AHU-0011 — OAT too low for mechanical cooling

Preconditions (host-enforced): Evaluate only in G36 OS#3, which Table 5.16.14.2 defines by actuator signature: heating coil = 0, cooling coil > 0, OA damper = 100%. Per §5.16.14.11 the host suspends evaluation while the AHU is not operating and for ModeDelay (30 min) after a mode change in any zone group the AHU serves; per §5.16.14.12 fault conditions not applicable to the current OS are not evaluated at all. Verdicts outside OS#3, and verdicts inside a transition window, are NO_EVAL — never healthy. sat_sp must be the setpoint the economizer is controlling to (G36’s SATsp for heating-coil and economizer control), not the cooling-coil setpoint SATsp-C. The 3.0 °C default threshold assumes a local OAT sensor at the unit; a site on a global OAT sensor must retune it before trusting the verdict.

Points: oat, sat_sp

Outputs:

• yFault — True while outdoor air has stayed more than oat_deficit_threshold below the supply air setpoint continuously for at least alarm_delay

Parameters:

Description

The unit is running mechanical cooling on top of a fully open outdoor air damper while outdoor air is already colder than the supply setpoint by more than the fan will add back. In OS#3 the dampers sit at 100% and the chiller trims what the economizer cannot deliver, which is legitimate — until OAT is more than dTSF below setpoint, at which point free cooling alone would overshoot and the unit belongs in OS#2 modulating back toward a blend.

Something is holding the machine in the wrong state: the sequence never changed over, a sensor is lying about which side of setpoint the air is on, or a heating coil is leaking warmth into a stream the chiller then removes — the expensive version, two subsystems paying to cancel each other. Within CLU-03 (Economizer Failure) this is the third face of the same changeover problem: AHU-0017 catches a damper that stays shut when outdoor air is useful, AHU-0009 an economizer still modulating after outdoor air stopped being useful, and this one mechanical cooling still running when the economizer alone had it covered.

Detection Logic

G36 §5.16.14 FC#11, applies to OS#3:

    OAT_AVG + eOAT < SATSP − dTSF − eSAT

rearranged to gap form:

    sat_sp − oat > eOAT + eSAT + dTSF = 1 + 1 + 1 = 3.0 °C

yFault = (sat_sp − oat > oat_deficit_threshold), sustained for alarm_delay

Block graph (rule.cxf.jsonld):

G36 spreads the two sensor error bands and the fan-heat correction across both sides of the inequality; the rearrangement collapses them into the single positive gapBig.t, so a host retunes one parameter instead of tracking which side each term sits on. Note the operand order: gap is sat_sp − oat, the reverse of AHU-0009’s, because the two rules look at opposite ends of the same band and each wants its own end positive.

G36’s comparison is already strict, so the rearranged GreaterThreshold reproduces it exactly and a gap of exactly 3.0 °C reads healthy in both. persist requires 30 minutes of continuous violation and any interruption restarts the timer; recovery is immediate on the tick the gap falls back under the threshold. Either input can open the gap — raising the supply setpoint does it as surely as a cold front.

Possible Diagnoses

Per G36 §5.16.14 Table 5.16.14.8, FC#11:

1. SAT sensor error — a supply reading biased high keeps the cooling loop calling for capacity the air does not need
2. OAT sensor error — a sensor reading high keeps the changeover logic from recognizing that free cooling would now overshoot
3. Heating coil valve leaking or stuck open — warmth upstream of the cooling coil is heat the chiller then removes, and the reason the unit needs mechanical cooling on air that arrived cold. The most expensive diagnosis here, and the one that also lights up FC#15 where that rule is deployed
4. Leaking or stuck economizer damper or actuator — a damper reported at 100% but physically short of it, or a return damper that will not close, delivers a warmer mixture than OAT implies

Energy Impact

COMFORT_ENERGY, LOW confidence, QUALITATIVE_ONLY — the reference’s §5.8.1 index row, which maps the fault to PNNL-25985 EEM-06 (OA damper faults). Mechanical cooling is doing free cooling’s job: every kilowatt at the chiller buys a temperature drop the outdoor air had already made. The waste is real and continuous but not computable here — two temperatures carry no airflow, no coil load, and no counterfactual for the correct state. An upper bound is the cooling energy spent over the fault’s duration; a host with airflow and coil data can substitute the direct measurement AHU-0017 uses. Cooling-dominant, and most common in shoulder seasons when outdoor air swings across the changeover point.

