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Playbook: Missing Reset Strategy (SAT / DSP)
Applies toAHU-0023 (SAT reset), AHU-0024 (DSP reset), AHU-0019 (SAT SP too low), AHU-0031 (excess static pressure), CLU-02; CHW-0002 (CHWST reset), CHW-0003 (CHW DP reset), HW-0006 (HW DP reset), HW-0008 (HWS temp reset) — plant-side additions by this library, the reference’s row names only the AHU rules
Fix complexityRemote fix (90%) · Controller upgrade (10%)
Typical time1–4 h remote (sequence programming)
Typical cost$0 remote / $2,000–$5,000 if controller upgrade needed
Energy impactEEM-05 (SAT reset): 1–4.4% site energy, 2.5% national median. EEM-12 (DSP reset): 1–3% site energy via cubed fan law. Combined, absent in 74% of buildings (PNNL 151-building study).

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 154–156).

Step 1 — Verify the fault

  1. Plot the SAT setpoint vs. outdoor air temperature over 7 days. A flat setpoint means no reset; one that varies without correlating to zone demand means misconfigured reset logic. PNNL-27338 AIRCx threshold: if MAX(sat_stpt) − MIN(sat_stpt) < 2 °F over the window, no reset is detected.
  2. Plot the DSP setpoint vs. the highest zone damper position. Flat setpoint with dampers well below 100% means no reset (AIRCx: MAX − MIN < 0.25 in. w.g.). Good operation: most zone dampers 50–75% open; bad: all near 100% or all near 0%.
  3. Check whether zone-level demand requests reach the AHU controller — if no requests arrive, the issue is at the zone level or the comms path.

Step 2 — Remote fix

  1. If the reset sequence exists but is disabled: re-enable it; verify all VAV boxes send requests to the correct air handler.
  2. If no SAT reset exists, program trim-and-respond per G36 §5.16.2: start 18 °C (65 °F); trim up +0.2 °F per interval when zones satisfied; respond down −0.5 °F per zone cooling request; range 13–18 °C (55–65 °F).
  3. If no DSP reset exists, program trim-and-respond per G36 §5.16.1: start 0.5 in. w.g.; trim −0.03 in. per interval when dampers not maxed; respond +0.06 in. per zone airflow request; max = design static, min 0.2 in. w.g.
  4. If trim-and-respond is active but ineffective: check trim interval (~2 min SAT, ~1 min DSP); ensure respond magnitude exceeds trim magnitude; check request thresholds at the VAV boxes.
  5. High-SAT heuristics (PNNL-27338): >60% of zone dampers above 90% open → SAT too high, lower it; >25% of zones with reheat valves above 50% → SAT too low.

Step 3 — On-site service (rarely needed)

  1. If the AHU controller doesn’t support trim-and-respond logic, upgrade the controller ($2,000–$5,000 including labor) — uncommon outside legacy pneumatic or first-generation digital controls.
  2. Verify zone controllers communicate with the AHU controller; check trunk cabling, repeaters, protocol converters (e.g. MSTP-to-IP routers).

Step 4 — Confirm resolution

  1. Monitor setpoints over 7 days after programming.
  2. SAT setpoint should modulate up during low demand, down during high demand; DSP setpoint should decrease when zone dampers are mostly closed.
  3. The fault should clear within one evaluation window (typically 7 days).
  4. Expected savings: ~2.5% of site energy from SAT reset alone (PNNL national median); DSP reset adds 1–3% via the cubed fan law (a 20% fan-speed reduction yields ~49% fan-power reduction).