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 5.6%o2 45ppm co 0 smoke After performing the maintenance we drained pressure from the system. Removed old autofeed combo and hi-vent. Installed new autofeed combo and hi-vent and filled system back up with water. Purged all 3 zones and started and tested the system. Had hot water coming back on all 3 returns. Unit was at 21psi when leaving.