The basic solar pool heating concept is very simple. We want to move swimming pool water through a black set of tubes that are in the sun so the water heats up. We don't want hot water coming out. We want the black thing in the sun to be as cold as possible because it collects more solar energy that way. We move a high volume of pool water through the solar collectors as fast as possible.

Automatic Controllers

Manual control means you have to turn a valve to send the water through the solar panels. Automatic means that the valve is turned electrically using a differential thermostat, sensors and a motorized valve. If water is moving through the solar collectors when it isn't sunny enough to add heat to the water, then heat is being lost by the collectors and you are cooling the pool. Under manual control this means that you are going to lose heat by having solar on at the wrong times. Sometimes, that's okay. If this is a small investment for a small pool then the extra cost of automatic controls may be better spent on more collector area to make up for the difference. When manually controlling a solar pool heater, normally you will cheat by simply setting the pool pump timer such that the pool pump only runs during the hours that the sun shines reasonably directly on the solar collector surface. If the weather changes or the pool gets up to temperature or you need a longer filtration time, you have to turn the valve but at least you don't have to turn the valve every morning and evening. With today's low flow variable speed pumps you may have to turn up the speed to accomplish filtration adequately over this shorter pump run time period.

The pool may have a pool automation system already and it may even have the solar brain built in already. If so, all that is needed is a sensor on the roof that sees the same sun as the collectors and a motorized valve to complete the automation component of the solar heater. The key component that none of the solar differential thermostat control manufacturers have ever clued into is the fact that the sensor on the roof, AKA the solar sensor, should look like the solar panel. Pentair, Hayward, Jandy and others use the same probe they use for the pool temperature sensor. These are affected by wind differently than the actual collector surface. Heliotrope uses an aluminum block which is better but still looks and behaves thermally - nothing like the collector. Solar sensors must be in the same sun as the solar panel, same shade, same pitch, same wind. This doesn't necessarily have to be next to the solar collectors.


Standard solar differential thermostats simply compare the sun sensor with the pool temperature and the setpoint (max. pool temp.) and decides if the flow should go through the solar collectors or not. The sensors are negative temperature curve 10K thermistors. That means that as temperature goes up their resistance goes down and vice versa. They are 10K ohms at roughly room temperature.

Hot Sun now makes our own sensors. The roof or solar sensor is a stainless steel probe that slips inside a piece of solar panel material and attaches to the roof usually with an adhesive.

The best choice for a motorized valve is a standard pool industry 3 way valve valves with a pool industry standard 24VAC actuator. Any solar automatic controller from any supplier or manufacturer works with these 24 VOLT AC motors and these motors (actuators) bolt on to any standard 3 way motorized valve.

Three-Way Valve

Valve Actuator

Pool Water & Solar Return Temp Sensors

One incompatibility in the industry that you have to watch out for is the "linear sensor". Compool uses these more expensive sensors in some of their control systems. Instead of a regular thermistor that follows a curve of resistance vs temperature, the relationship is linear.

Another odd ball item that exists is what is called a solar valve. This is a standard 3 way valve that has an internal check valve built in. The purpose is to allow water to drain down when solar is off. IN theory this makes sense. Allow the water to drain down to the pool so the collectors are empty so they don't freeze when the air temperature drops momentarily to 42F. That's right. A black radiator with the night sky can freeze at 42F because it is colder than the night air. This has been the demise of many a non warranty polypropylene collector. Unfortunately letting the water fall back through the tiny little check valve buried inside a 3 way "solar" valve doesn't mean the water will drain down. If there is pressure in the pool system (and there is) the water will only drain down to a level that corresponds to that pressure and often that level is not lower than the collector elevation. Sometimes a solar valve is an advantage but often its just false reassurance and a fussy check valve that could stick or fail. In the case of our product, the Powerstrip, we aren't worried about draining the solar collector every time night air temperature drops because our flexible collctor can freeze solid with no damage. We guarantee it. There's no advantage to a solar valve with Hot Sun's technology so we always use standard 3 way valves. Standard is good. If it leaks any pool serviceman can fix it or replace a part. Same with the actuators. Our solar systems last a long time and there will be maintenance issues at some point so let's not create a customized system that you can't get parts for. Powerstrip is built with longevity in mind.

