A usual location for mounting an O2 probe in boilers is downstream from the economizer. This location is ideal because it is close to the boiler, but the flue gas temperatures are usually below 1000°F (537°C). Some facilities mount one or more probes downstream from the "air heater". The air heater exchanges the heat from the flue gas to the fresh air used to fire the burners. There is always a possibility that a leak can occur allowing fresh air to infiltrate into the flue gas and reducing the rate of heat transfer and diminishing boiler efficiency. Likewise, the leaked air is not available for combustion, reducing the maximum firing rate of the boiler.
This problem is particularly acute in the rotary regenerative designs, which must use a rotary seal to separate the hot flue gases from the fresh combustion air. This type of air heater operates much like a revolving door. Plates in each section pick up heat from the hot flue gases, and, as the unit rotates, transfer this heat to the fresh air being preheated for combustion.