Seasonal input–output efficiencies of two heating plants with condensing boilers in cold climate institutional buildings (3300 m² school and 12,000 m² university building) were evaluated. Plant efficiency remained about the same or declined with load, contrary to typical lab boiler ratings, which show efficiency increasing at lower loads. These results occurred when the boilers were operated with return water temperatures largely in the condensing range. Heating plant load was often 25% or less of the rated load of a single boiler, resulting in heating plant input–output efficiencies well below rated boiler efficiencies.

Practical application : In-situ boiler plant input–output efficiencies can differ widely from manufacturer's boiler efficiency curves. For applications such as simulation for energy-efficient design, effective decision-making depends on accurately estimating real-world performance.