The service life of pipes, boiler systems, pumps and fittings in thermal hot-water and steam-boiler systems is determined to a great extent by the quality of the boiler water. Water contains different amounts of oxygen (O₂) and carbon dioxide (CO₂), depending on the pressure and temperature. Both gases cause particular types of damage to metallic boiler materials: Oxygen has an oxidising effect, and carbon dioxide (carbonic acid) is aggressive.

Special degassing methods can therefore be used to remove the two gases, oxygen and carbon dioxide, from the water or to reduce them to below the respective required residual concentrations (guaranteed value to be complied with).
The guaranteed value is defined as the remaining quantity of oxygen or carbon dioxide that continually reaches or undershoots degassing in the middle of a longer operational stage.

A differentiation is made essentially between the following processes for the degassing of gaseous contents in the field of water treatment:

Thermal degassing occurs at temperatures just above 100°C and operating pressures around 0.2 to 0.3 bar. Thus an oxygen content under 0.02 mg O₂/l and a carbon dioxide content of less than 1 mg CO₂/l are possible in the feedwater.

With vacuum degassing, water can also be degassed below 100°C by boiling. In this case, degassing takes place in a percolator under vacuum conditions. The gases are removed with the resulting water vapour. Vacuum degassing is used for treating supplementary water for heating systems.

Chemical degassing or subsequent degassing occurs through the addition of chemicals. These chemicals are dissolved in dilution water (minimum soft water quality, depending on water treatment) and are suitable for removing or converting the oxygen contained in the water. The reaction time differs when different chemicals are used. In general, the greater the water temperature, the shorter the reaction time.