Solar Water Heating FAQ

Solar Water Heating FAQ

How automatic is a solar pool heater?
Completely. All you do is set your pool pump’s time clock or automation system to run during the daylight hours, then set the solar pool heater’s automatic control temperature indicator to your preferred temperature. The control system takes care of the rest. Sensors compare the temperature at the solar collectors with you pool water temperature. Whenever the solar collector temperature is at least four degrees warmer than your pool water, the control system adjusts a motorized valve to divert pool water through the solar collector panels if your pool is not at the desired temperature.

And even if your pool cannot reach your preferred temperature setting during the coldest winter weather, your pool will always be warmer than a neighbor’s similarly situated unheated pool, without you spending a penny on expensive fuel.

How can I determine what size and type of pool heater is best for me?
Every situation is different and relying upon simple “rules-of-thumb” can lead to unrealistic expectations and unhappy customers. Among the many factors we consider when sizing a pool heater are:

  •     desired swim season length
  •     preferred water temperature
  •     type of pool use (exercise, kids playing, casual dips, etc.)
  •     therapeutic requirements
  •     screen enclosures and other direct shading of pool surface
  •     open space and windbreaks, especially along northwest to northeast exposures
  •     waterfront location
  •     distance between pool equipment pad and pool heater
  •     for solar, availability of sufficient unshaded roof or other installation location
  •     for solar, direction best available roof area faces
  •     willingness to use a pool blanket
  •     ability and willingness to pay increasing energy costs

I keep reading about the pool’s surface area. Why don’t you use the number of gallons in my pool for sizing?
Your pool’s water volume (gallons) does matter, especially if your system is installed during the winter, because the water temperature of an unheated pool during the winter months can be as much as 20 degrees below the desired temperature. In this case, dividing the estimated average daily Btus of heat input from the heating system by the pounds of water in the pool (water weighs 7.5 pounds per gallon) tells us how fast we can bring the pool up to the desired temperature.

On the other hand, in normal operation we are simply trying to replace the two to four degrees of water temperature lost overnight, and most heat loss occurs through evaporation at the pool’s surface. This is why we size pool heating systems in relation to the pool surface area.

What is the best way to handle and store a pool blanket?
Pool blanket reels offer convenience; however, many pool owners do not want the reel assembly sitting on their pool deck. For a typical residential pool, two people can very easily remove and fold a pool blanket in about two minutes.

To remove the blanket, each person stands at an opposite corner of the same end of the pool. Pull the corners of the blanket out of the pool, then step back and set the blanket down so that about three to four feet of the blanket is overlapping the pool deck. Next, each person should step forward along his or her side of the pool to a point roughly twice the length of the section of blanket resting on the pool deck (six to eight feet). From this point forward, simply repeat the process of pulling six to eight foot sections of the blanket back out of the pool and folding each section accordion-style on the pool deck.

With the blanket folded at one end of the pool, repositioning it on the pool surface takes less than one minute and is a simple matter of each person grabbing a corner of the blanket and walking forward to the far end of the pool.

I have heard about something called a liquid pool blanket.  Is this for real?
Yes. A liquid pool blanket is a patented solution that, when spread across the surface of a pool, reduces evaporation from a calm pool surface by about 40 to 50 percent. Of course, a swimming pool is not always calm, and actual reported savings on heating costs range from 15 to 35 percent.[1]

The liquid solution, marketed under the brand name Heatsavr®, is a mixture of alcohol and aluminum salt. Aluminum salt is a white flaky substance commonly used in cosmetics, soaps and toothpaste. It is safe, clear, tasteless, odorless and biodegradable. Alcohol is lighter than water, which allows the solution to form a thin film at the pool surface. A very useful property of the alcohol and aluminum salt mixture is that the molecules tend to rearrange themselves back into a thin sheet across the pool surface after pool activity, as the water calms. Also, the aluminum salt particles used in the Heatsavr® product are so tiny that they will not clog pool filters.

Liquid blankets are especially useful for larger residential and commercial pools and for unusually shaped pools, situations where handling a plastic blanket may be impractical. The Heatsavr® liquid solution is dispensed automatically, either from a metering pump dispenser or from a plastic container that floats in the pool. The most popular containers are shaped like tropical fish; the solution dispensed by one “fish” may cover up to 800 square feet of pool surface for two months.

While a liquid pool blanket is only about two thirds as effective as a plastic pool blanket, it may actually deliver better performance than a plastic blanket in real world situations if the inconvenience of removing and replacing the plastic blanket leads to inconsistent use.

We have an installed pool heater. How can we tell how well the system is working?
Of course, if your next door neighbor’s pool is unheated and has similar site factors (screen enclosure, windbreaks, etc.), you can simply compare water temperatures.

