Diverting Tees

Cam

Help! We are working on a heating problem in a single story building with a one-pipe hot water system. The system was installed in 1991 and until this winter has performed extremely well (even during the winter of 1994 when it was much colder for much longer). This winter they have been unable to get enough flow through their unit ventilator coils to run the fans. I have read through the Q&A's for diverting tees and still can't figure out what the problem could be. The problem is with ALL the units, not just a couple. One unit's coil was changed last summer, but this is the only work to the system since last winter. The system water has been treated regularly since the system's installation.

We have checked pump operation and DP, system pressure, air vents, relief valves, control and isolation valve positions and operation, expansion tank pressure, and the coil does heat up when blead so the coil is not blocked. Outside air dampers are closed and system strainers are clean. Unit coils are above the main and 2 tees are provided for each unit. Although the design was for a 20 degree temperature drop, the actual temperature drop on a design day was under 10 degrees.

Do you know of any history of errosion of the venturi cone in the tees or unusual scalling at the branch connection that would restrict flow? What are we missing?