Cast Iron Boiler Problems


Cast iron boilers operate without problems for most of their useful lives. In the rare instances when a problem does develop, it usually involves sensors, wiring or the metal body of the boiler. Troubleshooting these areas of the boiler allows home and property owners to take action before a total system failure occurs.


  • Over time, the thermostat of a cast iron boiler system may lose accuracy, leading it to maintain the wrong temperature or turn the boiler on or off at improper times. If the thermostat is programmable, its battery might have failed or its clock might be stuck. A wiring problem, such as a failed relay, short circuit or loose connection between the thermostat and boiler, can also cause boiler malfunctions.

Condensate Pipe

  • The condensate pipe takes condensation from the boiler and drains it to the waste water system. During cold weather, ice can form in the condensate pipe, leading to a blockage. Algae or mold can grow in the condensate pipe and cause an obstruction. When the condensate pipe becomes blocked, water flows back into the boiler, and safety switches will trigger the boiler to shut down. Bubbling sounds coming from the boiler or a flashing warning or fault light alerts owners to this problem.

Pilot Light

  • Cast iron boilers that are gas-fired are equipped with a pilot light. If the pilot light won't light, the vent pipe could be blocked. If the pilot lights but refuses to remain lit, the pilot light assembly or flame sensor could be dirty. Other causes of a pilot light going out include a bad thermocouple or a bad boiler control. The gas valve may also be faulty; this would disrupt gas flow, causing the pilot to extinguish.

Abnormal Temperature or Pressure

  • A boiler pressure or temperature that's too high or too low can cause the system to shut down. Air in the boiler causes pressure that's too low. A broken fan motor or a closed vent may also cause abnormally low pressure. High pressure or high temperature in the boiler could result from a broken circulator or control board. A malfunction of the heating temperature or flue temperature sensor may also cause abnormally high temperature readings.


  • Sludge buildup or operating at a low water level can cause cast iron boilers to overheat, which can result in cracks. If the boiler is allowed to get cold, cracks or gaps in the metal between the long rods and draw bolts can allow water to leak from the system. Weakness between the boiler's connecting joints and between the boiler and water pipes may also result in cracks and leaks.

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