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.

All combustion equipment must be operated properly to prevent hazardous conditions, catastrophic damage, and human and construction damage. The main reason for the explosion of engine room boilers is the combustion of flammable gases that have accumulated inside the engine room boilers. This can happen for a variety of reasons, such as fuel, air, or the combustion process is interrupted for any reason, the flame goes out, and flammable gases accumulate that cause re-burning.

Parallel controls use separate motors to regulate fuel flow and airflow, enabling each motor to be tuned throughout the boiler ignition range. During installation, to points are usually drawn to create a curve of airflow corresponding to the fuel flow. The air-fuel ratio can be varied along the ignition range to prepare the optimal ratio in different ignition conditions. Also with the use of electronic dampers, this method of control is very reproducible.

When a fuel defect occurs, the chemical energy of the fuel is not completely converted to heat, reducing combustion efficiency. This raises security concerns that unused fuel may ignite in the chimney and cause an explosion. Boilers must be adjusted to achieve complete combustion. One strategy to ensure complete combustion is to provide extra air. As shown below, a small amount of excess air increases the combustion efficiency, but a large amount reduces the efficiency.

Figure shows the combustion efficiency chart for natural gas fuel with electricity, which shows the relationship between excess air and flue gas temperature with combustion efficiency. For example, by following the Step line in the diagram, the amount of oxygen at % in the flue gas (as shown in the diagram equals approximately % of the excess air) and ° F as the flue gas temperature rises, the corresponding combustion efficiency is about . % is observed. At the same increase in the flue gas temperature by degrees Fahrenheit, the Step line shows that reducing the oxygen in the flue gas to % increases the combustion efficiency to about .%. The lower the percentage of oxygen in the flue gas, the less heat is transferred to the excess oxygen, resulting in increased fuel efficiency. The higher the efficiency, the more heat will be transferred to the inlet water instead of to the flue gas, thus reducing the flue gas temperature.