Fundamentals of HVAC Controls and Automation

A: —that's the most common mistake, assuming a demand problem when the actuator is already at the stop. A reversed or blocked pickup will flatten the flow feedback, so the PID keeps driving open even though the fan static looks fine. A warm-biased zone sensor would still modulate the damper and show changing flow, and SAT reset or VFD limits would hit multiple zones at once, not a single box pegged at 100%.

A: —you’re forgetting the basic air heating math. 1.08 × 20,000 × 15 × 8 lands in the low single‑digit MMBtu range before boiler efficiency. Fans matter, sure, but this question was about avoided sensible heating. Thirty would imply you never turn the plant down during the day, and 0.3 drops a zero.

A: —that’s where people mix up sizing with authority. At 240 gpm and Cv 120, the valve drop is (Q/Cv)² ≈ 4 psi, close to the 5 psi target. If the rest of the circuit is around 15–20 psi, authority sits near 0.2. Hitting design flow doesn’t magically give you authority of one, and 0.8 would mean the coil and piping barely exist.

A: —people assume a single proof covers all cases. A stuck‑true status tells the controller air is moving when it’s not, so the heating valve can open and cook the coil. It was never meant to handle motor protection, freeze stats, or tight shutoff, which are separate safeguards with different failure modes.