Temperature difference a cooling stage develops across its heat-transfer surface while it is actively operating — return vs supply air on an RTU or AHU, chilled-water return vs supply across a chiller's evaporator, or entering vs leaving water on a cooling tower. Measures how effectively the equipment moves heat — the earliest remote signal of degradation on both the air side and the water side of a chilled-water plant.
The temperature difference a cooling stage develops across its heat-transfer surface while it is actively operating. It reflects how effectively the equipment is moving heat, and it applies to three places in a cooling system — one on the air side and two on the water side of a chilled-water plant:
A healthy ΔT means heat is transferring as designed; a low or declining ΔT means it is not — the surface is fouled, the flow is wrong, or the refrigerant charge is failing. The signal reads the same way across all three, which is why one metric covers the whole cooling chain even though the equipment and the units differ.
The calculation is the same subtraction everywhere — the difference between the temperature going in and the temperature coming out — evaluated only while the equipment is running:
On the air side, when a direct compressor-on signal is not available, active cooling intervals can be approximated using power consumption data (kW above a defined threshold) or by filtering for intervals where Supply Air Temperature is significantly below Return Air Temperature (typically > 5°F difference). If only one temperature sensor is available, a proxy can be constructed using the available sensor and the space temperature, though this introduces additional uncertainty and should be noted in any reporting.
On the water side, ΔT is read directly from the loop supply/return sensors and gated on the equipment's run state (pump proven, tower on), so a stopped pump or an idle tower is never scored. Note the units differ by system: air-side ΔT is read in °F; chiller and tower ΔT are read in °C, matching how each is instrumented and specified.
Negative ΔT values almost always indicate a data or sensor issue — most commonly the two sensors swapped in the system mapping (return/supply on the air side, or entering/leaving on the water side) — and should be treated as invalid data rather than an operational reading.
The ranges below are reference examples only, split by equipment because each system has its own healthy band and its own unit. They should be reviewed and adjusted for each client and each plant based on equipment type, size, age, load, and climate zone.
Air side — RTU / AHU (°F). Multi-stage units typically show different ΔT ranges, with Stage 2 generally producing a higher ΔT due to greater cooling capacity.
| ΔT Range (°F) | Stage | Classification |
|---|---|---|
| < 0°F | Any | Invalid — sensor mapping issue. Treat as data error, not operational reading. |
| 0 – 10°F | Any | Possible issue — refrigerant, coil fouling, or extreme low-load condition |
| 10 – 18°F | Stage 1 | Expected range for single-stage or Stage 1 operation |
| 18 – 26°F | Stage 2 | Expected range for Stage 2 operation |
| > 26°F | Any | Elevated — possible airflow restriction (filter, coil, duct) |
Water side — chiller plant (°C). Evaporator and cooling-tower bands follow the chilled-water-plant reference pattern; calibrate per plant on chiller type, loop design, and load.
| Equipment | ΔT band (°C) | Classification |
|---|---|---|
| Chiller evaporator | 5 – 7 °C | Expected — chilled-water heat transfer as designed |
| Chiller evaporator | 3 – 5 °C | Warning — low flow or early evaporator fouling |
| Chiller evaporator | < 3 °C | Critical — fouled tubes, low flow, or a charge problem |
| Cooling tower | 4 – 6 °C | Expected — condenser heat rejected as designed |
| Cooling tower | 2 – 4 °C | Warning — reduced rejection (fouled fill, fan, or flow) |
| Cooling tower | < 2 °C | Critical — tower not rejecting heat effectively |
Reference ranges only. Air-side bands calibrate per client on equipment model, age, and climate zone; water-side bands calibrate per plant on chiller type, loop design, and load. A negative or near-zero ΔT on any surface is a data-quality flag first and an operational reading second.
The target percentage of sites with ΔT in the expected range should be defined and agreed upon with each client. A default of 85% is used as a starting reference.