The SWIM PC (Solar Water Internet Monitoring Programmable Controller)

Speaking of technology, Hot Sun has developed our own solar control system. After 25 years in the business we needed something better than a simple differential thermostat. Sometimes we are running a gas heater or heat pump as well as a solar heater. We learned through experience that simply having solar on whenever it could add heat to the pool left a lot to be desired when another heater was also trying to meet its thermostatic goals. Often we found the gas heater was firing when solar was on and the pool was only a little off setpoint and it was early in the day. The gas heater was doing work solar could have done. The strategies for optimizing solar and back up heater to work together to deliver the most energy with solar while maintaining a minimum temperature with gas turn out to be quite complicated. What is needed is online monitoring so the situation can be seen. What is then needed is online control so the back up heater can be disabled or its setpoint changed according to conditions.

For example you might see that the pool is getting up to 78 degrees during the day in October with solar and no cover so you might decide the allow the gas heater to heat the pool to 80 only in the afternoons and only Friday through Sunday. The online interface is key because it means you can check on the system from anywhere. You might adjust these parameters seasonally and use last season's data to make informed choices. You may realize its not or it is worth running gas in December. Our online system allows you the insight. You can see what is happening so you can schedule things accordingly. The big companies have tried to provide this kind of pool automation sophistication but they have failed so far. It takes real insight into what it is the customer really needs. We're a small company. We have that insight.

The SWIM PC should always be hard wired. If its outdoor its against code to have a line cord plugged into an open outlet all the time. If its indoor its in a pool room environment. You normally hard wire any solar controller and the most convenient place is usually to the existing mechancial pool timer. Often this is 220VAC and no 120VAC is available because there is no neutral wire. The SWIM PC can be powered by 240VAC or 120VAC. Factory setting is 240. You won't damage if it is wired to 120VAC

The SWIM PC wires to 3 sensors and normally a motorized valve. It also has an ethernet port. If internet is available the data will upload to the unit's own web page automatically every hour. If internet is down the unit will store up to 46 years worth of data on its own micro SD card. You don't need internet. Your data is stored for the life of the system. The micro SD card can be removed and the file on it e-mailed for free upload. Its what the whole clean tech industry has been missing and its the key to sustainable energy solutions gaining a market foothold in times of low natural gas prices. Solar pool heating remains one of the few alternate energy appliances that compete with low cost natural gas from a strictly economics perspective.

The SWIM PC can see the internet wirelessly. If the building has a wireless internet signal present then a wireless bridge or wireless booster (available locally for only about $60) can receive your wireless signal and from there port it to the SWIM PC ethernet port.

The SWIM PC represents the culmination of about 14 years work on Hot Sun's part trying desperately to bridge the technology gap and deliver this solution. Up until now there has never been a way to optimize the performance of a gas pool heater or boiler system with solar. Usually in residential applications solar is simply the only pool heater and that fact makes compatibility straightforward.

This demonstration interface allows you to try the system out. The user name and password are both "demo". It is a full spreadsheet on a web site accessible from anywhere in the world. We've designed our own programmable thermostat interface to be so user friendly you'll never need to read the instructions or look at a manual. This boiler override feature is the key to not only optimizing the use of the gas heater with solar, it's also the means by which to quantify the cost of doing so and the savings by locking the gas out at other times.

Monitored sites allow us to quantify the performance of solar seasonally and regionally. Check out many monitoring pages for many SWIM PC controlled systems in our CASE STUDIES section for residential and commercial pools.

Commercial pools operate a boiler at a setpoint. We haven't seen any commercial or municipal pool system with a programmable thermostat yet heating some municipal pools can be a major component of an entire municipality's natural gas budget. Just turning the gas down at night can save thousands in some cases. If swim team practices Wednesdays and Fridays then allow the boiler to fire starting at 5 am those days, otherwise lock it out until pool opening time and lock it out if solar is available.

The internet and data logging features of the SWIM PC are available to existing Hot Sun customers and new customers as part of Powerstrip solar pool heaters at very little additional additional cost compared to a basic differential thermostat. This is possible because your data is valuable to us. We're working on similar partnerships with other clean tech industries interested in exposing their truth.


If you don't need the advanaced capabilities of a SWIM PC controlled and monitored solar pool heater and your pool doesn't have a pool automation system with the solar brain built in then we recommend a very good solar controller, the SOLARTOUCH made by Pentair.