Responded to callback for no heat upstairs. Identified air lock in the system and resolved by bleeding significant air from purge valves where lines ascend to upper floor. Found and closed an incorrectly opened bypass valve which restored proper flow. System currently operational but noted several design issues including incorrectly piped expansion tank and questionable piping configuration. Recommended installation of conventional bladder tank with air scoop, auto fill, repipe on boiler return, and addition of glycol due to pipes running along exterior walls in this historic structure.
