AHU-0014 — Inactive cooling coil temperature drop

Preconditions (host-enforced): Supply fan running, and the unit in one of the two states where G36 requires the cooling coil to be off: OS#1 heating (Table 5.16.14.2: HC > 0, CC = 0, OA damper at minimum) or OS#2 free cooling (HC = 0, CC = 0, minimum < OA damper < 100%). In OS#3 and OS#4 the coil is commanded to cool and a drop across it is the intended result, not a fault. Suspend evaluation for ModeDelay (30 min) after any mode or operating-state change in a zone group the AHU serves, and whenever the AHU is not operating (G36 §5.16.14.11) — a coil coasting down still shows its drop. This binding reads the coil through MAT and SAT, so a unit with no MAT sensor cannot run the rule as shipped: install dedicated coil sensors and rebind, or omit. Silence the rule while AHU-0028 is active: a MAT outside the OAT/RAT envelope is not a coil entering temperature. When any gate is unmet the verdict is NO_EVAL, not healthy.

Points: mat, sat

Outputs:

• yFault — True while mat has stayed more than coil_drop_threshold above sat for at least alarm_delay

Parameters:

Description

In OS#1 and OS#2 the cooling coil is closed by definition — G36 identifies both states partly by CC = 0. Air crossing the unit meets the supply fan and nothing else, so it reaches the supply sensor about a degree warmer than it left the mixing box. When SAT instead reads several degrees below MAT, something is pulling heat out of the stream: chilled water past a valve that reports itself shut, or a DX circuit that never got the message to stop.

The waste is worst in OS#1, where every kilowatt the leaking coil removes is a kilowatt the heating coil is paid to put back — the AHU-0016 failure arriving through a different door. In OS#2 there is no heating bill, but chilled water is still being made and pumped for air the economizer was cooling for nothing. This rule and AHU-0015 are AHU-0016’s silent siblings: that rule reads the two valve commands and needs both past 5% open, so a valve reporting 0% and flowing anyway is invisible to it. This pair reads the temperature signature and does not care what the command says.

Detection Logic

drop   = mat − sat
yFault = drop > coil_drop_threshold,
         sustained continuously for alarm_delay

Block graph (rule.cxf.jsonld):

G36 writes the test as CCET_AVG − CCLT_AVG ≥ sqrt(eCCET² + eCCLT²) + ΔTSF*, footnoting the fan-heat factor as included or not depending on where the coil sensors sit. This library binds CCET := mat and CCLT := sat — the instrumentation most air handlers actually have — which brings the proxied epsilons (3 °C mixed-air, 1 °C supply-air, root-sum-square 3.1623 °C) and puts the supply fan inside the measurement, so the ΔTSF term applies: 3.1623 + 1 = 4.1623 °C.

Follow that arithmetic, because for a drop the fan works against the signal. Fan heat and coil cooling move the air in opposite directions, so mat − sat measures the true coil drop minus one dTSF and a measured 4.1623 °C is a real drop of about 5.16 °C. The shipped default is therefore doubly conservative — the direction the addendum says its defaults are chosen for — and sites that want the sharper test retune (see Deviations).

The comparison is strict, so a drop sitting exactly on 4.1623 °C reads healthy where G36 would report the fault. persist requires 30 continuous minutes and any interruption restarts the timer, which separates a leaking valve from a coil giving up the chilled water still standing in it after a state change.

Possible Diagnoses

Transcribed from G36 §5.16.14 FC#14:

1. CCET sensor error
2. CCLT sensor error
3. Cooling coil valve stuck open or leaking
4. DX cooling stuck on

Under this library’s binding, diagnoses 1 and 2 read as MAT and SAT sensor error, and they are the cheap ones to eliminate first. Diagnosis 3 dominates in the field and is why the card carries the stuck-actuator playbook: a two-way valve whose seat has eroded, or an actuator that has lost its close position, passes water at a command of 0% and no command-based rule will ever see it. Diagnosis 4 is the DX equivalent — a stuck contactor or a compressor a local safety has latched on.

