Depending on usage, and how hot the water is from the previous day, on a hot, sunny day, the tank temperature can rise to 180 F. or more, which is normal.
However, during very hot parts of the summer, especially under conditions of low use (e.g. you're on vacation) the temperature of the water in the collector storage tank can exceed the boiling point, which is not safe.
To prevent this, the solar collector comes with a bronze relief valve. Your conventional hot water heater inside your house also comes with such a valve. It is a standard safety feature.
The valve has 3/4" threads and a pipe (not included, you must obtain separately) should be attached to it, then run down along a stand leg and secured to it.
The pipe must be open on the bottom, facing the ground, so that if the relief valve opens to vent pressure or temperature, the dangerously hot water will be directed to the ground and not spray out in random directions, possibly injuring anyone nearby.
You should periodically test the valve by lifting the small silver lever on top to make sure water comes out under pressure normally and that the valve (and pipe directing the water to the ground, which you should add) is not blocked. You can do this as part of your regular maintenance such as hosing off the collector tubes to remove dust.
Note: To avoid overheating during hot summer months, for example, if you go on vacation or you get an unexpectedly hot heat wave, you can temporarily remove one or more heat pipes to lower your efficiency and therefore the temperatures in the storage tank. Then put them back later when you need more hot water.