The pressure difference between the combustion chamber of the boiler and the ventilator (also called the exhaust chimney) creates a suction that carries the products of combustion along with the boiler and directs it to the chimney. The natural suction of boilers depends on the natural buoyancy of hot gases to remove combustion products through the aerator and direct fresh air into the combustion chamber. Mechanical suction of engine room boilers includes compressed air, where the air is forced into the combustion chamber by a fan or blower to maintain positive pressure. Induction suction, where the air is drawn through the combustion chamber by a fan or blower to maintain negative pressure.

Cast iron boilers that are cross-sectionally cast iron are another type of engine room boiler that is used to heat commercial premises. These boilers do not use pipes, but instead, have cast-iron sections with water and combustion gas corridors. The cast iron parts are screwed together and look like a steam radiator. These parts are connected to each other by fireproof washers. These boilers are used to produce steam or hot water and are available in sizes from , to , BTU.

Modular boilers are small in size and capacity and are usually used to replace several small boilers instead of one large boiler. These modular boilers pass through standard ports and are portable through stairs and elevators. These devices can be arranged in different order to make the most of small places or embed other equipment in those places. These boilers can efficiently provide the required heating load.

The burner combines fuel and oxygen to create combustion conditions with the help of a combustion device. This combustion takes place in a combustion chamber and the heat generated is transferred to the heat exchanger water. The controls regulate the combustion process, the ignition rate of the burner, the required fuel, the amount of air required, the water temperature, the steam pressure and the pressure of the engine boilers.

Cast iron boilers are very useful because they can be collected in the basement and passed through smaller doors and gates. The most important drawback is that they are prone to leakage due to the connection of the parts by the washer when the washer becomes old or damaged by chemical reactions. Among the famous cast iron boilers in Iran, we can mention the boiler cast iron boiler and M.I. cast iron boiler made by Iran Pipe and Machine Building Company.