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Playbook: Low Chilled Water Delta-T
Applies toCHW-0004, related to CHW-0001; HW-0004 — library-authored HW-side addition, the reference’s row names only the CHW rule; AHU-0038 (coil-side hydraulic creep, same discriminators)
Fix complexityRemote + on-site
Typical time2–8 h (investigation across multiple air handler coils)
Typical cost$0–$2,000 (depends on root cause)
Energy impactLow delta-T is one of the most common and costly chilled water plant problems. It forces additional pumps and chillers to run to meet load, increasing pump energy by 30–100% and reducing plant COP. PNNL-27338 detects low delta-T for both CHW and HW systems when the supply-return differential falls below threshold at moderate loads.

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 163–164).

Step 1 — Verify the fault

  1. Calculate the plant delta-T: chilled water return temperature minus supply temperature.
  2. Compare to the design delta-T (typically 10–14 °F / 5.5–7.8 °C).
  3. If delta-T is less than 50% of design when the plant is above 30% load, low delta-T is confirmed — one of the most common and costly chilled water plant problems.
  4. Check whether the low delta-T is plant-wide or concentrated at specific AHUs. Trending return water temperature at each AHU isolates the worst offenders.

Step 2 — Remote fix

  1. Check for three-way valve bypass: where three-way valves exist on air handler coils, water can bypass the coil without picking up heat. Converting to two-way valves is a capital project but high-value.
  2. Check air handler chilled water valve control — valves should modulate smoothly between 0% and 100%. If valves are stuck partially open on multiple air handlers, that explains the low delta-T.
  3. Check chiller staging: too many chillers running at light load dilutes the plant delta-T. Adjust staging thresholds so fewer chillers handle the load at higher efficiency.
  4. Check CHW differential pressure reset (EEM-10): if DP is set too high, valves throttle excessively and water bypasses through the coils too quickly to transfer full heat. Resetting DP based on the most-open valve position saves 0.5–2% site energy.

Step 3 — On-site service

  1. Inspect air handler coils for fouling — dirty coils reduce heat transfer, which directly lowers the temperature differential. Clean coils at $200–$500 per coil.
  2. Check chilled water valve sizing — oversized valves tend to hunt and never fully load the coils. The worst offenders may need valve replacement.
  3. Check for air trapped in the chilled water piping — air locks reduce water flow through coils. Bleed air at high points in the piping system.

Step 4 — Confirm resolution

  1. Monitor plant delta-T over 2 weeks.
  2. Target: delta-T at or above 75% of design when the plant is above 50% load.
  3. The fault should clear and pump energy should decrease as fewer pumps are needed to move the same cooling capacity.