Sunday, March 31, 2013

A 2 Year Update on Gordon's Unique Khanh Solar Water Heating Collector

A couple of years ago Gordon did a solar water heating collector for his home that uses the Khanh design.

This design improves the performance of conventional water heating collector by extending the area of the collector to include an air heating collector that warms the area around the water heating collector to reduce heat losses from the water heating collector.

The design is explained in detail in Shurcliff's book: New Inventions in Low-Cost Solar Heating

Gordon's original article on his implementation of the design along with construction details is here..

Its been two years and Gordon has a good report on the things that worked and the things that did not work so well and had to be fixed or improved.

The report covers: performance, a new differential controller, pumps, backup water heaters, and some plumbing issues.... 

All good stuff to know if you plan to use this design.

March 31, 2013


  1. I noted in the 2 year update that an attempt was made to solve mold problems by adding bleach to the water. I can say for certain that this will spell disaster for the copper tube (along with any stainless steel, if present). Copper has such great longevity in aequeous environs because a protective oxide layer is created on its surface that prevents further 'corrosion'. This activity is encouraged by mildly acidic solutions and destroyed by alkaline ones. Bleach, an alkaline, will dissolve the layer, and then begin eating the copper tube itself. This is called 'pitting' and will cause pinhole leaks in the copper tube over time. The bleach solution should be removed. Tubing rinsed. Mildly acidic solution installed to recreate the protective layer. Then drained again. Install a pH neutral solution for long life. There are professional biocides that are compatible with copper. I use them in homebrewing. I believe iodophor would be compatible for long term. Its iodine based. But definitely not bleach. Never bleach.

  2. This is the reply to the comment above from Gordy

    LDave, thanks for the blog comment. It looks like you have faced these issues before. Quite frankly, I'm a novice at such reactions, and seem to have to learn the hard way all too often. Please note that I did not recommend the use of bleach, I indicated that it had been tried, and hinted that it the results would be monitored. It's very true that disinfectants and biocides can have some nasty implications on a variety of materials and that some of these implications can endure long after any biocidal utility. The collector/tank circuit of the BIS $1k SDHW system involves several differing materials, and an effort was made to try to identify the effect of differing biocides upon the differing materials – unfortunately it wasn't a clear cut situation. Chlorine was chosen as it tends to degrade rather predictably into salt (NaCl) – with generally predictable results. In the end, 'liquid domestic bleach' was chosen as most pool products contain persistent acidic stabilizers (corrosive to both copper and tin in solder). Additionally, most public health departments recommend its use for rural water systems.

    The tank water had initially been filled with filtered rain water – so the TDS should have been quite low and PH perhaps mildly acidic. Bleach was introduced once only, at a quantity to achieve slightly less than 3 parts per million (typical for potable water). This should degrade to slightly less than 2.5 ppm of salt. I may be wrong but I don't believe there will be much impact upon either the PH or corrosiveness of the tank water. What I do wonder is whether the process achieved much at all? One can expect at least one or two air changes per day occurring in the tank, thus free active chlorine would likely not be retained very long.

    However I do respect your view. Readers might have been thinking bleach had been introduced at concentrations typical of disinfective procedures as used industrially. These are typically 100 to 400 ppm, and I have no doubt that severe corrosion and pitting of copper, SS and tin could well occur. For example, our well water has a TDS of about 1350 ppm (including ~225 ppm NaCl) and it corroded through the electric water heater in just 4 years! Further thoughts?



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