Solar Pool Heating System  (C) Daniel Friedman Outdoor Swimming Pool Heaters
Choices, Troubleshooting, Installation & Repair

  • SWIMMING POOL SOLAR HEAT, OUTDOOR DIAGNOSIS - CONTENTS: Diagnosing why a 16-panel solar swimming pool heating system is not working - why is the swimming pool still cold? Solar heated swimming pool in Guanajuato, Mexico. Key steps in diagnosing a solar heating system. Function of solar pool heating system parts, controls, components. Proper operating pressure, temperature, flow in solar heating systems. Alternative methods for heating swimming pools.
  • POST a QUESTION or READ FAQs about design, installation, maintenance & repair of solar swimming pool heating systems

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Solar swimming pool heater troubleshooting:

Guide to outdoor swimming pool heater choices, methods, design, installation & troubleshooting. We explain the diagnosis of a poorly-performing outdoor solar swimming pool heating system. Sixteen solar panels installed on a flat roof are unable to raise the temperature of a swimming pool in Mexico.

This article describes the solar swimming pool heating system, followed by a discussion of diagnostic steps useful to determine why the solar heater performance was not satisfactory. Our page top photograph shows the 16 solar collector panels installed for a solar swimming pool heating system that was not working properly, viewed from the last panel pair (#15 and #16 in our sketch below).

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Types of Swimming Pool Heaters & Heated Water Sources/ Methods

Thermal hot springs La Grutas, Guanajuato Mexico (C) 2013 Daniel Friedman

[Click to enlarge any image]

A variety of methods are used to heat swimming pools both outdoors and inside. Our photo above shows a naturally-heated thermal spa, La Gruta, outside of San Miguel de Allende, in Guanajuato, Mexico. Thermal springs in the area feed a number of both public and private facilities.

Outside swim pool heating approaches include:

Use of a naturally heated water source such as thermal springs as a swim pool water supply.

At above left at La Gruta, a thermal hot springs located at Atontonilco, a historic site located about mid-way between San Miguel de Allende and Dolores Hidalgo in Guanajuato, Mexico. Hot water from thermal springs that are throughout the area is routed through a cascade of pools at this and similar facilities in order to provide a range of water temperatures.

Traditional hot spring fed Japanese bath (C) 1965 - 2013 Daniel Friedman

Naturally hot spring-fed baths are used world-wide where such waters are available, and may feed both outdoor (above) and indoor pools and spas.

At left we illustrate a traditional hot-spring fed bath at the home of family friend Keizo Ono, Tokyo, Japan, ca 1965.

Solar swim pool cover (C) 2013 Daniel Friedman

Using solar swimming pool covers as a passive heat gain for swimming pool water and to slow the loss of heated water during cooler night time temperatures or re-stated: solar covers provide solar heat gain into the pool water and they also help keep heat in the pool.

Our photo of a solar cover for a swimming pool (left) illustrates this application in New York where a pool cover can extend the season of swimming pool use.

Typically the solar cover is a plastic bubble-type material. The clear or light-colored plastic bubbles act as lenses to help focus sunlight and solar heat gain into the swimming pool water.

Solar pool covers add about ten degrees F. to the temperature of the swimming pool water during sunny summer months, and by slowing heat loss during cooler nights, the total benefit of a solar swimming pool cover can be up to a 90% reduction in the cost to heat the pool, or more reasonably, a 7j0% reduction of heating costs for the swimming pool if the average daily temperature is above 50 °F.

Naturally heated spa at Lourdes, San Luis Potosi (C) 2012 Daniel Friedman

Using an active or passive solar panel system to heat swimming pool water is discussed in detail below.

Using an LP gas, natural gas, or oil fired swimming pool heater to heat the pool is one of the more common methods for heating residential and some larger public swimming pools.

Shown at left is a heated "Jacuzzi Spa" swimming pool at Lourdes, near Santa Maria del Rio in San Luis Potosi, Mexico.

