Performed pm on oil boiler Brushed and vacuumed flue pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Performed combustion and smoke tests While performing the maintenance we found very high levels of sludge at the oil filter and also saw some water in the oil filter canister. Recommend getting a sludge and water additive for the oil tank. Also saw 2 hi-vent were showing signs of corrosion and were no longer working properly. After getting customer authorization we replaced both hi-vents. Drained pressure from the system. Removed both hi-vents. Filled system back up with water. Purged both zones of air. Started and tested the system. Had hot water coming back for both zones. When leaving boiler pressure was 20 psi.