However, if you don’t happen to have such a convenient comparison point, or if you simply want to better understand your pool’s temperature dynamics, the method described below will provide you with a pretty good approximation of what your pool’s current 24-hour average temperature would be without supplemental heat.

Go to Weather.com and enter your zip code into the Local weather search box at the top of the page.

When the results page for your local weather appears, you will see a row of boxes with links for different types of local weather data just below the page’s main header banner. Select the last box: Month.

Record the daily high and low temperatures for each of the preceding six days. You should have 12 temperature points.

Calculate the average for the 12 temperatures. The result will be a good estimate of the current day’s temperature of an unheated pool in your locale.

Remember that on a sunny day, a solar heated pool is usually at least three degrees warmer during the afternoon than during the early morning hours.

Also, keep in mind that Weather.com often publishes the same weather data for every zip code within a single county or large metropolitan area. However, actual air temperatures within the same county or metro area will vary: a bit warmer in urban surroundings and a bit cooler in rural surroundings. This is called a microclimate difference. You can get a good estimate of any microclimate difference applicable to your pool by comparing the Weather.com current local air temperature for your zip code with an outdoor thermometer reading. Just make sure the sun isn’t shining directly on the thermometer.

Can my solar pool heating system also be used to heat hot water for my home?
No. This requires two different systems. Home water heating water temperatures of 125°F to 140°F call for solar collectors constructed with metals like copper that conduct heat well, and insulation and glass cover plates to keep the heat from being dissipated into the air. Swimming pool solar collectors typically operate at temperatures of just 76°F to 95°F, so they can be constructed of polypropelene plastic and do not require insulation or cover plates.

Should I be concerned about roof penetrations? Is it possible that expansion and contraction of the solar panels over time might cause the roof penetrations to leak?
No. First, polypropylene solar pool heating collectors do indeed expand and contract throughout the day, so our mounting systems are designed to allow the solar collector panels to “float” inside the roof mounting straps, free to expand and contract as needed without putting any strain on the mounting hardware roof penetrations.

Second, conventional sealants can lose elasticity over time. This is not a significant issue for applications like window and bathtub caulking, but the extreme temperatures experienced on a roof surface are a different story. So our roof mounting penetrations are sealed with a special high technology sealant, originally developed for the aerospace industry. This sealant has been proven to maintain its elasticity over several decades.

Will expansion and contraction of the solar panels damage my roof?
No. Solar panel expansion and contraction occurs so slowly that it is not apparent to the naked eye. And such a slow rate of movement has no effect on your roof surface.

Will the heat generated by the black solar panels damage my roof?
No. Quite the opposite. Solar pool heating collectors actually protect the portion of roof they cover because the sun’s energy is being absorbed and carried away by the pool water circulating through the panels. Also, because a bank of solar pool heating collector panels typically covers a fairly large area of roof, it can keep your attic a bit cooler whenever the system is operating.

Should my solar collector panels be removed from the roof if a hurricane is approaching?
No. The mounting systems for all solar collectors approved for installation within Florida are engineered to withstand hurricane wind loads. It is not unusual to see completely intact banks of roof-mounted solar panels on a heavily damaged house following a hurricane.

I’ve heard that the U.S. Department of Energy determined that loose tube collectors are best for pool heating when they selected this type of solar pool heater for the Atlanta Olympic Games. Is this true?
No. This particular system was actually selected to cool the Olympic venue pool.[2] Here’s what happened. The swimming and diving competitions of the 1996 Summer Olympic Games were held at the Georgia Tech Aquatic Center, which was designed as an outdoor facility in order to increase spectator seating capacity from 2,000 to 15,000. (The Aquatic Center was converted to an indoor facility about five years after the Atlanta Summer Games.)

Pool water temperature for competitive swimming events must be maintained between 25° and 28° Celsius (77° and 82.4° Fahrenheit).[3] An outdoor pool’s temperature tends to match the 24-hour average air temperature for the preceding week, and Atlanta’s average daily air temperature in late July is 88°F. It isn’t unusual for temperatures to climb into the high 90s.

This meant the Aquatic Center pool water would be at least 10 degrees too warm for competition. So the primary goal of the architects and engineers was to come up with an effective method of cooling the pool.

Most non-metal solar pool heating collectors, including loose tube designs, radiate energy to a clear night sky at roughly the same rate. However, loose tube collectors have much greater convective heat transfer rates than flat plate designs because air is able to flow freely around and between the fluid passageways.

This is a really bad thing when you’re trying to heat a pool and the surrounding air is cooler than the water circulating through the collector’s fluid passageways. And even worse when the wind picks up.

On the other hand, a higher rate of convective heat transfer is great if your primary goal is to cool a pool at night. Thus, a loose tube heat exchanger design was a perfect choice to cool the Atlanta Olympic pool during the hot Georgia summer.

