85% That's the MAXIMUM efficiency you can get out of a non-condensing gas appliance. The other 15% of the combustion process is latent heat, and you must condense the combustion gasses to capture it. The 5% difference between 80% furnace and the maximum 85% theoretical non-condensing efficiency allows a safety margin so flue gases don’t condense.
Combustion Efficiency and Excess Air
Mobile Master Tech is SPOT ON with his advice.