Solar collectors for swimming pool heating:

This article discusses the question gas or air accumulation in solar collectors, and a possible solar collector corrosion problem with swimming pool heaters.

Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.

Overnight Gas (air) Accumulation vs. Corrosion in Solar Collectors

The question-and-answer article about the overnight accumulation of a gas (probably air) in a solar collector used for swimming pool heating, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

What Causes an Overnight Accumulation of Gas in my Solar Collector used as a Swimming Pool Heater? Corrosion?

Question: There is an overnight accumulation of gas in the solar panels that heat my swimming pool. When the solar heating system pump turns on in the morning, a small amount of gas (perhaps a quart) comes out of the discharge line.

I suspected a leak in the solar collectors but found none when the water was pumped at about 15 psi.

I am concerned that this solar collector gas accumulation may indicate corrosion involving the copper solar collectors and the chemicals in the swimming pool water. Is it possible that this is dissolved gas released overnight, owing to reduced pressure in the solar panels caused by the weight of water trying to return to the swimming pool? - R.H., Lake Charles, LA.

Answer: As you suspect, the gas in your collector is quite probably nothing more than air that has come out of solution in the swimming pool water left in the collector overnight.

Air developing in the solar collector in this installation would happen for two reasons:

• The water in the solar panels will heat up slightly even after the controller has decided that there is no more useful energy to be collected and has shut off the solar collector pump. Air is less soluble in warmer water than cold water, and will form bubbles that might accumulate in the upper portion of the collector overnight.
• If the solar collector is 12 feet above the level of the water in the swimming pool, the water is under a vacuum of about 5 psi less than atmospheric pressure. This vacuum will also draw dissolved gases (air) out of solution in the swimming pool water.

What About Solar Collector Corrosion Due to Chemicals in Swimming Pool Water?

According to Doug Root, of the Florida Solar Energy Center, there have been very few cases of actual corrosion of copper collectors by swimming pool water.

In the few cases that have been documented, the pool chemicals dissolved in the swimming pool water have been grossly out of balance - such as an excessive level of chlorine.

Technical Tip for Removing Air From Swimming Pool Solar Collector Systems

Air accumulating in water in hydronic home heating systems (hot water boilers) is a well-known problem for which at least two components are in common use that may help automatically purge unwanted air or other gases from inside the solar collector plumbing system: [Added, DJF, 10/2009]

• An automatic float-operated air purge valve
  
   is mounted at a high point on the hot water piping (the cylindrical device at the center of our photo at left|) in order to automatically purge any excessive air in the system.
  
  See AIR BLEEDER VALVES for an article about how these automatic air purge valves work, and
  
  see AIR-BOUND BASEBOARDS / BOILERS for examples of air-bound hot water systems.
  
  If your solar heating system does not already contain an air purge valve, and if the system has trouble from becoming air-bound, adding the air purge might solve the problem at very little cost.
• A small air expansion tank
  
   absorbs the initial pressure increase in the system when water is heated (and air is drive out of solution) to avoid opening and spilling a temperature/pressure relief valve when it's not appropriate.
  
  See EXPANSION TANKS for details. But watch out: this approach won't purge unwanted gases from the system.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission fromSolar Age Magazine - editor Steven Bliss.

Photo a rooftop mounted site-built solar collector at page top courtesy of Bob Cheney.

Also see SOLAR COLLECTOR OUTGASSING for more discussion of possible causes of outgassing from a solar collector. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

• Pool-Gas: Q&A on Overnight Gas Accumulation in a Solar Collector - original text. Use your browser's back button to return here

...

Continue reading a SOLAR COLLECTOR OUTGASSING or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SOLAR COLLECTOR AIR or GAS COLLECTION where we describe gas collection problems and solutions for solar heated swimming pools. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
• SOLAR ENERGY SYSTEMS - home
• SPA / HOT TUB / POOL MANUALS, PARTS, REPAIR, SUPPLIES
• SWIMMING POOL SOLAR HEAT, INDOOR
• SWIMMING POOL SOLAR HEAT, OUTDOOR DIAGNOSIS
• SWIMMING POOL LEAK DETECTION

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