Why do Cast Iron Boilers Leak/Crack?
Cast iron boilers are known for their durability, reliability, and minimal resistance to flow. However, over the years, many cast iron boilers have experienced failures not due to product defects but rather installation errors. It is important to recognize that while manufacturers can occasionally be at fault, all failures result from issues related to either manufacturing, sizing or installation issues. Blaming the manufacturer is often easier than addressing installation problems.
With over 50 years of experience as a service technician, service manager, and technical trainer, I have seen many boiler failures. This document discusses the causes of cast iron hot water and steam boiler failures based on my experience.
Boiler manufacturers assemble cast iron sections using metal push nipples or gasket-type materials at nipple ports, where water moves between sections. Each section may have multiple nipple ports that come in various sizes, particularly in steam boilers. Manufacturing issues can result in similar failures across multiple units. However, advancements in computer technology, casting processes, and machining equipment have led to improvements in cast-iron boiler quality.
Cast Iron Hot water boilers
First, hot water boilers will be addressed. Hot water boilers expand and contract as they heat and cool. Slow expansion and contraction are generally not problematic for cast iron. However, rapid expansion and contraction can cause push nipple and gasket leaks or stress cracks. This occurs when hot water returns to a cold boiler or cold water returns to a hot boiler. In residential applications, the larger issue typically involves cold water entering a hot boiler. The two main flow problems are extremely cold water entering the boiler or cool water entering at a high flow rate. Rapid and constant movement may also cause wear on push nipples or gaskets. Ideally, section movement should be slow due to gradual temperature changes. Another reason for stress cracks or thermal stress is an oversized boiler, which causes quick iron movement as the water heats rapidly, cools during normal operation, and fires up quickly again. This happens multiple times in a thermostat run cycle and hundreds of times in a winter season, causing more wear on push nipples and gaskets and additional stress on the cast iron sections.
To protect against thermal stress, it is necessary to control temperature and/or flow. Piping strategies can help protect cast iron boilers from these issues. A simple boiler bypass pipe provides minimal protection, as illustrated by this basic drawing.
A prevalent cause of boiler failures is corrosion resulting from the presence of oxygen within the iron boiler section. It is essential to remove oxygen from the heating system and ensure it remains excluded. The boiler triangle comprises three elements necessary for internal corrosion: iron, water, and oxygen. By eliminating one of these elements, the corrosion process can be halted.
To prevent oxygen corrosion, one of the three elements must be eliminated. Since removing iron or water is not feasible, oxygen must be removed. This can be achieved through proper near boiler piping and efficient air removal. Specifically, circulators should be placed on the hot supply side of the boiler, pumping away from the expansion tank connection. All automatic air vents at the system's high points should be replaced with manual vents. If your expansion tank does not have a rubber bladder, it is recommended to eliminate all automatic air vents. Each time this type of tank is drained, fresh water is added along with the make-up water. This introduces oxygen, minerals, and chemicals, which are not beneficial for the heating system. To minimize the addition of fresh water, strategic placement of service valves and timely repairs of heating system leaks are advised. The minerals tend to attach to the hottest iron surfaces, particularly around the flame area. Mineral build-up can create hot spots and lead to cracks.
Flue gas condensation occurs when hot flue gases pass through flue passages with cold water in the cast iron sections. Condensate forms when the flue gases cool sufficiently to reach dew point temperatures, similar to dew on summer grass, but this moisture contains hydrochloric, sulfuric, and carbonic acids. These acids corrode the iron, and as the cast iron sections warm up, the flue passes dry out. It is essential that the flue passes are dry before the boiler shuts down. The temperature at which flue gases stop condensing depends on various parameters. The industry standard is an outlet temperature of 140°F or an average water temperature through the boiler of 130°F. Employing boiler bypass can reduce the flow through the boiler, allowing higher iron temperatures to prevent flue gas condensation.
Maybe this will help understand why cast-iron boilers fail due to stress and condensation.
I did some research and found a boiler from the 50's with an input of 160,000 btu's. and we will assume the return has a flow of 7 gpm. As you see the boiler has almost six times more water than the return is bring back. With this ratio between return water and boiler water the internal temperature change is terribly slow. When the boiler is replaced with a new cast iron boiler today that boiler will hold about six gallons of water. With the same return flow as the old boiler had, there is one gallon more water returning to the boiler than the boiler holds. This will cause very quick and drastic temperature changes.
When a boiler bypass is added some of the flow from the return will flow through the bypass which means less flow through the boiler, so the boiler sees less stress and the iron heats up quick enough to avoid flue gas condensation. The flow through the bypass may be three times the boiler flow. In this example the flow through the bypass is 4 gpm which means the flow through the boiler is 3 gpm. The flow through the boiler is one-half of what the boiler holds. Again, it warms up faster than it would with the 7-gpm flow.
Steam Boilers
With steam boilers don't worry about flue gas condensation as there is no flow into the boiler until you start making steam. We still have the mineral and oxygen problems you experience with water boilers from adding too much make-up water due to steam or water leaks. The oxygen will cause pinholes at the water line as that is where the oxygen comes out of solution.
Chloride Corrosion - Chlorides are a significant contributor to leaks in steam boilers. When chlorides cause a leak, they create holes in the iron at the top of the boiler. Some of these holes can become quite substantial; instances have been observed where holes are large enough to fit a fist through. The water level in a steam boiler should be maintained according to the manufacturer's specifications, which typically is measured from the floor or bottom of the boiler jacket to the surface of the water in the sight glass tube, normally within 1/2 to 3/4 of the sight glass tube. This measurement is referred to as the "Normal Water Level" (NWL), indicating that the boiler is not completely full of water. The iron exposed above the water level experiences higher temperatures than the iron submerged in water, as the water absorbs some of the heat. Boiling within the boiler is highly vigorous, causing water to bounce off internal sections above the water level. As the water rapidly vaporizes, it leaves behind chemicals and minerals. Due to the cumulative nature of chlorides, they eventually begin to deteriorate the iron, and this process cannot be halted once it starts.
Avoid adding excessive make-up water from leaking air vents, valves, fittings, and piping. Refrain from using softened water; instead, use water prior to the water softener. Do not drain the boiler during the summer months. For an electronic Low Water Cut-Off (LWCO), do not drain any water from the boiler. If using a float type LWCO, drain no more than necessary once a month or as specified by the manufacturer's guidelines to achieve clear water.Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per-job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc and its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.