This spa has been in continuous use since February 1931 and adjoins a bottling plant where natural spring waters high in mineral content (Aguas Minerales de Lourdes) are bottled for consumption. While thermal hot spring water is available at the spa, including in private bathing rooms, the owners of the outdoor spa pool shown above have also installed a fuel-fired pool heater whose enclosure can be seen in the upper left of our photo.

Diagnosing a Cold Solar Heated Outdoor Swimming Pool - Installation Details

Swimming pool Shanti San Miguel (C) Daniel Friedman

Solar Heated Swimming Pool - Sources of Pool Warmth

Special thanks to Ken Wright, Hot Sun Industries, Inc. for assistance in diagnosing this chilly solar heated swimming pool. Mr. Wright is a solar heating expert who designs, sells, and installs solar heating systems. Brett Knobel is the owner/operator of Shanti San Miguel, located in San Miguel de Allende, Guanajuato, Mexico. The plumbing "looked right" for this solar swimming pool heater. Why wasn't the system working?

We started the diagnosis of this chilly "solar heated swimming pool" by a visual inspection simply to see what equipment, piping, collectors, controls, valves, pumps, etc. were installed. The solar installation is described below:

At the time of our inspection and despite daily use of the solar pool heating system, the swimming pool was cool.

The solar swimming pool heating installation was about six months old and, according to the owner, had never worked.

Solar Pool Heating System  (C) Daniel Friedman

About the only heat that this swimming pool was gaining in December 2009 was direct solar gain - sun shining on the swimming pool during the day.

Temperature of water entering the swimming pool from the solar collectors was about the same as the temperature in the pool itself - if indeed water was actually flowing through the collectors - something we needed to determine.

The swimming pool is about 12 x 20; 5 feet at the shallow end, 7 1/2 at the deep, about 100,000 liters in capacity.

At night the owner covers the pool to attempt to retain some of its warmth and turns off the pool pump.

Also see SWIMMING POOL SOLAR HEAT, INDOOR where we explain insulation, moisture, ventilation requirements.

See SOLAR COLLECTOR AIR or GAS COLLECTION where we explain gas collection problems and solutions for solar heated swimming pools.

Solar Swimming Pool Heating Panels

The solar swimming pool heating panels (photo at page top) appeared to be Solar Industries rigid polypropylene collectors made by Aquatherm Industries in NJ (no warranty - they private label everything).

The owner of the facility gave us the discouraging news that the original solar installer had left in bad graces. A second "engineer" had inspected the system, had the owners rotate the position of each of the sixteen panels 90 degrees, but had produced no improvement in the solar pool heater's performance.

Pool temperature did not change (C) Daniel Friedman

Solar Heated Swimming Pool Temperatures

The solar swimming pool heating system, sixteen 4 x 8 foot solar panels mounted on a rack above a flat roof, did not appear to be giving much heat to the swimming pool.

The pool temperature was 70 °F. when the heater started running (when sun hit the solar panels) and did not rise above 72 °F. during the day. We considered that this two degree temperature rise might be entirely due to direct solar gain on the pool itself.

We measured water temperature at the supply outlets into the pool from the solar heating system - and found the supply temperature to be about 72 °F.

So at most we were seeing a two degree temperature rise across the sixteen solar panels.

According to solar heating expert Ken Wright,

Solar pool heating is real simple. If it's black and its cold and it's in the sun then its delivering all solar energy it is exposed to to the swimming pool. End of discussion. [Almost]

So we set out to check the temperature of the panels across their widths in full sun and tell Ken where the hot spots were found, if any.

Other possibilities besides an air-bound system were improper plumbing, improper pool control setup, or simply a valve that was in the wrong position somewhere in the system. We also considered the less likely (with plastic piping) problem of a blocked pipe.

Solar swimming pool controller (C) Daniel Friedman

Solar Pool Heater Controller

A Sun Touch Solar Controller for Swimming Pool Heating System was found installed.

