Since boilers have so much piping, leaking is perhaps the most common problem users encounter. Problems with combustion are also fairly common, since boilers turn off and on so frequently.
Pipes and radiators used for boilers are designed to last for long periods of time, but metal fatigue and other potential issues can causes leaks to develop. As pipes warm up, they expand, and they contract as they cool down. This constant flexing leads to strain, and tiny holes can quickly expand due to this expansion-contraction cycle. Since pipes are often run behind walls, leaks can be difficult to detect.
Most boilers use natural gas, propane or oil, all of which require precise combustion. Over the years, the efficiency of the combustion cycle has increased significantly, leading to cleaner, more affordable operation. However, small defects and other problems can cause this combustion process to fail, and some units even have safeguards to stop the combustion process if it is not operating efficiently. Problems with the fuel line are common, but airflow obstructions prevent combustion as well. Pilot lights, which are lit at all times, are still used on older units, and the boiler cannot run if they go out. Newer models typically use more efficient electrical ignition systems, which also have a number of failure points.