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Why not use Prestone

rt_2 Member Posts: 86
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?


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Automotive glycol has a higer toxicity than propylene glycol. You can use it, but the heat exchanger has to be double walled, and you'd be wise to mark the tank that toxic glycol is in the system. There are also some compatibility issues with the metals in a solar system, versus an automobile.

    In some jurisdictions, the use of automobile antifreeze is against the code with out having some means of guaranteeing that the possibility of backflow into the potable water system is addressed. This may require the installation of a very expensive backflow preventer that has to be tested on an annual basis. In any case, you'd be way ahead to refer to the solar system manufacturers literature and comply with their requirements. I'd hate to see you clog the micro channels for fluid and lose the whole array....


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  • rt_2
    rt_2 Member Posts: 86
    Makes sense to me Mark. I figured that there had to be a reason but I'm no chemist. There are no micro channels in my system but I would be concerned about cross contamination. Thanks.
  • hot_rod
    hot_rod Member Posts: 22,256
    Actually many of the automotive antifreezes are now PG based for safety and liability. But they are inhibited for different conditions. Automotive glycols blend with silicates, for some reason. That can plug hydronic or solar components.

    With evac tube collectors you really want the best, highest temperature fluid you can buy. They stagnant well over 300°F. I've measured over 400F on a full sun, hot day!

    Glycol only breaks down if it has been abused. That being frequent over-heating, blended with bad quality water, or put into a dirty, uncleaned system. A good brand glycol, properly treated can last 10- 15 years without concern.

    Really ph is just one thing to check. Dark color and harsh smell tell as much or more than just ph, to indicate the fluid needs to be changed..
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream