a couple of quotes directly from Quietside 2009 DPW Installation Technical.
"Quietside DPWH can only make 176 DegF MAXIMUM inside the unit Typically temperature delivered into the loop is approx 165 DegF
We recommend designing system for a loop ∆T of 20 to 25 DegF to allow the system to inject enough 160/165 DegF water into the loop to maintain temperature and comfort"
"Return temps above 140 DegF back to the DPW significantly reduce efficiency and condensate production"
"DPW Water Piping
WE MUST PIPE THE UNIT AS FOLLOWS, NO IFS, ANDS OR BUTS
With a PRIMARY/SECONDARY LOOP with the unit injecting hot water into the secondary loop to be circulated by the loop pump(s)"
(due to the fact that the pre installed pump may not be able to overcome the loops)
Their design recommends that the primary loop will put out 160-165 degree water and you should design your loops with a delta T of 20-25. So the return water on the secondary loop may be 140 it will then mix with the primary loop of 160. So the water returning to the boiler is maybe 150? but they tell us that "Return temps above 140 DegF back to the DPW significantly reduce efficiency and condensate production"
It seems as though P/S piping can or will cause reduced efficiency.