At one time the hulking mass in the basement that heated the home with hot air or steam was known as the furnace. Today, a furnace moves air and a boiler produces steam or hot water. Oil fired boilers are constructed of steel or cast iron. Cast iron boilers are made up of sections, which are large hollow rings stacked together, like a box of donuts. Steel boilers can be of the same design or be of stamped steel welded together to form the sections or water chambers.
Gas burns at a lower temperature than oil, and gas boilers come in a variety of designs.
They can be similar to oil boilers, but usually put the flame closer to the metal, even immersing the water tubes in the fire. Some gas burners have a blast tube, similar to an oil burner, but most use atmospheric burners. This type of burner uses the pressure of the gas forced through an orifice to mix with air and create rows or fire, similar to the burner in a gas oven or grill. An aquastat serves two purposes. It controls the temperature the boiler operates at, and provides a low voltage transformer and relay to control the circulator.