Process heaters are typically located outside and subject to the weather. Although heaters are typically tuned at a given set of conditions, actual operating conditions vary significantly from season to season and sometimes even within a given day. Unfortunately, most heaters are not properly adjusted for actual operating conditions. Ambient air temperature, pressure and humidity all significantly impact process heater efficiency. This paper shows how changing ambient conditions can reduce efficiency if proper adjustments are not made. Combustion efficiency is related to air: fuel ratio and to air-fuel mixing. A general industry rule-of-thumb is that operating at 2-3% excess O-2 (dry basis) results in the best combination of efficiency and flexibility. At higher O2 levels, efficiency is reduced because the additional O-2 and N-2 absorb heat, much of which exits the exhaust stack. At lower O-2 levels, efficiency can be substantially reduced because some fuel is uncombusted. Low O-2 levels can also lead to soot and coke buildup on process tubes reducing heat transfer to the process fluid and reducing efficiency. Several examples demonstrate how ambient conditions affect heater efficiency. Calculations and graphs for a wide range of operating conditions demonstrate how efficiency can be affected by changes in ambient conditions for process heaters. (C) 2009 Elsevier Ltd. All rights reserved.