The Hidden Billion Pound Gorrilla in the Solar Water Closet;   S C A L E

All domestic waters contain dissolved solids. Hardness is the measure of concentrated dissolved solids in water.  When water becomes supersaturated with dissolved solids, precipitation may occur. Scaling is a particular type of precipitation where the precipitation occurs directly on a surface, such as on a heat transfer surface or pipe wall.   Most scale from domestic waters are 90+% pure calcium carbonate (CaCO3).

Scale buildup increases dramitically as the water temperature rises. Therefore water that is undersaturated at a low temperature may become supersaturated when heated to a higher temperature.  This is why, in areas with hard water, scaling tends to be worst in appliances that heat water, such as solar hot water systems, electric water heaters, coffee makers and other water heating appliances.

Solar Water Heating Hardness Map

It is ironic that most of the states that have the best solar radiation also have the worst water scaling issues.  As you can see from the map at top the majority of the U.S. has hard water.  Scale or calcium deposits will over time accumulate within the heat exchanger of a solar water system and greatly hinder its performance.  According to the Army Corp of Engineers 30% or more of a heat exchanger’s efficiency can easily be lost with just a slither of scale buildup.

A quality solar drainback system controls high temperatures and contrary to traditional water heaters that flash heat the water our system will over the course of a day gradually heat the water.  The lower temperatures of “all day” solar heating of the water causes much, much less sediment and scale buildup than a traditional water heater’s high temperautre flash heating.

The pipe at right was taken from a 15 year old glycol (antifreeze) system located outside of Austin, TX.  An inefficient heat exchanger, antifreeze/glycol design (does not have over heating protection) and hard water caused the buildup.  The amount of buildup shown in the picture at right can easily reduce the majority of a solar water system’s efficiency.

Solar Water Heater Scale Heat Exchanger

Water heater manufacturers as of late have vastly improved the heating elements within domestic hot water heaters.  In the picture at left “A” denotes the current stainless element, “B” is the older copper element and “C” is what “B” looks like after 10+ years within a hard water environment. 

Scale attaches to copper at a much higher rate than stainless steel and stainless is much easier to clean off scale than copper.

Many online solar water system resellers sell kits that will allow one to convert their exisiting water heater to a solar storage and backup heater in one.  As you can see from the pictures below many tanks have sediment buildup which can greatly hinder the installation, durability and future performance of the system.  Also most people do not have an 80 gallon tank which is truly what is required for homes with 2 or more residents.  To only activate the top element for backup in a small tank leaves the home owner with 15 to 18 gallons of consistant hot water… a little more than a shower.