When the pump is started, there is a big flush of water, which then subsides.
The engineer claimed the pump pressure is normal.
We do not have a vacuum breaker.
The collectors are always cold. (This sounds normal, but no temperature rise is being seen in the swimming pool).
The swimming pool pump runs from 9:45 to 5PM every day.

Swimming pool pump pressure (C) Daniel Friedman

Solar Pool Heater System Water Pressure

Pressure at the swimming pool pump looked dangerously high to us. The pool filter had been freshly backwashed when we took this photo of the system pressure.

We were a little concerned about bursting a plastic pipe or other component. But according to the owner's engineer, this pressure, about 36 psi, was considered normal.

Wright and other solar swimming pool heater experts point out that we don't need much pressure to circulate pool water through the solar collectors, just enough to overcome the pressure created by the height of elevation of the collectors over the swimming pool. In fact, circulating water at higher pressure through the solar collectors can damage them and will reduce the efficiency of the system as well.

Swimming pool solar heating system piping details(C) Daniel Friedman

Swimming pool and solar pool heater piping:

The sixteen flexible solar panels are arranged in four banks of four 4x8 panels each.
Each bank of four panels is fed from the swimming pool pump, by a 1 1/2" PVC pipe manifold.

Our photograph (left) shows the 1 1/2" PVC supply manifold connected to solar panel No. 1, the first in the series. At each solar panel quad or bank, swimming pool water enters the input end of two solar panels in parallel, flows out of the bottom of this panel pair into the top of the second panel pair of the quad, and exits at the bottom of the second solar panel quad to pass through a return pipe to the pool pumping system.

The four "quads" of solar panels are piped in parallel, that is, swimming pool water flows through groups of four panels in parallel, and groups B, C, and D are provided with control valves to turn the flow on or off. Control valves permit turning off flow through three of the four solar panel banks.

The solar panel banks, while installed roughly level, are not all at the same elevation. The last four panels are roughly 6" lower than the middle six solar panels, and the first four panels are roughly 4" lower than the middle six panels.

Because each quad of solar panels has its own water feed piping in, these level differences may be unimportant. But other piping and connection details may have left air trap points in the system.

High and Low Points in the Solar Panel Swimming Pool Heater Installation

Solar swimming pool heater - solar heater piping sketch (C) Daniel Friedman

The installation of each quad of solar panels is roughly flat, but piping was found to be uneven, with some solar panel quad bank input or outlet manifolds tipped as much as 3" out of level.

But the levels of the quads of solar panels varied; the first quad, "A", comprised of panels 1, 2, 3, 4, was installed about six inches below solar panel quads B and C (the center eight panels in our sketch). Finally, solar panel quad D (panels 13, 14, 15, 16), was installed about four inches lower than the center quads B and C.

The panels were also not well supported on their steel frame, and we observed ponding after rainfall that might reduce the solar gain of the system during periods of intermittent rain and sunshine.

Ultimately the solar heated water returns to the swimming pool through eight jets installed around the pool sides.

There were no air bleeder points in the piping system, and no vacuum breaker drain-down valve. The system is not designed to drain down when the pump is turned off. A single check valve was located near the pool pump.

Swimming pool filter (C) Daniel Friedman

Swimming pool filter system (photo above left).

The swimming pool pump is visible, lower right, and the control valve routing water through the solar heating system or the gas fired pool heater is at the center right of this photo.

The swimming pool pump runs from 9:45 AM to 5PM every day.

A check valve is installed down-stream from the pump [need to confirm this location] but the system was not designed to "drain down" at night.

"When the pump is started, there is a big flush of water, which then subsides." reports the owner. [This may be diagnostic.]

Swimming pool valves, controls(C) Daniel Friedman

This swimming pool pump output flow control valve intends to switch between routing water through the solar heating system and a separate Hayward LP gas fired swimming pool heater.