Energy Impact

CRITICAL_WASTE, HIGH confidence, DIRECT_MEASUREMENT, savings 0.5–5% of site energy mapped to PNNL EEM-03 (leaking coil valves) — the §5.8.1 index row, the only energy statement the reference makes here. DIRECT_MEASUREMENT is honest in a way it is not for the abbreviated comparison rules: the two temperatures the rule already reads are the measurement.

waste_kw = supply_airflow_m3s × 1.2 × 1.005 × ((mat − sat) + dTSF)

The fan’s rise is added back because the measured drop under-reports the coil’s work by that much; design airflow is the one substitution. HIGH confidence because a sustained drop across a coil commanded shut has no benign explanation other than a sensor, and the sensor case shows up as a drop that does not move with load. Heating-dominant despite being a cooling fault, following the operating states: OS#1 is a heating state, and the hours a leaking chilled-water valve does the most damage are the hours a heating coil is fighting it.

Emissions Impact

PROXY_EMISSIONS, scope 1+2, both library-assigned since the §5.8.1 index publishes no emissions column. The unwanted cooling is purchased electricity at the chiller or DX compressor (Scope 2), and in OS#1 the heating that cancels it follows whatever the plant burns — Scope 1 for gas, Scope 2 for electric resistance or a heat pump. On an all-electric site the exchange collapses to Scope 2, and when the cause is a sensor there is nothing to attribute. Avoided-emissions basis: marginal operating emissions rate (MOER) for the electric half, static combustion factor for the fuel half.

Deviations

• The reference card is abbreviated; G36 is the normative text. The HVAC FDD Reference carries AHU-0014 only as a §5.8.1 index row — no equation, internal variables, vectors, severity, diagnoses, or preconditions. Detection logic and the diagnosis list are transcribed from ASHRAE Guideline 36 §5.16.14 FC#14 as it appears in Addendum u to Guideline 36-2018 (First Public Review, 2021).
• CCET and CCLT are bound to MAT and SAT. G36 leaves the instrumentation open (§5.16.14.5) and Table 5.16.14.5 supplies the proxied epsilons. The consequence is that the rule sees the whole air path from the mixing box to the supply sensor: the fan is inside the measurement (handled by dTSF) and so is any duct heat gain between coil and sensor (not handled — it biases the drop downward and makes the rule quieter still). A site with dedicated coil sensors rebinds the two boundary inputs at deployment and retunes coil_drop_threshold with its own sensor errors; because the fan is then outside the pair, that retune also drops the dTSF term.
• The fan-heat term is included, and for this fault it works against the signal. A measured 4.1623 °C at the threshold is a true coil drop of 5.16 °C, where the sensor bands alone would justify reporting at 3.16 °C — the shipped default demands a leak 63% larger than the noise floor does, and the cost is real misses of modest leaks. Two worked retunes for sites that want the sharper test: dedicated sensors bracketing the coil with the fan outside the pair drop the term entirely (3.1623 with a 3 °C entering band, ≈1.41 with a matched ±1 °C pair); keeping the mat/sat binding but accepting G36’s noise floor on the true drop gives sqrt(10) − 1 = 2.1623.
• G36’s ≥ becomes a strict >. CDL Reals offers only strict comparisons, so a drop of exactly 4.1623 °C reads healthy where G36 reports the fault. Measure zero on a real temperature signal, and it errs toward silence. A host binding coarsely quantized temperatures should retune the threshold down rather than rely on the signal overshooting.
• The threshold is a rounded constant, not a root-sum-square computed in the graph. sqrt(3² + 1²) + 1 = 4.16227766…, shipped as 4.1623 — high by 2.2 × 10⁻⁵ °C, four orders of magnitude below the resolution of the sensors feeding it, and one number to retune instead of three.
• Instantaneous samples instead of 5-minute rolling averages. G36 computes every §5.16.14 signal as a 5-minute rolling average with 1-minute sampling; this library consumes instantaneous points and lets the 30-minute AlarmDelay stand in. Not equivalent — persistence resets on every compliant tick, so an oscillating drop (a short-cycling DX stage) can hide indefinitely, while the steady leak of a failed valve seat reads the same either way. (Honesty note from AHU-0002.)
• Operating states, ModeDelay, and the not-operating suspension are host-side preconditions. G36 scopes FC#14 to OS#1–#2 and suspends evaluation after a mode change in a served zone group and whenever the AHU is off; none of it is in the graph, per the library’s stance. G36 attaches no “omit if no MAT sensor” qualifier to FC#14 — it contemplates dedicated coil sensors — but this library’s binding needs MAT, so the qualifier applies to the shipped rule and lives in preconditions.
• Severity 2 is the library’s. The §5.8.1 index carries no severity column. Severity 2 puts this fault with AHU-0016 and AHU-0020 rather than the 001-range comparison rules at 3, which is where CRITICAL_WASTE and HIGH confidence point. G36’s Level 3 alarm grading is a priority scheme, not this library’s 1–4 scale.
• The energy profile is the index row’s; the runtime formula, climate sensitivity, and emissions block are the library’s, reasoned from the operating states the fault is evaluated in.
• persist.delayOnInit = true (Modelica/CDL default is false), the library’s standing choice: a violation already present at load waits out the full 30 minutes instead of alarming on the first tick after a controller restart.

