Do you know how your house stays warm on cold winter nights?

Most American households are heated by either a furnace or a boiler. Do you know the difference? Furnaces heat air, and boilers heat water. That heat from the boiler is distributed to the rest of the house via steam or hot water in pipes and radiators.

In order for boilers to run efficiently and effectively, there’s some pretty important chemistry going on. For example, what prevents the steel boiler from corroding? After all, water famously corrodes steel – especially hot water, since the temperature speeds up the reaction.

In this article, we’ll find out how chemistry is keeping you warm this winter. We’ll answer that question, and we’ll also find out what prevents solids from precipitating out and creating deposits known as “scale” and “sludge.” Most importantly, we’ll learn the critical role that sodium hydroxide and other chemical additives can play in keeping your boiler running smoothly and effectively.

The problem with untreated boiler water

First, let’s take a look at the input. The water that goes into the boiler system, known as the feed water, likely has some contaminants. These could be ionic impurities, insoluble impurities, gases, or even microorganisms. All of these can potentially create challenges to boiler maintenance.

As impurities build up, they approach the limit of solubility and begin to film precipitates. “Hard” water, containing calcium or magnesium salts, can cause the precipitation of scale, or deposits, on the sides of the boiler.

Boiler water may be assessed via conductivity tests, which measure the ability of the water to conduct electricity. High conductivity indicates a high concentration of impurities in the water.

Pre-treating the feed water is certainly an important step in boiler maintenance. Another important step is to treat the water already inside the boiler. This can help preserve the operative efficiency of your heating system.

But perhaps the most critical parameter of all is pH. The alkalinity of the water has a big influence on which reactions are taking place. Let’s take a closer look.

The role of pH

Boiler water should be maintained at an alkaline pH. There are a few reasons for this.

Reason #1: Water will definitely cause steel to corrode – especially hot water, since the increased temperature speeds up the chemical reaction. The best way to mitigate this process is by maintaining an alkaline pH. The recommended pH is usually slightly above pH10. This can be achieved by adding a base like sodium hydroxide, or potassium hydroxide.

Reason #2: Maintaining an alkaline pH also helps to make sure that the chemicals are precipitating out in a harmless, easy-to-control form. Let’s take a look at what happens to hard water at alkaline versus acidic pH.

Hard water contains metals like calcium and magnesium. Phosphates are typically added to help remove the calcium. If the pH is alkaline, hydroxyapatite forms. It falls to the bottom of the boiler where it does not pose a problem. It can easily be removed later during a maintenance procedure called “blow out.” If the pH is acidic however, a harmful calcium phosphate deposit will form.

Reason #3: An alkaline pH helps to ensure that excess silica remains in solution. Silica is often naturally present in boiler water, and is even considered to be useful for proper maintenance. Silica combines with magnesium to form a harmless magnesium silicate sludge which can be removed during blow out. In fact, in cases where there is not enough silica naturally present, it is added to achieve this result. However, in the case where there is too much silica in the water, an alkaline pH keeps it solubilized.

Adding sodium hydroxide is the most typical method for controlling the pH of the boiler water.

Boiler chemistry in a nutshell

Boilers are a common type of heating system in many American households. The chemistry of boiler maintenance is straight-forward, but absolutely crucial to keeping boilers running smoothly and efficiently. One of the most important maintenance chemicals for boiler water is sodium hydroxide. The role of sodium hydroxide is to maintain the alkalinity of boiler water – which is important for three reasons. First of all, the alkalinity slows down the process by which hot water corrodes the steel boiler. It also ensures that calcium phosphate is forming a non-adherent sludge that can easily be removed during a blow out maintenance procedure. Finally, it keeps naturally-occurring silica particles harmlessly solubilized.


Hagan, Richard B. Carey. “Boiler water treatment and condensate return line protection.” (1959).

Kenny, Steve, Dave Pope, and Western Dry Kiln Association. “Basic water treatment of steam boilers.” (2000).

Panigrahi, Bhabani Shankar, and Krishnamurthy Ganapathysubramanian. “Boiler Water Treatment.” Mineral Scales and Deposits. Elsevier, 2015. 639-655.

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