The boiler plant should be designed and operated to produce the maximum amount of usable heat from a given amount of fuel. Combustion is a chemical reaction of fuel and oxygen which produces heat. Oxygen is obtained from the input air which also contains nitrogen. Primary and secondary air should be allowed to enter the combustion chamber only in regulated quantities and at the correct place. Defective gaskets, cracked brickwork, broken casings, etc. will allow uncontrolled and varying quantities of air to enter the boiler and will prevent accurate fuel/air ratio adjustment. A waste heat exchange directly from flue gases to combustion air using static tubular, plate, or rotary exchangers can be implemented. A generally superior means of meeting a fluctuating boiler load demand is a system of modular boilers which can be fired independently. Fuel burning equipment allowed to become dirty and out of adjustment becomes increasingly inefficient with continued usage.