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December 13, 2004: Frozen antifreeze! 

When I re-filled the collector loop last fall, I started with about 3 gallons of propylene glycol antifreeze (DowFrost HD) that I had available.  I mixed this with water in a 5 gallon bucket and pumped it into the collector loop.  Well, I forgot that my loop takes more than 5 gallons now (since adding 2 collectors last year).  So I kept adding water to the bucket until the loop was full.  I ended up with a mixture that was less than 40% antifreeze.  I figured that this would be OK.

Well, this morning my collector loop froze - no circulation - even in full sun!  The ambient temperature dropped below -13F last night.  The high for yesterday was no more than 9F.  I checked the specs for DowFrost and found that a 42% mixture only protects to -10F.  Oops!

I made a quick run to the local plumbing supply and got a 5 gallon pail of Hercules Cryo-Tek-100.  By using my fill pump to aggressively push the ice around in my plumbing I eventually got the collectors un-frozen.  The collectors themselves were stagnating at 130F while the shaded and insulated return pipe was well below freezing!  I drained a few gallons from the collectors and dumped it outside,  (remember this stuff is non-toxic) then I pumped in a couple of gallons of the Cryo-Tek.

It's interesting that the DowFrost that I used originally looked like green Gatorade, while the Cryo-Tek looks like fruit punch!  The mix now looks like prune juice!  But hey -- it works!  I'm good to -40F now. 

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