PTR Discharge in Hot Weather

Your solar hot water system has built in safety features.
One such feature is the Pressure and Temperature Relief Valve (PTR)
On Rheem and Edwards passive (tank on roof) solar hot water systems this valve will open at 850Kpa or 99 deg C.

During times of very hot weather it is not uncommon for this valve to open and release excessively hot water.  It will close again once it has discharged enough water for the tank to cool down.

You can also turn off the water supply and wait a few hours before turning it back on. In rare situations the valve wedges itself open. Usually opening and closing the silver lever on the end of the valve will remedy this. If you are getting on the roof, all relevant safety precautions must be taken.