Solar water heating systems can make water in your tank scalding hot. Depending on the type of system you have, there may not be any temperature regulation inherent in the system. Unregulated solar heating can cause a dangerous situation where the water at your faucets is unacceptably high and dangerous, particularly to children and the elderly.
I came across an excellent description of how a Thermostatic Mechanical Mixing Valve works on the Watts website. Watts is a leader in the manufacture of brass valves.
When a differential temperature controller is used in a solar water heating system, the circulation pump should shut off when the tank reaches the desired water temperature. If the set point is not set too high, the homeowner should be protected from overheated water. However, in a system where a photovoltaic panel operates a pump directly, tank temperatures may become excessively high, and a mixing valve is advisable. A high-limit switch can be used to reduce the likelihood of this happening, but a mixing valve is a more reliable way to provide a constant maximum temperature.
Unfortunately, mixing valves do fail. This can be a frustrating problem resulting in no water available at faucets suddenly. Worse yet – valves can fail in the open position, causing a dangerous scalding condition. The only solution is to replace a failed valve. In Fafco Solar’s experience in Southwest Florida, water quality and high temperatures are the leading causes of failure. When the tank temperature gets very high, scaling and buildup can occur, and mixing valves fail. Some valves are better than others, providing more reliability. The Watts MMV-M1 and M2 are good valves. We have also received reports from FAFCO’s engineers that the Honeywell AM101 is a reliable valve.
While these valves are not cheap, they are a helpful and often necessary part of a solar water heating system. In fact, recent plumbing code changes require their installation with or without solar water heating systems in some jurisdictions.