In a fairly sophisticated solar pool system, water is pumped out of the pool, through the strainer, and through the filter. Alter the filter it passes through a check valve. This check valve is more than just an option. Its purpose is to keep water that’s draining out of the panels from back-washing the filter (Figure 99).
Once it has passed through the one-way check, the pool water reaches a critical tee. This tee separates the upper panel loop from the ordinary circulation loop. When the pool temperature is sufficiently warm — or when there’s no sun to heat the panels — water circulates only through the filter loop. A gate valve in this lower loop, immediately after the tee, determines which way the water will flow (Figure 100).
Whenever this gate valve is closed, water is forced upward through the panel loop to collect heat. When it’s open, there’s normal circulation through the lower loop. This gate valve can either
Figure 99. An electronically controlled valve like this one makes an automatic solar pool heater possible. When there’s heat in the panels, this valve is instructed to close. And when the pool water is hot enough, it opens again, circulating water only through the strainer and filter. (See appendix.)
be operated manually or controlled by a differential thermostat such as the one mentioned in the last chapter (Figure 101, next page).
Water moves up through the panels and comes out again — we hope a few degrees warmer than when it went in. Near the top of the
system it will pass through that same small, float-type air vent that appears in a forced circulation domestic solar heater. The panels and panel loop will be draining and filling constantly, and the air vent will allow air to escape from the lines each time the upper loop is filling as well as let air into the lines when that loop is emptying. From the panels the heated water simply circulates through the rest of the upper loop, into the last part of the lower filter loop, and back to the pool again.
The all-important gate valve is opened or closed depending on the temperature of the water and on the weather. Many people do it by hand. If you want a system that works automatically, you can stick in a remote-control "solenoid" valve that can open and close at a command from a differential thermostat.
A common circulation pump for a pool works constantly unless it’s either switched off manually or controlled by a preset timer that shuts it off and turns it on again at certain times of day. So
the differential thermostat, which controls the upper loop, is not wired to the pump at all. The pump just keeps running.
But a sensor in the line leading out of the pool to the strainer, measures the temperature of the pool water. If the sensor says the water’s 82 degrees (or whatever temperature you want it to be), the solenoid-operated gate valve stays open and no water goes to the panels.
When the pool temperature falls below 82, and a special sensor box near the panels indicates that the temperature and sunshine there is strong enough, the differential thermostat tells the gate valve to close, and water goes into the upper loop to be heated. Whenever the two sensor temperatures get within a few degrees of each other, the valve opens and the panels are by-passed.
That’s how it works. Simple. Enjoy. Oh, one last thing: Don’t forget about the sun. It’s always there, and when it shines, it’s powerful and useful, as we’ve seen. It can also give you a wicked burn. So keep your shirt on. Especially when you’re mounting panels.