Technically plasticiser will leach out of system where water is always changing ,hence more brittle (less spring back to keep things sealed)
Sealed circuit (ie-Boiler) , plasticiser cannot escape gets re-circulated
( PURE Theory no experiece with plastic plumbing--)
Loss of plasticiser and enbrittlement == fact
Trade offs a) How hot
b) How much water thru system
c) How many years of life
d) How many hours a day pump runs
e) Price of install
Life whole system needs to last -vs- water thru system for effect to happen
Copper rules !