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 8.7o2 2ppm While performing the maintenance the customer stated they heard air in the heating pipes. Could not purge air out of the heating loop because 2 isolating valves for both halves of the loop are corroded. Did not want to touch them due to fear of leaking water. Need to return to install 2 new isolating valves on the return piping. Also found some fittings by the mixing valve are heavily corroded and need to be replaced. Took pictures to work up an estimate for total repairs. Needs 3/4 valves