🔍 Key Concepts

• Positive positioning relays (P) and how they drive the three dampers together • The normally open (N.O.) vs normally closed (N.C.) symbols on the exhaust, outside air, and recirculation dampers in the RA‑0009 diagram • How a single‑zone unit maintains a roughly constant supply air volume while changing the mix of outside, exhaust, and recirculated air

💭 Think About

• Look at the pneumatic lines from the positioning relays: when the actuator signal increases in one direction, which damper(s) are shown as tending to open and which as tending to close? • For minimum ventilation, which damper must stay at least partly open, and which damper must reduce flow when more outside air is introduced so the total flow through the fan does not change? • If the system wanted to go to 100% recirculation (no outside air), what must happen to the exhaust and outside air dampers together?

✅ Before You Answer

• Verify which damper is tagged as minimum outside air bypass (B.P.) and how it is drawn relative to fully shut/fully open positions • Trace the control air from the positioning relay to see which dampers are arranged to move opposite to each other and which move together • Confirm that the total air through the supply fan is effectively constant, so opening two dampers generally requires closing the third