Notes

In OS#1 this rule overlaps AHU-0005, which tests the same two sensors in the same direction against a narrower 3.0 °C threshold and therefore alarms first on any leak large enough to trip both; the value of this rule there is its diagnosis list, which names the cooling coil and the DX circuit directly. It stands alone in OS#2. The threshold asymmetry between the two is about how sensor bands compose — linearly for AHU-0005, in quadrature here — and the fan-heat term then moves opposite ways, leaving this rule the quieter despite the tighter bands. Start at the sensors, since a MAT reading high or a SAT reading low produces this trace with nothing wrong in the mechanical room; an active AHU-0028 should already be suppressing the rule. Then isolate the coil and watch the drop disappear, and let the stuck-actuator playbook separate a failed actuator from an eroded seat.

Test Vectors

9 scenarios, clock step 300 s over 5400 s.

{
  "schema": "cxf-library/vectors/v1",
  "clock": {
    "step_s": 300,
    "horizon_s": 5400
  },
  "scenarios": [
    {
      "name": "heating_state_with_the_cooling_coil_off",
      "description": "OS#1: a 18 \u00b0C mixture leaves the unit at 19 \u00b0C. The only thing the air crossed between the two sensors was the supply fan, so it gained dTSF = 1 \u00b0C and the cooling coil took nothing back. `drop` = mat \u2212 sat = \u22121 \u00b0C, the healthy signature of an inactive cooling coil on a draw-through unit",
      "inputs": {
        "mat": 18.0,
        "sat": 19.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 5400,
          "equals": false
        }
      ]
    },
    {
      "name": "drop_exactly_at_threshold",
      "description": "Threshold edge: mat \u2212 sat = 4.1623 \u00b0C exactly, the composed sqrt(eCCET\u00b2 + eCCLT\u00b2) + dTSF allowance. `dropBig` is a strict `>`, so a drop sitting precisely on the allowance reads healthy \u2014 G36's `\u2265` would report it (see Deviations)",
      "inputs": {
        "mat": 20.0,
        "sat": 15.8377
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 5400,
          "equals": false
        }
      ]
    },
    {
      "name": "drop_just_over_threshold",
      "description": "Threshold edge, other side: mat \u2212 sat = 4.2623 \u00b0C clears the strict comparison on the first tick and the alarm asserts one alarm_delay (1800 s) later",
      "inputs": {
        "mat": 20.0,
        "sat": 15.7377
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 5400,
          "equals": true
        }
      ]
    },
    {
      "name": "chilled_water_valve_leaking_during_heating",
      "description": "OS#1 with the cooling valve commanded 0%: a 20 \u00b0C mixture arrives at the supply sensor at 12 \u00b0C. Adding the fan's own 1 \u00b0C rise back, the coil is pulling roughly 9 \u00b0C out of air the heating coil is simultaneously paying to warm. AHU-0016 cannot see this \u2014 the cooling command reads zero and the water is flowing anyway",
      "inputs": {
        "mat": 20.0,
        "sat": 12.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 5400,
          "equals": true
        }
      ]
    },
    {
      "name": "sat_sensor_reading_low",
      "description": "Same 6 \u00b0C signature with the chilled-water plant off for the season and the valve verified shut: nothing is removing heat, and the supply sensor is what is wrong. The rule reports the contradiction and cannot say which of the two sensors is lying \u2014 G36 lists both sensor errors ahead of the valve in its diagnosis order",
      "inputs": {
        "mat": 21.0,
        "sat": 15.0
