http://www.uponorpro.com/calculator/
For 1/2" pex at 100*F
1 GPM through 200' = 8.022 feet of Head loss = 3.45 PSI
So the pump is pressurizing the supply manifold 3.45 PSI greater than the return manifold.
3.45 PSI will push .67 GPM through 400'
So when we double the length of a loop, 2/3 as much water flows.
http://www.pmmag.com/ext/resources/P...0813_Siggy.pdf
See Figure 3
Going from 1 GPM to .67 GPM reduces heat from 8,200 BTU/hr to 7,400 BTU/hr = 10% less