What is “Maximum steady state efficiency”?

This figure represents in percentage, the absolute best amount of useable heat the fireplace can produce. Maximum steady state efficiency is determined by specific test conditions that include burning the fireplace until the fireplace and flue gas (exhaust) temperatures reach equilibrium, which means that there is no further change in temperatures. Temperatures, taken from a specific location, reflect the amount of heat being exhausted, which is then subtracted from the predetermined input of the burner. The remaining amount of heat represents the highest amount of heat that the fireplace can transfer into the surrounding area. Example, our natural gas stove, the GDS50 has a BTU input of 44,000 BTU. The maximum efficiency is 84%. 44,000 x 84% = 36,960 BTU output.