The manufacturer and dealers of this particular loose tube collector system promote the Georgia Tech Aquatic Center project as their flagship large-scale “solar heating” installation, and often point to the Department of Energy selection process for this project as evidence of the product’s solar heating superiority. Nevertheless, the fact remains that the loose tube collector array installed at the Georgia Tech Aquatic Center was specifically designed for cooling. The pool has a steam heat exchange system for primary heating.

A salesman told me that solar collector panels with two-inch header pipes are better than models with 1-1/2 inch headers. Is this true?
No. The argument is that a two-inch solar collector panel header improves efficiency by allowing more water per minute to flow into the fluid passages of the heating surface. While it is true that two-inch pipe has a higher saturation (maximum) flow rate than 1-1/2 inch pipe, a single bank of solar panels is never installed with more than about 480 square feet of total solar collector panel area. (Larger solar systems are broken into multiple panel banks.) Solar panels designed for swimming pool heating temperatures function best at a water flow rate of about 1/10 gallon per square foot of solar panel surface area per minute. So for the best thermal performance, we would never want to flow more than about 48 gallons per minute (1/10 gpm per square foot x 480 square feet) through a single panel bank, regardless of the pipe size. 48 gallons per minute is well below the saturation flow rate of 1-1/2 inch pipe.

Some solar collector panels require larger headers to partially offset the increased back pressure created by a plenum chamber design (see below). Unfortunately, some of the companies that sell plenum chamber collectors teach their salespeople to compare the costs of 1-1/2 inch and two-inch Schedule 40 PVC pipe at building supply outlets like Home Depot and Lowe’s, to justify higher prices for their solar pool heaters. But this is a meaningless comparison because headers comprise only a fraction of the material in a solar collector panel.

Where do we get our temperature information?
An unheated swimming pool’s water temperature will usually match the average air temperature for the preceding week. If the pool is warmer than the air after a cold front passes through, the pool will give up heat energy through evaporation until the air and water temperatures are the same.

Think of it this way: You run your air conditioner during the summer to cool your home. If you were to open all the doors and windows on a hot summer day, the indoor air temperature would rise to match the outdoor air temperature within a fairly short time. At some point, the indoor and outdoor air temperatures would be the same.  Your pool goes through the same balancing act, only in reverse.

So the unheated pool temperatures you see on the this page are the same as the 10-year average air temperatures for each three-month season at the latitude and longitude of the reference city.

Our data comes from 10 years of NASA weather data. Unlike measurements taken at an airport, the NASA data represent an average for an area measuring one degree of latitude by one degree of longitude. Naturally, actual temperatures within this area will vary: a bit warmer in urban surroundings and a bit cooler in rural surroundings.

What is a pool blanket?
The most popular pool blankets are similar to blue plastic air bubble packing material. To increase durability, the pool blanket’s plastic contains UV inhibitors and is two to three times thicker than the plastic used in air bubble packing material.

Will a solar pool blanket heat my pool?
Yes, but the term “solar” pool blanket is a misnomer. A pool blanket keeps heat in your swimming pool by stopping evaporation at the pool’s surface. Evaporation is by far the greatest cause of heat loss from a swimming pool. If evaporative heat loss is reduced, an unheated pool will stay warmer longer and less energy will be required to keep a heated pool at a given temperature.

On the other hand, a pool blanket does not help the pool surface absorb more solar energy. In fact, a blanket actually blocks and reflects a very small amount of the solar energy that would normally be absorbed by the pool surface. But this effect is small and insignificant compared to the pool blanket’s dramatic ability to stop heat loss off the pool surface.

Are there any other benefits to using a pool blanket?
Yes. By blocking evaporation from a pool’s surface, a pool blanket also conserves water and reduces consumption of pool chemicals by 30 to 60 percent. The precise amount naturally depends upon how many hours the cover is in place.

I tried one of those liquid pool blanket “tropical fish” this winter. It didn’t heat my pool and I ended up with a white residue on my pool walls. What’s going on?
First, as we explain above, a pool blanket does not heat your pool. A pool blanket allows your pool to stay at a higher temperature by stopping heat loss from the water surface, but it will not add heat to your pool. For that, you need a pool heater.

As for the white residue, that’s aluminum salt (see the previous question). If you add Heatsavr® solution to pool water that is colder than 70°F (21°C), the aluminum salt will solidify and collect at the water’s edge. This may also occur if you add too much liquid pool blanket solution in relation to your pool’s surface area. Don’t worry, though. It’s completely harmless, biodegradable, and will go away on its own.

How well do solar collectors work during cloudy weather?
A solar pool heating system will typically collect about half the solar energy of a clear, sunny day on an overcast day. If you have ever had the experience of going to the beach on an overcast day and still getting a sunburn, you understand this phenomenon. Clouds block many of the visible wavelengths of sunlight, but much of the heat energy still gets through.

