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Glycol Concentration

I've been asked to maintain a solar system, specifically to change out the glycol in the solar panels. The glycol is necessary not so much for freezing, but to increase the boiling point during the summer when there is no heating demand. From what I can tell, there is no "heat dump" like a swimming pool to shed this heat. What I'm trying to find out is what concentration to make the glycol; 50% or greater?

By the way, my supplier sold my a product called Intercool Biogreen to use in place of a glycol compound.

Any help would be greatly appreciated.

Often wrong, never in doubt.


  • hot_rod
    hot_rod Member Posts: 14,566
    You do need some freeze protection? If so use the lowest % possible. Most glycol manufacturers suggest 35% as the lowest blend as the inhibitor package starts to get too weak under 35%.

    Some of the bio, plant based glycols have a higher oerating temperature. maybe the one you show id corn of bean based? Try to keep any glycol below 300 intermittent temperature or it can break down in a year or two.

    As for over-heat protection, run a higher pressure in the system. At 30 psi boiling point of water is about 274°F.

    It's not unusual to see Euro systems running 60psi which gives you around a 300°F boiling point.

    A typical plate style collector will stagnate around 340F on a full sun, 85 degree day.. If the system has low or inconsistent loads and you expect to see stagnate, glycol will not last long, so water only if at all possible.

    If you raise the full pressure the relief valve needs to be adequate. Solar relief valves can be purchased up to 120 psi.

    Also the expansion tank may need to be upsized if you run higher pressure.

    Another couple options, if you use straight water all the new electronic solar controllers have a freeze protection options. It simply turns the pump on if the collector drops to 40°F. It is adjustable. This function is intended for systems that may see infrequent freeze potential.

    They controllers also have over-heat protection that starts the pump if the collector reaches a high limit of 230°F , also adjustable.

    Another handy feature is nighttime cooling.
    Let's say the tank got to 160 or 180F, during a low or no DHW use day. The pump runs again at night and can cool the tank back down to outdoor ambient temperature, usually limit it to 110- 120. The collector becomes the heat dump after the sun goes down. It works really well. That gives the system a lower starting point if no one uses DHW after a solar day.

    Ideally you would use all the energy you harvest and not have to spend energy dollars to dump :)

    Converting to a drainback would eliminate any over heating issues also. If the system is not completely sloped to be a safe drainback, some installers add a weak glycol to protect any low spots that may trap.

    An updated controller would be a great option to offer them, let me know if you need a demo.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    HR, I am GLAD I was successful in recruiting you for our ASSE series 19000 solar installation standard. My how times have changed as it pertains to solar controllers...

    Alan, when I first saw your post I thought it said glycol constipation... Age does strange things to our eyes and brains, doesn't it :smiley: Say hey to your lovely wife.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Wow! I've been digesting that all week. You should run a blog: "What Hot Rod Says". Excellent knowledge and I appreciate the wonderful insight.

    Mark: We keep setting a place for you at out table, but you haven't show up yet.
    Often wrong, never in doubt.
    Mark Eatherton
  • matt_sunwaysolar
    matt_sunwaysolar Member Posts: 61
    If the system is designed for space-heat, although it will be more expensive, switching to a drainback system will probably be best in the long-run, as Hot Rod suggested. You'll need a DB Tank and probably an additional pump (or a larger one) for the solar loop. But, you can stop worrying about overheating and most freezing too. No check valves in DB Systems.

    If you're sticking with a pressurized system and the system is designed for space-heat, you'll almost definitely want to upsize the expansion tank too (as HR also suggested). Most times, you'll want a tank 3 or more times larger than most standard kits come with. Also, best to locate the expansion tank on the "cold line" going to the collectors and in-between the collectors and the check valve on that line. This will protect the glycol a bit more, but ultimately, if it's stagnating consistently in the summer, the glycol's not going to last more than a few years anyway.

    What's the collector to storage ratio? What's the system designed to do? Type of collector?
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