Parts of a Boiler


The two main parts of a boiler are the furnace, which provides the heat, and the boiler proper, in which water is turned to steam. Several devices have been built into boilers such as the fusible plug and the Hartford loop, invented by Hartford Insurance Co. to reduce boiler claims.


  • The shell contains a mix of water and steam. If water input fails and the shell heats dry, the temperature could become so high that new water boils on contact with the shell, causing an explosion.

Fusible Plug

  • A fusible plug is a screw with a tapered hole in the center-smallest side inside the boiler. It is a safety device. The hole is blocked with a metal of low melting point so steam is not released at normal working temperatures.


  • Hand-holes are covered with metal plates. They allow access to the boiler interior for inspection and cleaning.

Safety Valves

  • Safety valves are used to relieve pressure and avoid an explosion from steam pressure.

Hartford Loop

  • The Hartford loop is a plumbing configuration that provides backup water in the event a water-return pipe's rupturing leads to a low water level in the boiler.

Low-Water Cutoff Valve

  • A low-water cutoff valve cuts off the fuel source if the water gets too low.

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