We wondered if it would make sense to provide another option to use both heating systems in parallel, routing solar output through the heater and thence to the pool.

NOTICE: We have not heard final resolution from the property owner who was to follow the diagnostic suggestions provided below. We will add that information here when we know the last chapter of this investigation of why the swimming pool stays too chilly.

Solar Swimming Pool Heating System Diagnostic Guide

Readers may find it helpful to follow the solar pool heater diagnostic discussion below. We don't promise to hone in on the specific problem immediately - after all, we are diagnosing a non-heating solar pool system by email between two countries.

An on-site inspection by an expert is nearly always more complete, quick, and reliable. But reading our discussion (below) describes the sequence of investigation, discussion, and ultimate repair of the solar pool problem we have introduced above.

Our diagnostic discussion of this non-working solar pool heater is provided below, with the most recent comments listed first. Discussion was between Ken Wright, a solar pool heating expert, and Daniel Friedman, an experienced forensic investigator. Both investigators agreed to diagnose the pool heating system problem without compensation from the owner in exchange for her permission to use photographs of her system.

[Ken Wright to D Friedman (12/23) provided these solar pool heating system diagnostic questions and suggestions.]

Is water actually flowing through the solar collectors? In diagnosing this non-heating solar swimming pool heating system, it is important to consider the fact that maybe the water isn't even flowing through the collectors. [That's why one of the first steps in diagnosing a too-cool solar heating system is to feel all of the solar panels, looking for temperature differences. A hotter panel is probably one that has less or no water flowing through it.]

Solar Heater Diagnosis - Install a clear check valve: We always install a clear check valve on the return pipe so there is no doubt in the novice owner's mind as to whether or not the flow is going to the solar panels when it is sunny and the pool isn't up to temperature.

Is the solar pool heater controller set properly? Solar pool heater controllers have test modes meaning water could be just flowing to solar all the time, cooling the pool at night.

Are open valves really open and allowing water to flow? Or solar may not be working at all. Once we ran into a motorized valve that turned on the outside but the shaft inside was broken.

Are the solar heating system, pump, controls operating at correct pressure? The only pressure gauge on the system we are discussing (see photos above) is at the pool filter and may not accurately represent the delivery pressure at the swimming pool pump. We need a pressure gauge at that location too. When we have really high pressure like that, we usually suspect the pressure gage is faulty. [Or its location is not measuring system pressure. See WATER TANK PRESSURE GAUGE for more about these devices.]

A good clue as to what is happening without a pressure gage is to look at the flow. A better way is to install a $10 pressure gage from the pool store. You can see swirl in the skimmer basket if there is good suction side flow. You can of course feel a strong force of water at each of the inlets to the pool.

At the subject swimming pool, we felt (by hand) good flow at the eight ports that deliver water into the swimming pool. -- DF

Pumps also make weird sounds of they are overloaded. Pool system water pressure has nothing to do with solar collector performance, only flow. Pressure and flow depend on each other of course. It's useful as a solar technician to understand what pressure is. Everyone in the business thinks they understand but very few have the slightest clue really. Videos at Mr. Wright's website provide a tutorial on this topic.

High pressure can damage solar collectors: Pressure will cause the collectors to fail for sure in time. How much time? 6 months, 2 years. We have one system like this rigid polypropylene system in question in San Diego under 9 psi and its 5 years old. First we had to return to tighten up rubber removable couplings, then again. That was year one.

Then in year 2 we had to replace the rubber couplings with reinforced ones. Then in year 3 of the collectors sprang leaks. I wish the pool builder we'd fought with this over 3 years earlier had still been around so we could rebut his claim that he does this all the time and there is no problem with this small pressure.

The pool builder wanted to subcontract out for solar for the customer meaning he was doing the solar design yet he wasn't taking responsibility for the solar design. There is no such thing as solar design in his mind and he convinced our customer.

