Performed a heating pm on an oil boiler. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. 10.2% o2, 18 ppm, 77.7% efficient System is operating properly. While performing the maintenance, noticed that water is starting to migrate into the air side of the expansion tank. The expansion tank will need to be replaced.