With the most common high-temperature hydronic heating, it's closer to 80% efficiency when running steady-state, and lower than that as the duty cycle decreases

With the most common high-temperature hydronic heating, it's closer to 80% efficiency when running steady-state, and lower than that as the duty cycle decreases (if the system is oversized or for the 99% of the time you're not at the design temp). The remaining energy escapes as hot, uncondensed flue gases. Modulating/condensing boilers can get up to like 95% if you're running with ~100-120F water temperatures, but at that point you can also readily run with an air-to-water heat pump.