Three years later there we were changing the solar design, re plumbing the way solar tied in so the pressure was regulated. No more problems since. No pressure, no leaks. But all the while the system heated the pool just fine.

Excessive pressure and air blockage in solar pool heaters: Excess pressure contributes to air blockage issues. The reason is we now have a compressed bubble of air in the solar collectors. We have to flush that out upon startup. Flushing a compressed bubble of air down the return pipe to the pool is more difficult than a zero pressure bubble of air.

A second explanation of the effects of high pressure air bubbles in any piping system we learned from oil burner heating school: a compressed air bubble expands to a much greater volume when the pressure on the system drops, making it still more difficult to "push" this air bolus around through the system at next pump start-up. --DF

This is where some of the finer points of solar troubleshooting and design come into play but if the pressure is low like it should be this issue almost disappears.

Solar pool heater with no vacuum breaker and no drain-down: In the case we are discussing here, with no vacuum breaker the system is full of water all the time. The danger is that with the pump off in mid summer with the pool up to temperature the collectors could get as warm as 187 degrees. PVC plumbing will collapse and so may the collectors. Depends how much suction pressure which depends on how high the roof is.

No drain down and missing check valves can convert a solar pool heater into a night time pool chiller: On low elevations its not as critical to have a vacuum breaker, however the fear that water is migrating backwards cooling the pool is always a real fear.

A vacuum breaker is as simple as a one way valve teed off the pipe feeding the collectors. It doesn't let water out and it does let air in if there is a negative pressure. Ask the pool store for a 1/2 pound spring check valve. Don't ask for a vacuum breaker. They won't have one. At Hot Sun we use an actual brass vacuum breaker. (This valve is sold by HotSun).

Because we could not measure more than a degree or two of temperature difference between water leaving the pool and water returning to the swimming pool, we didn't think that a night-time pool chilling effect was the problem--DF.

Add a clear swing check valve to the return pipe on this system. Then we know 100% for sure when and if there is flow.

Solar Pool Heater: Initial Diagnostic Questions Posed to the Solar Heating Expert

[Daniel Friedman posed these questions to Mr. Wright.]

  1. The problem system that we are diagnosing is a solar pool heater in San Miguel de Allende, Mexico (described and photographed at the beginning of this article). Also pertinent to the operation of any solar collector system are the average temperatures in San Miguel - which we report separately at WEATHER in SAN MIGUEL de ALLENDE.

    We have as a solar swimming pool heating system, a flat rooftop system mounted on a metal grid using flexible solar panels. It's a new installation, the original installer is off the scene, a second "engineer" has looked at the system and had the owner change the direction of flow of all of the solar panels, but keeping the system flat - it didn't work before and didn't work after his suggestions and the owner is stymied.
  2. This is a flat roof 16 flexible 4x8 profiled surface panel installation using flat plate style collectors, controlled by a SunTouch auto controller (Since the pool never reaches a cutoff temperature we were not sure what this control is doing - it might be improperly set.).
  3. Flat solar collector panels are piped in 4 sets of 4 4'x8' panels (16 panels total), no visible vacuum breaker, no temperature sensors, pressure sensors, flow sensors on the piping system. A manual isolation valve sends water from the pool pump through the solar panels or through a gas fired pool heater. (Not the layout I'd have recommended). A single check valve is located near the pump.
  4. At our initial site inspection we saw an inlet water temperature of 70 °F and water returning to the pool at just under 72 °F on a sunny December day (air temp in the high 70's or above).
  5. Pressure at the filter looks dangerously high (past the "Red zone") and over 20 psi at the filter - this may not be the delivery or flow through pressure on the system but it needs investigation;
  6. The solar heating panel system looks flat and may be air bound.
  7. We were not sure that the piping arrangement is correct. We had some concern that the outlet pressure from the first panels in the group might be providing backpressure on the outlet end of subsequent panel gangs, fighting flow through subsequent panel sets in the group.
  8. There is poor panel support, ponding (rain is seasonal here) but we doubt that the ponding would explain the very low temperature rise across the panels.
  9. The solar collector panels that we felt (we did not check every one - a critical diagnostic step) all felt about the same temperature, and all felt cool, but they had not had much sun yet at the time of our inspection.
  10. A Hayward pool heater fired by LP gas is installed but not in use.

