You are likely experiencing problems due to the ethylene glycol breaking down. Over time it breaks down to glycolic acid and lowers the pH. I highly recommend that when you refill the system use dowtherm sr1 if you are looking for EG and dowfrost HD if you are considering PG. These are designed for heating and cooling loops. The corrosion inhibitor package in them is far superior to automotive antifreeze. Dow will test the antifreeze for free, they determine the freeze point as well as whats called the reserve alkalinity. This is the buffer present to help combat the glycolic acid formation. The pH in the system should be between 9 and 10.5. I work in the industrial water treatment industry and install 10,000's of gallons a year, and regularly see this type of decomposition. The deposits you found were likely zinc carbonate, the result of the zinc anode ionizing and combining with the free alkalinity in the water, thus lowering the reserve alkalinity. Another thing to consider is only lower the freeze point to what is necessary for your area as the addition of glycol significantly reduces the heating or cooling capacity. A 30% mix brings the freeze point to about 0f. It also reduces the efficiency by 30%. The heat transfer ability is reduced and pumping flow is lost due to the higher specific gravity. You may also want to install a cartridge filter in your piping to help remove the suspended solids from the system, usually a 5 micron filter is used for this. Make sure the housing and filters are rated for the temperature of the water. I hope this is helpful.
