The problem
Early underfloor heating systems used plastic pipes that were permeable to oxygen. Technology has since advanced to the point where it is now possible to produce underfloor heating pipes that are practically diffusion proof.
Valves, threaded joints, circulating pumps, regulators, automatic bleed devices and faulty expansion tanks, however, are still potentially important sources of oxygen uptake.
Oxygen diffusing into the heating water, too low a pH value and raised electrical conductivity of the system water can all lead to corrosion and blockage of the heating system from corrosion products.