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 5400,
          "equals": true
        }
      ]
    },
    {
      "name": "modest_drop_below_shipped_default",
      "description": "A real but small leak: measured mat \u2212 sat = 3.5 \u00b0C, which after adding the fan's 1 \u00b0C rise back is a true coil drop of about 4.5 \u00b0C. The shipped 4.1623 \u00b0C threshold is measured against mat \u2212 sat, so it demands a true drop of ~5.16 \u00b0C and stays silent here. A site that retunes to sqrt(10) \u2212 1 = 2.1623 (keeping the mat/sat binding but dropping the fan-heat term from the allowance) catches this leak; the shipped default is deliberately the quieter one",
      "inputs": {
        "mat": 20.0,
        "sat": 16.5
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 5400,
          "equals": false
        }
      ]
    },
    {
      "name": "transient_clears_before_alarm_delay",
      "description": "Transient: an 8 \u00b0C drop persists for 1200 s while chilled water still standing in a just-closed coil boils off, then SAT returns to mat + fan heat. Anything shorter than alarm_delay never reaches the operator",
      "inputs": {
        "mat": 20.0,
        "sat": [
          {
            "t": 0,
            "value": 12.0
          },
          {
            "t": 1200,
            "value": 19.0
          }
        ]
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 5400,
          "equals": false
        }
      ]
    },
    {
      "name": "fault_clears_on_recovery",
      "description": "A sustained 8 \u00b0C drop alarms at 1800 s; the leaking valve is isolated at t = 3000 and SAT climbs back to mat + fan heat. `TrueDelay` has no off-delay, so the alarm drops on that tick",
      "inputs": {
        "mat": 20.0,
        "sat": [
          {
            "t": 0,
            "value": 12.0
          },
          {
            "t": 3000,
            "value": 19.0
          }
        ]
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 1500,
          "equals": false
        },
        {
          "output": "yFault",
          "from_s": 2100,
          "to_s": 2700,
          "equals": true
        },
        {
          "output": "yFault",
          "from_s": 3300,
          "to_s": 5400,
          "equals": false
        }
      ]
    },
    {
      "name": "oscillating_drop_never_alarms",
      "description": "A DX stage short-cycling on a 10-minute period swings the drop between 8 \u00b0C and \u22121 \u00b0C. Its mean is well outside the allowance and G36's 5-minute rolling average would report the fault, but persistence restarts on every compliant tick, so this rule stays silent \u2014 the limit of substituting TrueDelay for averaging (see Deviations)",
      "inputs": {
        "mat": 20.0,
        "sat": [
          {
            "t": 0,
            "value": 12.0
          },
          {
            "t": 600,
            "value": 21.0
          },
          {
            "t": 1200,
            "value": 12.0
          },
          {
            "t": 1800,
            "value": 21.0
          },
          {
            "t": 2400,
            "value": 12.0
          },
          {
            "t": 3000,
            "value": 21.0
          },
          {
            "t": 3600,
            "value": 12.0
          },
          {
            "t": 4200,
            "value": 21.0
          },
          {
            "t": 4800,
            "value": 12.0
          }
        ]
      },
      "expect": [
        {
          "output": "yFault",
          "from_s": 0,
          "to_s": 5400,
          "equals": false
        }
      ]
    }
  ]
}