How well will my solar pool heater work during cold weather?
Solar pool heating collectors typically deliver excellent performance in Florida during cold weather because the sky is very clear during winter high pressure waves. On the other hand, increased evaporation from your swimming pool surface can significantly reduce your pool temperature during cold fronts. A pool blanket can help keep the heat from escaping.

How long will it take for my pool to heat up after my installation is completed?
This depends upon what time of your system is installed and will be greatly accelerated if you use a pool blanket to keep the added heat in the pool. For most solar pool heaters and with a pool blanket in place, an unheated pool will usually come up to temperature within three days or so during the spring and fall.

How long will polypropylene solar collectors last before I need to replace them?
Millions of square feet of polypropylene solar pool heating collector panels have been operating in the field since the late 1970s. You can easily expect 20 to 30 years of service from high quality solar pool collectors.

What is ideal direction and tilt angle for a solar pool heating system?
Solar pool heating collectors installed in the northern hemisphere should ideally face south. As long as the solar collectors face within 45 degrees or so of true south, heating performance will be pretty much the same as for due south. Solar pool heating collectors can also be face east or west and deliver the same performance as south-facing collectors by increasing the collector area (assuming enough roof area is available.

As for tilt angle, the ideal for winter heating is to tilt the collectors at an angle equal to the degree latitude plus 15 degrees. However, solar pool heating collectors are usually mounted directly on a sloping roof for aesthetic reasons. While this typically results in a more shallow tilt angle, which may result in a slight performance loss during the winter months (between two and eight percent), the difference doesn’t really matter because most Floridians don’t swim during the coldest winter weather.

Is it true that solar collector panels with “flow-balancing” plenum chambers perform better than other solar collector designs?
No. The idea is that flow-balancing plenums—secondary water chambers between a solar collector’s headers and the flow passageways of its heating surface—provide more balanced water flow throughout a bank of solar collector panels, and thus more efficient heat transfer.

But this is a solution to a non-existent problem. A basic rule of fluid hydraulics is that flow rates will vary in parallel pipes so that the head losses are equalized through each flow path. In plain English, if the diameters and lengths of the individual parallel flow passages in a solar collector are identical, the flow rates through these passages will be identical. This is always the case in a correctly installed solar pool heating system.

The only practical effect of additional flow restriction in a properly installed solar pool heating system is increased workload for the pump.

Here’s how this idea got started. During the 1970s, a solar collector manufacturer developed a process for heat-welding the heating surface of a polypropylene solar collector to the collector’s header pipes. The patented[4] process involved fusing strips of plastic—called flanges in the patent application—along the length of each header pipe, encasing the the ends of the heating surface. The flange design cut manufacturing costs by reducing the number of steps needed to attach the heating surface—and its many individual fluid passageways—to the headers. Unfortunately, the new process created a secondary water chamber along the length of each header, which significantly increased flow restriction through the collector.

An old saying holds that you should find ways to turn your weaknesses into strengths. At some point, someone came up with the idea that the additional flow restriction would ensure that water spread out more evenly among all of the fluid passageways in the solar collector’s heating surface. And so it was that the drawback of substantially increased flow restriction was magically transformed into “flow metering” and “flow balancing.”

Two major solar pool heater manufacturers use the plenum chamber design in their solar collector panels today. Their dealer salespeople sometimes use the example of house central air conditioning ducts to illustrate the need for a flow-balancing plenum chamber. But this is a poor analogy because there is usually great variation in the length and size of air conditioning ducts branching to the different rooms within a house, so there is indeed a need to balance the air flow between the rooms. This variation does not exist within a bank of solar collector panels.

References
United States Patent No. 6,943,141, “Process for making a liquid evaporation retardant solution,” September 13, 2005. The Heatsavr® patented mixture of alcohol and aluminum salt is based upon the science of Langmuir monolayers, materials that tend to form into one-molecule thick sheets when spread onto an aqueous surface.

Rigsby, G.G. “Its Olympic goal is a cool pool,” St. Petersburg Times. March 13, 1996.

Federation Internationale de Natation (FINA) Rule 2.11.

United States Patent No. 3,934,323, “Solar heat exchange panel and method of fabrication,” January 27, 1976.

United States Patent No. 6,943,141, “Process for making a liquid evaporation retardant solution,” September 13, 2005. The Heatsavr® patented mixture of alcohol and aluminum salt is based upon the science of Langmuir monolayers, materials that tend to form into one-molecule thick sheets when spread onto an aqueous surface.

Rigsby, G.G. “Its Olympic goal is a cool pool,” St. Petersburg Times. March 13, 1996.

Federation Internationale de Natation (FINA) Rule 2.11.

United States Patent No. 3,934,323, “Solar heat exchange panel and method of fabrication,” January 27, 1976.

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