have boiler system in an apartment complex, it feeds 8 units. in one unit there is no heat. the plumber has drained the system and replaced the t-stat. when the system gets drained and filled back up there is heat for a while and then no heat. the system was repiped due to a fire, is it possible that the hot supply line that goes to the unit got plumbed back to the hot supply line and not the return. thanks