Solar pool heating involves using the sun to heat your pool water.

In the "old days" people used to simply connect a long black hose to the output of their pool pump with the other end dumping into their pool. Then the middle part of the long hose is strung out along the ground or thrown up on a roof. The pool pump runs the pool water through this black hose and back into the pool. Being black, the hose absorbs the suns rays and becomes hot, heating the water. The result is solar heated pool water. Though I say "old days", this type of system is still quite common.

Today there are well engineered products you can buy that replace this hose with flat panels designed to be long lasting and very efficient. These panels are made of a black rubber or plastic and contain many tubes for the water to flow through. The black panels turns the suns rays into heat that is absorbed by the water. There are also products for automating the flow of water so that it flows through the panels only when needed and only when the sun is out. I've worked on at least one installation where the customer was replacing their homemade hose system with this off-the-shelf type.

Note: All photos on this page are of actual installations I was involved with while working for Ottawa Solar Power.

A sample system. The pool pump and other necessary components are cleverly hidden in the vine covered area beside the house.

How it works

The following illustrates how the above control valve and check valve work together to control water flow. Notice the different positions of the brown blocking piece in the control valve. Notice also in the check valve how the plug (triangular object) is pushed to the left by the water returning from the panels. Because of the plug's shape, any water coming from the other direction would force it to the right, plugging the hole. There is also a spring (not shown) helping to pull it to the right.

Control and automation

The control valve can be either a manual one or an automatic one. With a manual control valve, if you want the pool water to be heated up and the sun is out, then you'll have to turn the handle on the valve yourself to get the water to flow to the panels. Then, at night, you'll have to manually turn the handle again to stop the water from flowing to the panels otherwise your hot pool water will flow through the now cool panels, give its heat to the panels and then go back into the pool as now cooler water.

An automatic control valve with a control box and two temperature sensors handles all this for you. The diagram and photo below illustrate these components. One temperature sensor is on the roof, positioned where it will get sun whenever the panels are getting sun and be shaded whenever the panels are shaded. The other temperature sensor monitors the temperature of the pool water by being inserted into a pipe that contains water coming from the pool. The automatic control valve has a motor to open and close the valve. The control box monitors the temperature sensors and controls the valve's motor. It has a switch and a dial on it. The switch normally has three positions: turn on the automatic control system, turn it off, and do a test. The dial lets you tell the system what temperature you want the pool water to be.

If the temperature of the pool is cooler than what you selected via the dial then the control box will use the temperature sensor near the panels to determine if allowing water to flow through the panels will heat the water up. If the sky is cloudy then the temperature sensor will be cool and the control box would do nothing. However, if the sun is out then the temperature sensor will be hot and the control box will turn the control valve to let water flow through the panels.

an actual setup

Close-up of automatic control valve. The top part contains the motor.

Using a second heat source

Sometimes a second heat source, such as an air heat exchanger or possibly a natural gas heater, is used in addition to the solar heat. Typically the piping would be installed such that the water is first heated by the solar panels since the heat from the sun is free. Then the pipes would run the water to the second heat source. The control system for the second heat source would measure the temperature of the water coming to it and decide if it needs to do any more heating. If the water was already heated by the solar panels then the second heat source would do nothing, thereby saving energy.

Using a second heat source

Whether or not a second heat source is needed depends on your environment. In my experience, on systems I've worked on, people either request that we remove their old heat source from use so they can get rid of it, or ask to have it left in just in case it's needed.

Variations

There are, of course, variations to all of the above. For example, in some systems the panels might be laid out on the ground or installed on a wooden support structure which is on the ground. Check with whomever you buy your system from first though. Some systems absolutely must be installed such that the panels are on an angle with the bottom lower than the top so just laying them on level ground may not work. On the other hand, some are designed for just that. Naturally, for automated systems, that means that one of the temperature sensors would no longer be on the roof but instead be mounted somewhere else near the panels. Also, some automated systems can be connected to a home computer.

There are also many variations in the types of panels but they all work the same as illustrated above. They all have bottom and top pipes with many small tubes for water to flow from bottom to top. The differences lie in what material they're made of, typically rubber or plastic, how the small tubes are manuafactured, how much material is surrounding the tubes for additional heat storage, and how the panels connect to each other.

Your system may include a trench. If your pool pump is far away from the house you probably won't want the pipes to run above ground from the house to the pump. In that case a trench will need to be dug and the pipes put in it. Of course the trench would then be buried and likely covered in grass so that no one will even know it's there.

Maintenance

Here in Ottawa, Canada, any water still in the pipes during the winter would turn to ice and possibly damage the pipes, creating leaks to be fixed in the spring. Draining the water out of the system in the fall to avoid this problem is the only maintenance needed.

Whoever installed your system may have designed it such that any water in the panels or pipes will naturally drain downward to the pump without any places where the water will have to run upward. If you look very carefully at the photos of solar panels on this page you'll see that at least the bottoms of the panels are all tilted in the same direction so that even the set of all panels is self-draining. Likewise, any pipes running horizontally along the roof are sloped.

The first step to draining your system is to turn the control valve to the position that allows water to flow up to the panels. The control valve in the following diagrams (the part on the left in each diagram) has two positions. The diagram on the left shows the position for allowing water to flow up. With an automatic control valve this is usually the test position on its control dial.

Some automatic control valves can be turned by hand. For manual control valves, this is of course easy to do. I've repaired one valve where the valve cracked because the piping arrangement near the valve allowed water to sit in the valve over the winter. Had the customer turned the valve such that the blocking piece was not blocking anything then the water would have completely drained out of the valve and it would not have cracked. In fact, if your valve has a third position where the blocking piece is not blocking any water flow at all then that would be ideal. In the above diagrams, the blocking piece can actually be rotated such that neither direction is blocked (not shown.)

But for water to drain from the pipes and panels, air must be able to get in to replace it. If a vacuum breaker valve has been installed in your system then this will take care of it by allowing air in when water is being sucked out. Not all systems have a vacuum breaker valve though.

If there is no vacuum breaker valve then simply turning off the pump and setting the valve position will not be enough. In that case, whoever installed your system will also have included some locations where you can either open a pipe right up or at least have some plugs which you can unscrew, letting air in. You'd then leave the pipes open and/or plugs off for the winter.

Pipe ends after draining. They're left open for the winter and will be closed again in the spring.

The best way to make sure you got all potentially damaging water out is to blow and/or suck the water out with an air compressor or shop vac (vacuum). This is done by opening the pipes in at least two locations, hopefully parts were installed in places along the pipe run for doing this, and blow/suck at one open location. For more assurance you could then go and do the same to the other open location.

A shop vac. This one can both blow and suck.

Some systems can't be self draining. Examples of these are ones where the pool is at a higher level than the pool pump, where a trench is involved and where the roof is not just one flat surface but has obstructions such as those caused by dormer windows which means the pipes may have to run upwards over the obstructions at points. In this case blowing/sucking out the system will be an absolute necessity.

If it's possible that rodents or birds might enter any open pipes and start making a home, you might want to cover the open pipe ends with a piece of cloth for the winter.

In the spring you simply close the open pipes, turn the control valve such that water will NOT flow up to the panels, let the pool pump run for a few minutes, then turn the control valve such that water will flow up to the panels. Plenty of air bubbles will flow into your pool for a minute or so as air is flushed from the system but this will stop once the system is filled with water. If you have an automatic control valve, set it to automatic. Depending on how much sun you get, your system should heat up the pool within a few days to a week. Enjoy your solar heated pool.

A few more sample installed systems