Hot Sun Industries Inc., San Diego, California
Plumbing and Controlling Solar Swimming Pool Heating Systems
Solar swimming pool heating makes sense. Generally the requirement for heat co-incides with good weather. The existing swimming pool provides a large storage and comes equipped with a pump capable of handling the high flow we want for an efficient solar heater. Residential and commercial pools often have a nearby roof surface available large enough to accomodate a large solar heater. All we have to do is divert the filtered pool water through a bank of collectors whenever the sun shines. If only life was so simple! The bulk of the system is actual solar collecting surface so swimming pool heaters provide great econonics.
These diagrams are shown in the solar off mode. Worse case design conditions occur when solar is off. The isolation and drain valves allow the system to be isolated and drained (winterized) so the pool pump can be run through freezing conditions. These drains should be located at the lowest points in the plumbing and all piping including the bottom headers of the solar panels should slope (tilt) towards those drains so the water runs out by gravity. The check valve prevents the water from running backwards through the filter (backwashing it) when the pool pump is shut off. It also prevents the pump from losing its prime if it is above pool level. The vacuum breaker allows air to enter when solar shuts off. This allows the system to drain back to the pool and prevents negative pressure in the collectors and plumbing. Without it the collectors would always be full of water and reverse flows would be induced at night cooling the pool. At night solar panels lose heat to the night sky by radiation so if they are full of water you are likely to have a cooling effect if the collectors are full of water and not operating. There are many subtle ways your system can underperform but we'll make sure its all set up perfectly though simple checks done as part of each installation including do-it-yourself installations of course.
Tilting the collectors to drain is important and co-incidentally avoids air entrapment that can cause poor performance as depicted in this animation...
This animation explains a tricky concept that is a common source of frustration for solar installers. You have to let the air migrate up out of the system. Air in water wants to rise. You need that second return pipe to make sure all the air can rise up to the highest point where it can be flushed to the pool.
Note that most of our standard plumbing schemes include a bypass valve that allows us to not have to send all the water to solar when solar is on. With this capability your roof can be 400 feet away and we don't have any trouble handling it from a mechanical perspective even using 1.5" pipe all the way. Relative to pool system flows we don't need much for efficient collector operation. Collectors can operate at 2-9 GPM each. You need 2 to get good enough efficiency and over 9 GPM you're starting to see pressure from too much resistance to high flow. 400 feet of 1.5" PVC pipe with 10 panels needing 20 GPM won't develop a significant pressure drop. Smaller pipe means less air to flush thru upon startup and less surface area for heat loss thru piping. The industry standard for 4'x12' polypropylene grid style collectors is a 2" header and even we recommend 2" pool system plumbing always! That's the pipe coming out of the filter carrying the full flow. In these cases we often plumb with 2" but most of our collectors use 1.5" header pipes. If we plumb in 2" we split into banks of 1.5". Our Powerstrips use 1.5" headers as do many of our Powerpro collectors.
Automatic Controls
A popular control system for a solar pool heater is a differential thermostat as depicted in the schematic of the system at the start of this page. It senses solar radiation (roof sensor) as well as pool temperature and diverts water to the collectors when appropriate through a motorized valve automatically. The pool temperature can be limited to a set maximum temperature if desired. The Flo-Verter valve has no seals (doesn't need them if the solar panels drain down naturally) which makes it more reliable long term.
We'll use positive sealing Jandy or Compool 3-way valves and actuators to control solar when we need that seal. If we're using the pressure to drive the flow then we need to seal the flow to solar off when we want solar off. Ideally we want the pressure to be low so we can use a non positive sealing Flo-Verter valve. No seals to stick. Solar drains down freely and sometimes we can establish year round operation through freezing conditions. Every situation is different and there are often compromises most of which stem from the fact that there are no engineers in the pool industry. Once there was one and he said use 3/4Hp pumps (or 1/2HP) on residential sized pools and after that he wasn't needed anymore. Nevertheless I've seen pool pumps go from 1/2 and 3/4 HP to 1 then 1.5 and now even 2HP as the standard over my career at this since 1986. See pools for an extensive discussion on this important topic.
If you want manual controls we'll often sell you a Jandy or Compool valve (not motorized) with the system so that if you want to automate later you can just bolt a Jandy or Compool motor(actuator) to the valve without any re-plumbing. In these cases you sometimes actually want to drill a small hole in the valve spool so water can drain down past it. You get a pressure gage with every kit and are instructed to check collector pressure as part of the initial start up procedure. That's an important step that can create extra work like this. Communicate with Hot Sun what you find and work with oiur staff to make sure your solar heater is set up right for the long term. Its easy to set solar up to heat the pool. It's another thing entirely to set it up to last.
Note that using a timer to semi-automate solar is a viable alternative to the cost of automatic controls on smaller systems where the cost of automation is harder to justify. If solar is manually controlled and the valve is turned to "solar on" then simply running the pump only for the hours of the day that sun is on the solar panels means solar is on when heat can be gained and off when it can't. The only downside is the pool will get too hot at some point and you'll have to turn the valve. Also if the weather changes you may have solar on when it shouldn't be. In colder air conditions and wind (and sun blocked by clouds) you can lose significant heat having solar on when it shouldn't be. Several sophisticated monitored sites that we were recently involved in showed data that proved automatic controls are more beneficial than we once thought.
A type of controller sometimes employed is one which turns the pool pump on as well as turns a valve when solar is to be gained. This is useful if the pump is on a timer to only run a few hours a day. You don't want to spend a lot of money running a big pump every day all day if you don't have to. The solar control turns the pool pump on by bypassing the timer if the pump is not on when the controller wants solar on. This strategy allows the pump to be off when solar's not on thereby allowing the pump to operate on its normal 3 hour a day cycle during bad weather or when the pool is warm. We seldom sell this type of control but it is available. Keep it simple. Don't use an oversized pump in the first place for filtering. Also consider that with the pump off you aren't able to control chemistry and you aren't taking advantage of solar gain to the pool's top surface directly. The hot water forms a layer on top meaning you are maximizing the heat loss from the pool. You need the pump on during the day anyway!
Now that you've been on the home page and learned all about which collector type is best for your situation and learned all about plumbing and auto controls on this page , you're ready to go to the sizing pages and learn all about what a solar heater can do for your pool. You will find a form you can fill in so that I can give you my recommended sizing and a full quote.
Sizing your collector system. We've posted many regionally specific curves showing what you can expect in your area with verifiable proof.
If you'd like an education on solar heating in more general terms check out the pages buried here I wrote them in 1995 when I first published this web site.