The wall-panel radiators that can be part of a modern hydronic heating system are typically manufactured from thin sheets of steel that are pressed and welded to form the radiator.
Water and oxygen cause steel to rust (corrode).
Corrosion increases as the temperature increases.
Corrosion extracts the oxygen from the water that is contained in the sealed circuit of pipes that carry the hot water to and from the radiators.
This corrosion stops only after all the oxygen has been used.
If the pipes are made from a type of plastic that allows oxygen to pass from the air through the plastic and into the water then corrosion of the steel radiators does not stop.
Plastic pipes can be manufactured with several layers.
One layer can be made from a material that is not permeable to oxygen.
"Barrier" pipes include a layer that is made from Ethyl Vinyl Alcohol EVOH or a layer made from aluminium (PEX-Al-PEX) to form an "oxygen barrier".
Cross linked PolyEthylene (PEX) and PolyButylene (PB) pipes are manufactured both with and without an oxygen barrier.
The names of manufacturers include Tiemme-Cobra-PEX, Rehau, Hepworth, Iplex
If the layer of EVOH was sprayed onto the outside of a pipe the layer is easily damaged during transport and construction.
The diameters of many pipes used in Australia are different from the diameters that are used in other countries.
PB barrier pipe is not available in Australia.
Barrier pipes are designed to admit less than 0.1milligram of oxygen per Litre per day.
Oxygen permeability values for pipes without a barrier are
for PEX 3.2mg/L per day
for PB 6.5mg/L per day
Chemicals can be added to the radiator water circuit to reduce or to prevent corrosion.
The names of three corrosion-inhibitor products are:
Fernox F1 (a clear liquid costing $85 for 500mL),
Sentinel X100 and
Rad-Protex (a black liquid costing $75 for 500mL).
To inhibit corrosion, sufficient sodium molybdate can be added to achieve a concentration of 2 to 4 parts per million in the water.
Farmers use this chemical as a fertilizer.
The pH should be controlled to be between 7.0 and 8.5.
Add vinegar to decrease pH.
Add Dipotassium phosphate or sodium bicarbonate (baking powder) to increase pH.
