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A Detailed Review of Vacuum-tube Solar Pool Heating Systems

As a CET - Certified Engineering Technologist (mechanical) (a professional & accountable designation) with 35+ years in the solar business designing solar systems and solar collectors, I have gained a great deal of knowledge about how and how well solar collectors work. As the past chairman of the CSA standard committee on solar collectors (CSA-F378) I have worked with others in the industry to establish the processes for accurately measuring the performance of solar collectors and assuring that they are safe and durable. It is with this background that I offer the following comments on the use of “evacuated-tube” or “vacuum-tube” solar collectors for heating outdoor swimming pools.

Evacuated tube collectors on a surface area to surface area basis are less-capable of heating outdoor pools than unglazed collectors. Evacuated tube collectors are a bad choice for heating outdoor pools in almost every circumstance, for the following reasons.

1) For a given surface area, evacuated tube collectors will deliver less heat to the pool when you actually want to heat the pool, than unglazed (plastic) solar collectors. This fact, of course, flies in the face of what the suppliers of the evacuated tube collector retailers claim.* The reality is this: outdoor pools lose heat at a rapid rate when the air temperature gets much cooler than the pool. This loss of heat is so large that only a large gas-fired pool heater will be able to keep the pool from cooling off - or reheat it quickly to a desirable temperature. Evacuated tube collector retailers try to claim that their product is much better at heating the pool, than unglazed solar collectors, when the air gets cold. The fact is neither type of solar system (if conventionally sized) will have the ability to heat the pool when the air gets cold (below 10 degrees C, 50 degrees F) because the pool is losing so much heat to the air. (BTW - a solar blanket makes a big difference to the heat loss, but you will still need a properly sized pool heater.) Another reality is: 99% of pool owners don't swim if the air is cold or if the weather is miserable - so it really doesn't matter if a solar collector can heat a pool when it's cold outside. But - the solar system must have the capacity to re-heat the pool quickly when the weather gets nice!

* A typical example of the miss-information, specific-case presented as typical, deliberate misleading, incorrect data (otherwise known as lying or alternative-facts) is a 10 page PDF from an Alberta-based solar component supply company which "compares" their evacuated tube product with "black mat" style of pool collector. Granted, rubber "mat" collectors are among the lowest efficiency pool collectors available, but even they are FAR more efficient than evacuated tube collectors - when you actually want to be using the pool. The numbers presented for the comparison are totally without merit. Their claim is under sunny conditions the evac-tubes deliver 5500 BTUs per hour and a 40 square foot mat collector only 2500 BTUs per hour. Test results from accredited solar test agencies show that 40 sq. ft. mat collector is capable of delivering up to 8742 BTUs per hour - and the better (plastic) solar pool collectors of the same size can deliver up to 10500+ BTUs per hour. It should also be noted that the rubber and plastic collectors cost one-fifth as much per square foot and don't have the other disadvantages of evacuated tube collectors listed below...

2) Because pools require a lot of heat, solar pool heating systems must be large. The smallest size a solar system should be is a minimum of 50% of the pool's surface area if using a high efficiency unglazed solar collector. Evacuated tube collectors are built with gaps between the tubes and these gaps miss catching the energy from the sun. Some suppliers will babble about how evacuated tube collectors "track the sun" because they are round. This is techno-junk. When compared to unglazed solar collectors this is complete garbage - unglazed solar collectors do not have a glazing that reflects sunshine so they don't need to track the sun. Unfortunately the retailers of evacuated tube collectors know very little (if anything) of the science of solar energy, and simply make-up or parrot information they don't understand. The result of the gaps between the tubes is evacuated tube collector systems need to be physically larger to have the same peak heating capacity as unglazed solar collector systems.

3) Because evacuated tube collector systems actually need to be larger, they may or may not fit on a roof that will fit an unglazed solar collector system.

4) Evacuated tube collectors are MUCH, MUCH heavier than unglazed solar collectors. Because of this, municipalities require a building permit for them to be installed. A building permit will require that the roof be inspected by an engineer before and after the installation, who must verify that the roof can support the weight of the Evacuated tube collectors system (as well as Winter snow loading) and that the system has been installed properly. All of this costs money and takes time. Fly-by-night evacuated tube collector retailers will attempt to skip these requirements.

5) Because of how the majority of evacuated tube collectors are built, they can reach very high temperatures when not running, and therefore they must use copper piping between the evacuated tube collectors and a pool water heat exchanger. Plastic piping will not stand up to the high temperatures an evacuated tube collector can generate if the pool water is not cooling it off (the temperatures will easily melt plastic pool piping). You cannot put pool water directly through most evacuated tube collectors because pool water corrodes the metals in them or etches the glass. Pool water can also cause thermal shock in evacuated tube collectors causing them to fracture, implode or explode.

6) Because of their higher costs, most evacuated tube collector retailers make their systems smaller than unglazed solar collectors systems to keep the cost down. If they properly sized them they would actually be (much) more expensive.

7) Evacuated tube collectors are made of untempered glass tubes. They can be broken by baseballs, hockey sticks, large hail, falling branches and may other things. They should never be located where the broken glass - which will be made up of large sharp shards and splinters - could fall on the pool deck or get into the pool.

8) Evacuated tube collectors are dependent upon maintaining a vacuum (many components contain two vacuums) to maintain their performance. History has shown that the vacuums are lost in many - if not most - tubes within 10 years.

9) Evacuated tube collectors systems will be expensive to remove and re-install when the roof they sit on needs to be reshingled. Due to the tube's fragility, it is very possible that some will be broken. Finding replacement tubes will likely be very difficult due to the non-standardized nature of the product, and the fact they have only been available for the past 8 to 9 years. Several unglazed solar collector products have been available for over 40 years.

Some evacuated tube collector retailers try to suggest that their systems can be used to heat the pool and the domestic hot water. It may be technically possible - if the installer really knows what he is doing - however… First, this will automatically require that a building permit be obtained to install the system. Second, it will require that the solar system be made larger (more collectors area) because the heating load is now larger - so the solar system must have an area at least 50% of the pool's surface area -PLUS the area required for heating the hot water (typically another 6 square metres or 60 square feet of collector). This approach will make the system much too large for the solar hot water system in the Winter when there is no pool heating load. This will necessitate adding a "heat dump" or a space heating component to the design and tens of thousands of dollars in cost - for a small contribution to the space heating load. This takes a project that should typically cost $4,000 to $6,000 upwards of $30,000 (if properly designed & installed). There are much better and cheaper ways of achieving the same goals. (For example two separate solar systems - and unglazed system for the pool and a glazed system for the hot water.)