In a well-designed system that doesn’t suffer much overheating, the propylene glycol HTF may last for 15 years or more. However, if a system is poorly designed, has component failures, or is underused, it is likely to overheat more often. Over time, propylene glycol that is overheated breaks down and becomes acidic. Old, “burned” glycol has a dark, almost coffee-like appearance and a foul odor, and will need to be changed more often.
Glycol breakdown is primarily due to inhibitors added to reduce the corrosion. Because different manufacturers use different inhibitors, the initial pH will vary. Check the manufacturer’s product data sheet for this information. When diluted to a 50:50 mix, the initial pH will typically be between 8.5 and 9.0.
Test the pH of propylene glycol each year to determine if the HTF needs to be changed. Testing kits are available from suppliers of propylene glycol, solar equipment distributors, and other sources such as chemical suppliers. As the pH falls to 8 or less, the likelihood of corrosion increases. A pH below 7 indicates that the HTF has degraded and corrosion is likely already occurring—the old HTF should be drained, the loop thoroughly flushed with water or a detergent, and fresh propylene glycol solution added.
Disposal methods for propylene glycol vary by region. Flushing used antifreeze down the drain is the least-preferred disposal option, but it may be the only option available in some areas. Your wastewater treatment facility should be contacted to ensure that they can accept the waste. Some recycling facilities accept used glycol solutions, both propylene and ethylene glycol used in cars.