I was sent on a service call for "high discharge temp alarms" on a single story office building. I found a building with 4 roof mounted ahu's, each having a chilled water coil and high eff. "bag" air filters. each ahu had both supply and return fans controlled by vfd's, each ahu 20-30 tons ? They all are ducted into the same supply duct which makes a single loop around the building. They all pull return air from a plenum ceiling. The building operator had shut down 2 ahu's to save energy because the building was approx. 30% occupied. The 2 ahu's that were operating were running sf vfd at 100% and rf vfd at 80% moving approx. 14,000 cfm each.
chilled water valves were at 100%, but supply air temps were averaging 67deg. I started the other 2 ahu's and vfd's slowed to approx 28%, chilled water valves came under control as supply air temps dropped. All seems well. I've never seen this type of system before.
1) Do all 4 ahu's need to run even under low load?
2) should return fan vfd set point be a % of supply fan vfd? If so How much?