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.