Final answer:
A high-efficiency condensing boiler improves energy efficiency by reducing the temperature rise and condensing flue gases to extract more heat.
Step-by-step explanation:
A high-efficiency condensing boiler achieves improved energy efficiency primarily by reducing the temperature rise and condensing flue gases. This process allows the boiler to extract more heat from the fuel being burned, which would otherwise be lost in a non-condensing boiler through the flue. In a condensing boiler, the hot flue gases pass through a heat exchanger where they cool down to the point where they condense, releasing latent heat that is recovered and used to heat the space. This is in contrast to allowing the hot flue gases to escape unused.