My issue is during the hottest part of the day I get hot water from the cold side of all faucets. The water is very hot and does not cool off by keeping the water running.

Background:
I have an over sized solar water heating system, three large collectin panels and 2 80 gallon tanks. Over the last few years every summer I also have issues with the pressure relief valve going off on the system. The water temputure can get up to 190 measured at the water outlet from the storage tank with hot water running. Two years ago the plumber said the tube inside the storage tank that feeds water from the bottom of the tank had broken, they replaced it. All was well but it was late summer and no problems over the winter, I live in Hawaii so it never really gets cold!

Last summer I heard a loud rattle coming from the tank when I turned on the hot water, after that the pressure valve starting popping again, plumber replaced the pressure valve and told me"to use more hot water" what the?..they did not recheck the inside pipe as previously replaced.

Shortly after that I started having issue with very hot water coming out of all faucets during the hottest part of the day.

If I close the cold water supply to the water heating system the hot water stops and I get cold water.

It does not do this at night or early mornings.

Any suggestions? Thanks for your time.

A check valve on the cold water inlet, and a thermal expansion tank would likely take care of the problem.

Thanks for the reply, my concern is that while I've let the problem go for the last few years on and off I've owned the house for 8 years and this has only been an issue for the last few years. So, I feel something has changed/broken. Looking to educate myself with possible causes/fixes so I can either fix myself or at least be knowledgeable when speaking with a plumber.