Cast iron boilers are commonly used to provide heat for hydronic hot water systems and steam heating systems. Although normally manufactured in the same way, cast iron boilers in hot water applications and steam heating applications exhibit differences in their most common failure modes.

Cracked Sections

Cracked sections are the most common failure mode for cast iron boilers. The cracking is influenced by thermal shock and corrosion. Thermal shock is the extra stress that rapid temperature changes exert on the brittle cast iron. Thermal shock commonly occurs when large amounts of cold water are suddenly added to a hot boiler and vice versa.

Cracked sections result in boiler leaks and the eventual shutdown of the boiler. Although cracked sections are replaceable, the repairs involve downtime and many labor hours.

Our data suggest that cracked sections happen more frequently in hot water boilers than in steam boilers. The difference can be attributed to the different boiler piping arrangements.

Low-Water Condition

When the boiler water level drops below the minimum, all of the cast iron sections are exposed to “dry firing”. During dry firing, metal losses the cooling protection from water and can become damaged.

This creates a low-water condition and can cause extensive damage, including a boiler explosion. To prevent boilers from low-water conditions, all boilers should be equipped with low-water cut-off (LWCO) devices that shut off the burner when the water level is below the minimum. When LWCOs are not tested or maintained properly, low-water conditions and failures may still occur.

Our data shows low water conditions on steam boilers cause the most significant losses in terms of total dollar value.

• Steam boilers are more prone to low-water conditions because the water only occupies a small portion of the total volume in the heating system.
• Hot water boilers and hydronic heating systems operate with the boiler and hydronic piping completely filled with water.

If either of these failure modes occurs, unfortunately, it will be too late to save the boiler. Luckily, there are loss prevention measures that can be taken to prevent these types of failures from occurring. First, knowing the differences between hot water and steam heating applications can help to prioritize loss prevention steps and identify any energy efficiency improvements. We will be sharing five other loss prevention tips for cast iron boilers in an upcoming article this month.

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