High-efficiency heating systems, including boilers, are typically at or above 90 percent annual fuel utilization efficiency, or AFUE. They operate similarly to older systems but contain a number of components that increase efficiency.
Older, inefficient boilers were typically built with little consideration for efficiency, and they often used large but durable parts. While these components provided robust operation, they also heated up while the system was in use, wasting heat in the process. In addition, large piping was often preferred for its ability to handle build-up at the cost of efficiency operation. Advanced technology enables better reliability using compact, efficient components.
The combustion process in systems has improved as well. Different fuels have different ideal fuel-air mixture ratios, and an optimal ratio leads to far better performance than heavy or lean mixes. Systems that are able to ensure optimal airflow have led to significant improvements over time.
Condenser boilers are more complicated than older types of boilers but are better able to capture heat created from combustion. Combustion optimizations also lets systems use sealed combustion chambers, which provide a combustion environment that is largely unaffected by environmental factors and better able to capture and distribute heat. As boilers approach the maximum theoretical limit of extracting heat from fuel, many are looking to ductwork, insulation and other heating components to provide even better efficiency.