Requirements for the feed water of steam boilers and hot-water boilers

 Parameter Feed water > 0.5 to 20*
Feed water > 20*
Makeup water
pH value at 25 °C
> 9.2
 > 9.2
> 7.0
Conductivity at 25 °C [µS/cm]
< 6000  < 3000
< 1500
Total hardness (Ca + Mg) [mmol/L]
 < 0.01
 < 0.01
 < 0.05
Iron [mg/L]
 < 0.3
 < 0.1
 < 0.2
Copper [mg/L]
 < 0.05
 < 0.03
 < 0.1
Silica [mg/L]
 pressure-dependent
Oxygen [mg/L]
< 0.05
 < 0.02
 –
Oil/grease [mg/L]
< 1
< 1
 < 1
Organic substances**
 –

* operating pressure [bar]

** Organic substances can be degraded to products that increase acid conductivity, corrosion, and deposits. Furthermore, they can lead to foaming and/or the formation of deposits. Their concentration in boiler feed water should thus be kept as low as possible.