Boiler Pre-Treatment Strategies & Options

Last updated: May 22nd, 2019

What is Pre-Treatment?

Pretreatment of the makeup water is critical to a boiler system.  As Calcium and Magnesium are inversely soluble with temperature (meaning the hotter the temperature, the more likely Ca and Mg will come out of solution and form scale), it is imperative to make sure there is no hardness in the makeup water to a boiler.  Silica is another ion that can form scale problems in the boiler. Alkalinity can cause corrosion in the condensate system. Here are a few common ways to pretreat the makeup water.

Pre-Treatment options:

Soft Water:

Soft water is the most common way to pretreat makeup water to a boiler.  It is important to make sure the softener is sized for the correct flow rate and hardness level of the makeup water.  Maintenance includes maintaining salt in the brine tank and checking hardness levels throughout the cycle of the softener to make sure the water is soft at all times.

Here is a great article that explains how a softener works and how to troubleshoot hardness issues

Reverse Osmosis:

A Reverse Osmosis or RO unit is typically seen on a larger boiler system and it has many advantages. Not only will it remove hardness (Calcium and Magnesium) from the makeup water, it will also remove about 90% or more of all other dissolved solids (silica, carbonate alkalinity, etc), making it an excellent choice for systems looking to save water by increasing Cycles of Concentration of a boiler.  (More on Cycles of Concentration here). Be aware that when using this type of pretreatment, you also remove most of the alkalinity in the makeup water, making it corrosive. Caustic, or sodium hydroxide is typically fed to the boiler to replenish the OH-Alkalinity and place the boiler back in a non-corrosive condition. (Click here to find out more information on typical control ranges of a boiler).  The advantage of removing the alkalinity from the makeup water is that it also removes the potential for CO2 (Carbon Dioxide) from entering the steam system. When CO2 enters the steam system, it will resoluablize with the condensate and form carbonic acid, lowering the pH and cause corrosion issues. An RO is a great option for food grade applications where a steam treatment cannot be used to control pH in the condensate. 


Deionized water:

Deionized water is also typically seen in a larger boiler application with little makeup water demand.  Where an RO unit can remove approximately 90% of dissolved solids in the makeup water, a DI unit can remove up to 99%. Their benefits are similar to that of an RO unit, but corrosion potential is even more extreme. DI systems are expensive to operate and requires strong acids and bases to regenerate.


Once pre-treated, the water will then go into a DA (Deaerator) or feed tank.  More on what DAs/feed tanks do here.