Customer had no heat in home, her relief valve had popped so unit was turned off, found expansion tank had failed and filled with water, also relief valve no good, as well as the air vent clogged, also shut off valve for top floor broken so no way to purge top floor, sweat in new ball valve, replaced expansion tank, air vent, relief valve and purged system, let boiler run through two cycles, and felt radiators. All supply and returns got hot. System operational and working.