I have a 30 year old passive-roof mounted solar hot water system. There is a circulating pump and storage tank in the basement. The pump is controlled by the temperature differential between roof water and tank water. The system used to have a silicone heat transfer fluid. When I found out what it was going to cost to replenish it, I decided to switch to water based antifreeze. That was about 10 years ago. The system has worked fine since then. This summer, I have been getting accumulation of antifreeze in a bucket set under the pressure relief valve pipe. It has been perhaps a half gallon each time, but over the course of the summer, the 5 gal bucket has filled with fluid. I am wondering if a pressure relief valve can get old and tired and release too soon, or if there is something else that could cause this release. Thanks- Dave