Dear Bob Z.,
What is the best way to prevent scalding in solar water heating systems?
As solar contractors we must be diligent when designing solar water heating systems. I have witnessed many systems over the years where storage tank temperatures have exceeded 165°F in the summer and the customer had not left on vacation. Although we want temperatures to be high enough to destroy bacteria (specifically Legionella bacteria), we do not want to cause scalding burns to small children, elderly or handicapped clients. Many experts recommend turning water heater thermostats down to 120°F to reduce the risk but unfortunately this puts your storage tank right at the high range for breeding Legionella.
Of course turning the thermostat down on a solar system only shuts the back-up element or burner off at 120°, it does not stop the solar from continuing to heat the water unless you have a differential control and set the high limit switch to cut out at a specified temperature. Therefore, here is the dilemma, let the water get hot enough in the tank to kill bacteria or set it down to protect the vulnerable.
I chose both! Installing a thermostatic mixing valve as a standard procedure gives you the best of both worlds. A tank that can rise above 140°F to kill bacteria and a mixed hot water output to the house that protects your clients. Oh... did I mention we carry them in stock at all UMA Solar warehouses. (Sorry for the shameless plug.)
Variables Attributable to Third Degree Burn*
Water Temp. F° Exposure Time
120 9.5 minutes
125 2.0 minutes
130 30 seconds
140 15 seconds
150 1.8 seconds
158 1.0 second
*From studies conducted by Lewis & Love
(1926); Wu. Yung-Chi, N.B.S. (1972);
Dr.M.A. Stoll, for U.S. Navy (1979)