Emissions Impact

QUALITATIVE_EMISSIONS, LOW confidence, Scope 2. The waste is chiller or DX compressor electricity plus fan energy already being spent. Diagnosis 3 adds on-site combustion at the boiler, which would be Scope 1 — but that heat is inferred rather than observed, and the coil it feeds is commanded shut in OS#3, so the emissions this rule can honestly claim are the purchased-electricity ones. Avoided-emissions basis: N/A.

Deviations

• The reference card is an index row, so this card is built from G36. §5.8.1 gives the code and the name and nothing else. The equation, OS#3 applicability, four diagnoses, and internal-variable defaults are transcribed from ASHRAE Guideline 36 §5.16.14 as it appears in Addendum u to Guideline 36-2018 (first public review, 2021).
• Severity 3 is library-assigned. The reference publishes none; the value matches this chapter’s scaffold row and every other G36 001-range card here. G36’s Level 3 alarm grading (§5.16.14.16) is a reporting priority rather than a ranking, but it points the same direction.
• Energy profile follows the §5.8.1 index row (COMFORT_ENERGY / LOW / QUAL, EEM-06, savings “sensor-dependent”), and the published row wins where one exists. EXCESS_CONSUMPTION is a defensible alternative — setpoint is being met, so the unit is comfortable and wasteful rather than uncomfortable — and is recorded, not adopted. The emissions block is library-assigned.
• Combined-epsilon threshold. G36 puts eOAT on the measured side and dTSF and eSAT on the setpoint side, which would bind one card parameter to three block parameters. sat_sp − oat > eOAT + eSAT + dTSF puts one positive number on one CXF path, algebraically identical, with the composition recorded in the parameter description. Same move as AHU-0028 and AHU-0009.
• The default assumes a local OAT sensor. G36 gives eOAT as 1 °C at the unit and 3 °C for a global sensor. A site feeding this rule from a campus or weather-service OAT must set oat_deficit_threshold to 5.0 °C (3 + 1 + 1); leaving it at 3.0 makes the rule fire on sensor disagreement G36 considers within tolerance.
• No boundary deviation for this fault. FC#11’s comparison is already strict (<), unlike the ≥/≤ forms elsewhere in Table 5.16.14.8 (FC#5, FC#12, FC#14, FC#15), so GreaterThreshold on the rearranged gap reproduces the source exactly.
• Instantaneous samples instead of averaged signals. G36 compares 5-minute rolling averages sampled at 1-minute intervals; this rule compares raw samples and leans on the 30-minute persist delay. Not equivalent — persistence resets on every compliant tick, so an oscillating OAT can hide indefinitely, while the steady offset this rule is for reads the same either way. (Honesty note carried from AHU-0002.)
• Operating-state gating and NO_EVAL are frontmatter, not graph. G36 scopes FC#11 to OS#3 (§5.16.14.9c), suspends evaluation for ModeDelay after a mode change, and suspends it entirely when the AHU is off (§5.16.14.11). The engine is status-blind and the graph computes fault-given-valid-data only (precedent AHU-0029). A host that evaluates in OS#2 will see this rule assert on every cold morning — cold outdoor air with the dampers modulating is free cooling working.
• persist.delayOnInit = true (Modelica/CDL default is false): a gap already open at load waits out the full 30 minutes rather than alarming on the first tick after a controller restart. Library-wide choice, per AHU-0016.

Notes

The instructive property of this rule is only visible next to its pair. AHU-0009 and AHU-0011 test the same two points against the same epsilons and both account for the same 1 °C fan rise, but FC#9 subtracts fan heat (threshold 1.0 °C) while FC#11 adds it (3.0 °C): fan heat narrows the usable free-cooling band from the top and widens it from the bottom. A site on a global OAT sensor recomputes both — 5.0 °C here, 3.0 °C for AHU-0009 — and they do not scale together. The two cannot fire at once, since one needs OS#2 and the other OS#3; a unit that alternates between them across a day is saying the changeover point is in the wrong place or the OAT sensor is unreliable. Check that sensor before anyone edits changeover logic — step 1.2 of the economizer-failure playbook.

