What is a significant disadvantage of using older boiler systems?

The significant disadvantage of using older boiler systems lies in their lower energy efficiency and higher emissions. Over time, many older boiler systems were designed without the advanced technology and materials that contribute to optimal fuel combustion and heat transfer. As a result, these systems typically have much lower efficiency ratings, meaning they require more fuel to produce the same amount of energy compared to modern boilers.

Additionally, the inefficiency in combustion can lead to greater emissions of harmful gases such as carbon dioxide (CO2), nitrogen oxides (NOx), and sulfur dioxide (SO2). This not only impacts environmental quality but can also lead to higher operational costs due to increased fuel consumption. Thus, the inefficiencies present in older systems directly contribute to their significant disadvantage in terms of cost and environmental impact.

The other choices do not accurately reflect the drawbacks of older systems. They are often heavier due to outdated materials, may have longer startup times caused by older design technology, and do not use natural gas more effectively than newer models.