Steel boilers, as their name implies, are made of steel and are usually made of horizontal cylinders. Steel boilers have a combustion chamber in which pipes are placed around this chamber. The pipes that are installed around the combustion chamber of steel boilers are made of a special alloy so that they do not burn or damage in the face of a direct flame. These pipes are called (fire pipes). Steel boilers are divided into friends from the point of view of heat transfer, which are as follows.

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.

Engine room boilers are high-pressure tanks designed to heat water or generate steam that can be used to heat the environment or to heat the building consuming hot water. In the heating of most commercial complexes, the source of heat for the engine room boiler are burners that work by burning natural gas. Diesel burners or fuel oil (rarely) can also be used. In some heating installations, the use of steam is preferred. Such as: absorption cooling systems, kitchens, laundries, disinfectants and steam-powered equipment.

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.

In facilities, in addition to limiting, the system activity sequence is also important to achieve energy efficiency. Nowadays, with the general use of VAV (variable volume) systems in commercial complexes, simultaneous heating and cooling and overheating of the primary air are often ignored. Applying boiler restrictions based on outside air temperature, for example when the temperature is above . degrees Celsius, is an effective way to prevent this situation.