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I recently had to replace my 80 gallon indirect water heater tank because of a weld leaking water. My system is a closed loop system with 50 evac tubes. I blew down the entire system of all the old glycol. That glycol was around 7 years old so I thought it would be a good time to replace it. Not sure if it needed replacement. I've checked it for correct PH and the freeze point was still low. Much lower than I would see here in NH. I had to buy 6 gallons of glycol at a cost of about $100.00. It's up and running right now. I was thinking, instead of using glycol, why is it that people don't use something like Prestone antifreeze used in automobiles?