and 131 a remain off in step 305 the crossover damper 117 is closed to direct 100 of the flow of combustion air from the fan 101 a to the air heater 119 a and to shut off the flow of combustion air to the air heater 119 b in step 307 the crossover damper 125 is closed to direct 100 of the flow of flue gas from the boiler 121 to the air heater 119 b and to prevent flue gas from flowing through air heater 119 a once steps 301 303 305 and 307 are performed the b side air heater 119 b will increase in temperature rapidly because it receives none of the relatively cool combustion air flowing through the combustion air conduit 109 as a result the flue gas flowing through the flue gas conduit 129 and all components in contact with the flue gas increase in temperature relatively quickly relative to a conventional boiler system thus the flue gas and the components in contact with the flue gas are raised above the acid dew point much more quickly than in a conventional boiler system and as a result acid mist opacity and corrosion is reduced steps 301 303 305 and 307 may be performed in any order however in one preferred embodiment steps 305 and 307 are performed prior to steps 301 and 303 and combustion begins in step 308 after steps 301 303 305 and 307 are complete once the gas exiting air heater 119 b reaches 300 u00b0 to 320 u00b0 f in step 309 the crossover damper 117 is throttled open to unrestrict the flow of air from fan 101 a.