Test Vectors

8 scenarios, clock step 300 s over 7200 s.

{
  "schema": "cxf-library/vectors/v1",
  "clock": {
    "step_s": 300,
    "horizon_s": 7200
  },
  "scenarios": [
    {
      "name": "mech_cooling_with_mild_oa",
      "description": "Normal OS#3: 16 \u00b0C outdoor air against a 13 \u00b0C setpoint. Outdoor air is warmer than the supply target, so the dampers at 100% plus mechanical cooling on top is exactly the right arrangement.",
      "inputs": {
        "oat": 16.0,
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 7200,
          "equals": false
        }
      ]
    },
    {
      "name": "oat_far_below_setpoint",
      "description": "5 \u00b0C outdoor air against a 13 \u00b0C setpoint: the gap is 8 \u00b0C, well past the 3.0 \u00b0C threshold. Free cooling alone would overshoot, so the chiller is removing heat from air that was already cold enough. Alarms after the 30 min delay.",
      "inputs": {
        "oat": 5.0,
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 7200,
          "equals": true
        }
      ]
    },
    {
      "name": "edge_gap_equals_threshold",
      "description": "Strict boundary, clear side: sat_sp \u2212 oat = 3.0 \u00b0C exactly. GreaterThreshold is u > t, so sitting precisely on the composed epsilon-plus-fan-heat budget is not a fault.",
      "inputs": {
        "oat": 10.0,
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 7200,
          "equals": false
        }
      ]
    },
    {
      "name": "edge_gap_just_over_threshold",
      "description": "Strict boundary, fault side: sat_sp \u2212 oat = 3.1 \u00b0C clears u > t and alarms after the delay. Pins the same edge as edge_gap_equals_threshold from the other direction.",
      "inputs": {
        "oat": 9.9,
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 7200,
          "equals": true
        }
      ]
    },
    {
      "name": "fan_heat_widens_the_band",
      "description": "Teaching case for the sign of dTSF: a 2.5 \u00b0C gap reads healthy because fan heat is added to the epsilon budget (1 + 1 + 1 = 3.0). Outdoor air 2.5 \u00b0C below setpoint arrives only 1.5 \u00b0C below it after the fan, which mechanical cooling can legitimately still be trimming. An epsilon-only threshold of 2.0 \u00b0C would have called this a fault.",
      "inputs": {
        "oat": 10.5,
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 7200,
          "equals": false
        }
      ]
    },
    {
      "name": "setpoint_raised_opens_the_gap",
      "description": "Outdoor air holds at 11 \u00b0C while the setpoint is raised from 13 \u00b0C to 15 \u00b0C at t = 600 s. Weather did not change; the unit now asks for supply air that free cooling alone would overshoot. The delay runs from the setpoint step, so the alarm lands at 2400 s.",
      "inputs": {
        "oat": 11.0,
        "sat_sp": [
          {
            "t": 0,
            "value": 13.0
          },
          {
            "t": 600,
            "value": 15.0
          }
        ]
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 2100,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2700,
          "to_s": 7200,
          "equals": true
        }
      ]
    },
    {
      "name": "transient_cold_snap_never_alarms",
      "description": "A 1200 s excursion to 5 \u00b0C \u2014 shorter than the 1800 s alarm delay \u2014 then back to 16 \u00b0C. The persistence timer resets on the first compliant tick and nothing is reported.",
      "inputs": {
        "oat": [
          {
            "t": 0,
            "value": 16.0
          },
          {
            "t": 600,
            "value": 5.0
          },
          {
            "t": 1800,
            "value": 16.0
          }
        ],
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 7200,
          "equals": false
        }
      ]
    },
    {
      "name": "fault_clears_on_recovery",
      "description": "Sustained 8 \u00b0C gap alarms at 1800 s; outdoor air warms back to 16 \u00b0C at 3600 s and the alarm drops on that tick. Recovery has no delay \u2014 only the assertion does.",
      "inputs": {
        "oat": [
          {
            "t": 0,
            "value": 5.0
          },
          {
            "t": 3600,
            "value": 16.0
          }
        ],
        "sat_sp": 13.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 3300,
          "equals": true
        },
        {
          "output": "yFault",
          "from_s": 3900,
          "to_s": 7200,
          "equals": false
        }
      ]
    }
  ]
}