). Also, you read the title, you know where we’re going with this.
To make low-temperature solar thermal collectors, we need something that can withstand UV light, be somewhat flexible for ease of installation, and ideally, be black: in comes polyethylene pipe, a flexible-ish black pipe meant for irrigation. Smaller pipes gives you better surface area per water volume, so the smallest size easily available, half-inch, was used.
Then came the question of area: how much roof space do you need to fill with panels to get good temperatures? My reflex answer is as much as you can, but there are some difficulties with this approach:
- The more panels you put, the bigger the pump you will need to push water through them, and the higher the operating pressure you will need.
- Water is heavy and your panels will be full of it; be careful how much weight you place on your roof.
- For this application having panels in the shade is not really harmful, but it will be wasted space and pressure; try to put only as many panels as you actually need.
Figuring out how many panels you need to heat up a pool is rather difficult: you will most certainly end up partially eyeballing it. However, there are some important facts you need to consider:
- How big is your pool and how much of a temperature difference you actually want.
- The angle of your roof and the direction your roof is facing.
- The height of your roof and the power for your pump, as it will dictate your flow rate.
For us, what made sense was around 500m of total poly pipe exposed to the sun, we also had a roof that was readily accessible right next to the pool pump. That number is somewhat arbitrary and more to do with how we went about doing it, but it ended up working out in the end.
Designing the panels
To make panels that would actually work, we set the following criteria:
- The panels must be small and light enough to be lifted to the roof by a single person.
- The panels must be able to be replaced if necessary.
- The panels must be arranged in such a way as to have the smallest possible impact in flow rates and pressures.
Because we went with half-inch poly pipe, putting panels in parallel was pretty much mandatory, so we decided to make lots of small panels we could haul up into the roof and then connect into an array; after some quick calculations we realized that a flat spiral a meter in diameter would have roughly 50m of pipe, which meant we could build 10 lightweight spirals: the pipe would be tied using steel wire to a wooden cross every four turns, and after many, many hours of rolling, we had our panels.
10 panels also turned out to be a bit of a magic number, as it meant that doing 5 sets of two panels would equal to roughly the same cross-sectional area of the 50mm pipe coming to and from the pool, which meant pressure loss would not be that bad. The total internal volume of the panels was around 350L, which meant the waterline would recede by around a centimeter. This was the winning combination.
Connecting it to the pool filter
There are three key features regarding the connection to the pool: first, the water circulating through the panel must have already passed through the filter, as to prevent blockages. Second, the user must be able to control not only whether the panels get water or not, but how much water gets up to them, to be able to control the temperature without sacrificing too much flow and pressure. Third, attention must be taken in order to get the shortest runs of pipe possible; every fitting and every jump in height reduces flow and pressure.
With all of this in mind, and blessed with a roof just next to the pump house, the output of the filter was teed off in two places, with a ball valve installed in the middle: this will be our mixing valve, allowing us to mix cold water from the pool with warm water from the panel in order to control the temperature. Then, the first tee in the chain would be connected to the panel valve, and then up to the panels, in which there are five manifolds in order to hook up the poly pipe spirals, with a matching set of inputs downstream after the panels. The return from the panels would enter into the second tee, and then back to the pool.
There are some considerations here: A ball valve for mixing is not the most precise way of controlling temperature: something like a gate valve gives you more control, but they are a lot more expensive and you can still adjust the temperatures just fine with a little finesse on the valve handle. Also, when the pump turns off, a vacuum forms inside of the panels, as the water descends from gravity and nothing replaces it. For these panels, I found that the back pressure from the return lines was enough to break the vacuum and prevent an implosion, but for taller roofs, I would recommend adding a vacuum breaker (essentially a valve that opens when the pressure inside of the panels goes below atmospheric and lets air in) just in case.
And, well, that’s it! By opening the panel valve and slowly closing the mixing valve, water will start to go up the panels, and heat capture will commence.
Using the system in practice
Bernoulli’s equation of hydrostatics tells us that if we increase the height of a fluid, it’s pressure must go up. For us, this means that there will be a minimum operating pressure in which the panels will actually get water, otherwise a column of water will peacefully reside in your pipes without overtopping the highest point in your system. The same equation gives us the answer:
Pcritical [Pa] = ρ[kg/m3] · g[m/s2] · h [m]
Where P is the minimum pressure you need, ρ is the density of the water, g is gravitational acceleration, and h is the difference in height between your pump and the tallest point of your panel. You can also kinda eyeball this: close the mixing valve bit by bit until you start hearing water coming back through the return pipe, and then back off until you can’t hear it anymore: that’s your minimum pressure.
With panels this thick, passing the entire flow of water through the panels is somewhat unnecessary: there are diminishing returns once the water starts heating up, so you want to close the heating valve just enough so that the panels don’t get above ambient temperature (so you don’t lose heat) for maximum performance of both the panels and your pump. If the pool gets too hot, then losing heat is what you need, so just open the mixing valve a little bit more and in a day or two you will have a cooler pool.
Unforeseen circumstances
Great, your pool is now warm! Unfortunately, this is not without consequences. For one, warmer pools lose water by evaporation a lot faster than cooler ones, so expect to fill it up more often, be mindful of your water bill. Also, warmer pools are much more attractive to algae, which grow a lot faster in these waters: maintaining good chlorine levels, incorporating some sort of electrolysis cell for adding copper ions, and cleaning the pool regularly are a must, unless green and turbid water is what you want.
After much experimentation, I have found to be the winning combination: one and a half tablets of TCCA per week, addition of copper ions via electrolytic cells, and weekly vacuuming and sweeping is enough to keep algae at bay. Remember that the actual quantities are going to be dependent on your temperatures and volume of the pool.
Albeit not my case, if you happen to live in a place where freezing temperatures are common, it’s very important that the panels are drained during the winter season: usually popping open the cap on the filter and the drain plug on the filter for a couple of hours is enough, otherwise prepare for burst pipes and cracked joints. On that same vein, remember to paint your PVC pipes every so often, UV light is not nice to polymers, so try to avoid exposure if possible.
On a more humorous note: my panel usually drains almost completely at night, which means every morning the pump removes all of the air out of the pipes, which results in a very unique noise every morning: my pool is farting!
A five-year retrospective: closing thoughts
This project turned out to be a huge success not only for my household, but because it made me learn many useful skills not only building it, but designing it: the art of the educated guess cannot be understated, and sometimes the only thing you need to succeed is some ballpark back-of-the-envelope calculations. By applying some high school physics and a bit of blood, sweat, and tears, we ended up with a pool which regularly hits 28ºC and beyond, and it became a centerpiece of our beautiful garden. If you want to get into some low-stakes plumbing, the low pressures and big pipes are a great way to get started, and even a large pool can be done for relatively cheap, definitely more so than hiring someone to do it. Best of all, you’ll be doing it in an environmentally friendly way.