Solar Pool Heater: Initial Diagnostic Questions & Advice Posed to the Owners

[Daniel Friedman wrote these questions and suggestions to the owner after an initial site inspection.]

  1. Check the solar collector surface temperatures: go on the bank of solar collectors on a sunny day, perhaps at 1 or 2 PM (or at solar "noon") when the system has been operating, and feel the solar collectors that you can safely reach. You should feel EVERY solar collector surface and compare the results. If you find that some collectors are cool (correct conditions) and others quite hot (improper) then the hot collectors don't have water flowing through them.

    When operating with water flowing through the solar collector, the panel will feel cool to the touch across its entire width. Near the feed or inlet side of the collector temperature may feel close to swimming pool temperature. Near the outlet end of the collector the temperature will feel warmer but should be only 5-10 degrees higher than the inlet temperature.
  2. Swimming pool dimensions and capacity: Please give us the dimensions of the swimming pool itself, length, width, depth or average depth.
  3. Air in the solar pool heating system: When you turn on the solar system and pool pump in the morning, do you see a big flush of air coming out of the roof panels and back down through the system into the pool?

    You should see a big rush of air, stopping after a few minutes. If not, the system is not draining down when shut off, and/or it may be air-bound. We need to look into the air bound question because the solar pool heating system is installed roughly flat on the rooftop, but with some uneven, tilted piping (high spots) and with some banks of solar collectors at different levels than others.
  4. Water pressure in the solar pool heating system and solar collector panels: did your engineer do any pressure checks on the system? Your pump pressure looks dangerously high at the filter. Provided the filter is clean there should not be high backpressure.

    At the solar panels themselves, pressure should not be more than a few psi. The panels work best at very low pressure. If someone has set the pressure up to try to force the system to work, it's not helping and could risk injuring someone if a component breaks while someone is nearby. Here are comments from an instruction manual for a solar pool heating system similar to the one discussed in this article:

    It's important to understand that unglazed solar panels such as Powerstrips are made of thermo-plastics, plastics that can withstand high temperature, but plastics cannot withstand high pressures like metals can when combined with the high temperatures.

    Stagnating in Hot Sun and no wind, solar heater collectors of this type will get very hot. In making sure your solar system is compatible with your pool mechanical system we are making sure the system will operate stress free. This means minimizing the pressure on the collectors. Any nonmetal solar collector will fail prematurely if installed under pressure regardless of the pressure capability the manufacturer claims. The fact is there is a lot of plumbing on the roof and a lot of water flowing and installing a solar system under pressure is asking for trouble in the long term.

    Most pool/roof combinations are suitable for a problem free solar heater and your dealer will always check first to make sure. Start up instructions at the start of this manual tell you to check the pressure in the collectors with a pressure gauge.

  5. Solar pool heater drain-down: Related to 3 above, did your engineer install a vacuum breaker on the system to allow air in and the system to drain down when off? For flat roof installations the vacuum breaker is on the pipe feeding into the solar panels.

    We don't need to worry about freezing panels (and thus don't need to drain-down for that reason, but we were concerned that without seeing air moving through the system and out when the pump turns on, it may be air bound somewhere.
  6. Be sure the pool pump is off at night or that you turn off circulation through the solar panels so you are not accidentally COOLING the pool down at night through the panels.

    Even with the pump off at night, if the system does not drain down when turned off and lacks effective check valves, the night time exposed solar collectors may actually be cooling the swimming pool by circulation that occurs by natural convection - working "backwards".


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