An installer is asked to assess a system in which an aging steam boiler supplies several cast-iron radiators. After inspecting the radiators, he proposes to convert the system from steam to hot water and replace the old boiler with a mod/con boiler. The installer pipes in the new boiler to the existing black iron piping system as shown in Figure 1, and installs thermostatic radiator valves (TRV) on each radiator to provide room-by-room zone control. The circulator is wired to run continuously during the heating season. Although the installer’s intentions are good, can you spot some potential problems with the make-over and suggest some improvements? 

One of the problems with the proposed piping layout is the circulator placement. It’s pumping toward the connection point of the expansion tank. This results in a drop in pressure between that point and the inlet of the circulator. It may even be possible that the pressure in this portion of the system goes sub-atmospheric when the circulator is operating. That’s an invitation for air to enter the system.