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The pump that you need.....
when you find that single loop of 3/8" pex that is 4000' long buried in concrete
This pumps about 951,000 gpm and is one of several installed at a pumping station called IJmuiden in the Netherlands. Used to keep the western part of the country dry.
and actually it would not work well for the endless radiant loop application as its range of head is only up to about 16 feet.
Link where I saw this:http://pressurewashr.com/the-worlds-most-powerful-water-pump/