Filling and refilling hot water heating systems introduces oxygen into the system. Air bubbles or air pockets can remain in the highly branched pipe network even during venting. This can cause in corrosion, which can result in leaks, a reduction in efficiency and even system failure. VDI Technical Rule 2035 Sheet 2 prescribes that hot water heating systems must be protected against this corrosion. Dosing systems are installed in a mixing-intensive position in front of the boiler in the heating water return. The injection nozzles must be able to withstand temperatures of up to 110 °C.
During filling or refilling of the heating system (for systems over 100 kW), the dosing stations feed trisodium phosphate and sodium sulphide into the circulation. The addition of trisodium phosphate leads to an increase of the pH value and a reduction of the water hardness. The pH value should be between 8.2 and 9.5. This value should lie between 7.5 and 8.2 if aluminium or aluminium components are used.
Sodium sulphide is dosed to perform chemical oxygen scavenging. A sulphide residue of 5 to 20 mg/l must be maintained after first filling. The buffer generated in this way is sufficient to bind the oxygen released by the ageing water. The lower the temperature of the heating water, the greater the amount of sodium sulphide which is required (no oxygen is present in water at a water temperature of 100 °C). We recommended checking the pH value, phosphate content, the sodium sulphide, total water hardness and the conductivity value immediately after filling. Further checks should be performed after approximately one week and approximately six weeks, and then at least once a year.
If the desired values are not reached, dosing can be increased manually via the dosing systems. Softened or de-mineralised water must be used for filling given a high degree of water hardness or frequent changing of the heating water. Compliance with these water values is essential to prevent damage caused by corrosion and limescale and to comply with the boiler manufacturer’s warranty conditions. Lutz-Jesco dosing systems enable pH value increases and oxygen scavenging to be performed without difficulty.