Cooling DeltaT is one of the four HVAC optimization metrics Keedian tracks. The table below shows how moving this metric impacts each of the four customer value drivers the product is designed to improve. These are the same four levers expressed on every HVAC product page as Expected Outcomes — the metric pages explain the mechanism; the product pages express the magnitude.
| Value driver | Impact strength | How Cooling DeltaT moves this lever |
|---|---|---|
| Energy savings | Direct, strong | A degraded DeltaT means the system runs longer and harder to deliver the same cooling, burning excess kWh per ton delivered. Restoring DeltaT (coil cleaning, refrigerant correction, airflow fixes) recovers efficiency — typically one of the largest single energy recoveries on a struggling RTU. |
| Avoided truck rolls | Direct, primary | DeltaT is the canonical early-warning signal for cooling-side degradation. Catching the drift early — and grading its severity — enables a scheduled, scoped maintenance visit instead of an emergency "no cooling" dispatch. Root-cause discrimination between refrigerant loss, compressor wear and coil fouling requires the additional sensors that come with Optimized (CT clamp per RTU, refrigerant pressure, outdoor air). |
| Asset lifespan | Direct, strong | Operating at low DeltaT forces the compressor to run harder and longer to compensate; refrigerant undercharge specifically causes compressor overheating. Catching DeltaT degradation protects the most expensive RTU component and extends useful life. |
| Customer experience | Indirect, predictive | DeltaT degrades before temperature compliance fails noticeably — when capacity drops, the system can still hold setpoint for a while. Acting on DeltaT prevents the comfort failure that would otherwise arrive days later. |
The table below summarizes the principal alarms that fire directly from Cooling DeltaT. Each row links to the full operational detail (trigger, preconditions, action plan, human role, linked SOPs) in the SOPs catalog. These are single-metric alarms only — composite FDD that combines multiple metrics with control state and weather context will appear in a future release.
The alarms below are air-side (RTU / AHU). On the water side, evaporator and cooling-tower ΔT are surfaced on the central plant through the Chiller Plant Operational Compliance and Plant Efficiency (kW/ton) metrics, carried by the Chiller Plant Monitoring add-on.
| Alarm | Description | Severity | Tier | AI executes? | Value drivers | SOP |
|---|---|---|---|---|---|---|
| Low DeltaT | DeltaT below the floor during active cooling — refrigerant-side / capacity family. | High | Essential | Hybrid | Energy savings · Asset lifespan · Avoided truck rolls · Customer experience | Open SOP → |
| High DeltaT | DeltaT above the ceiling during active cooling — airflow-side (clogged filter, blower, dampers, ducting). | Medium | Essential | Hybrid | Energy savings · Customer experience | Open SOP → |
Roadmap (single-metric): DeltaT degradation trends — sustained downward drift vs site baseline that surfaces refrigerant slow leak and coil fouling earlier than the threshold breach. Composite alarms in development (multi-metric FDD): refrigerant slow leak, coil ice, economizer stuck closed, capacity vs load mismatch, compressor failure imminent.
The action plan for each alarm lives on its own SOP page in the SOPs catalog — with the diagnostic steps, human role, value drivers, and escalation criteria specific to that alarm. The list below maps each ΔT-derived alarm to its SOP; this page keeps only the cross-alarm items that don't belong to a single SOP (data quality, external context).
If ΔT values look implausible, escalate internally to the technical team before treating any alarm as real. Common issues to investigate: sensors swapped in the integration mapping, ΔT calculated without filtering for active cooling intervals, single faulty sensor reporting incorrect values. Do not report the affected site as non-compliant until the data issue is confirmed or ruled out.
If ΔT readings reflect low-load conditions (mild weather, off-peak hours) rather than equipment behavior, note the context and monitor over the following month. If ΔT returns to the expected range as conditions change, no action is needed. Include a brief note in the MBR if the pattern was visible at the portfolio level.
Per-alarm escalation criteria live in the Escalation block of each SOP in the SOPs catalog. The patterns below are metric-level — they are read from the portfolio view, not from any single alarm firing, and don't belong to a single SOP.
Escalation criteria and communication protocols should be defined as part of the operational agreement with each client. The guidelines below are a general reference and should be adapted to the specific terms, SLAs, and relationship dynamics in place for each account.
Temperature difference a cooling stage develops across its heat-transfer surface during confirmed active operation — return vs supply air on an RTU/AHU (°F), chilled-water return vs supply across a chiller evaporator (°C), or entering vs leaving water on a cooling tower (°C). Measures how effectively the equipment moves heat — low or declining ΔT may indicate fouled coils or tubes, low flow, or a failing charge; on the air side, high ΔT may signal airflow restriction. Negative values always indicate invalid data and are caught by automated alert. Reviewed monthly in MBR and weekly for worst performers.
SOPs linked to this metric will be documented here. Each SOP will describe the step-by-step operational procedure for a specific scenario identified in the Alarms and Actions sections above.