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TDS limit in glycol systems

150 ppm TDS is a ball park limit for TDS concentration in hydronic systems with condensing boilers. When you add glycol, the TDS measurement goes through the roof. How do you tell if your TDS is high if you have glycol in the system?


  • Jamie Hall
    Jamie Hall Member Posts: 22,916
    Interesting. I'll have to think about it. The TDS which you are interested in is the ionic contribution -- salts, hardness, that sort of stuff. How are you measuring TDS? Conductivity?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,542
    I noticed this on a system recently. It started with water at 120, went to almost 500 with 50% propylene glycol.
    @hotrod posted some info that made me very confused so I forgot about it for a bit :/
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 21,853
    Depends what the glycol was blended with. From the manufacturer it is blended with DM water, which is very low TDS.

    Then any inhibitors added to pure glycol will raise TDS.

    Dow show conductivity in metric units, for pure PG.

    Also a link to a Lenntech conversion table to get to ppm which is a more common unit of measurement for conductivity.

    Ph would be another number to watch with glycol, it will indicate the inhibitor breakdown.


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream