What temperature sensors should I specify for an AHU?
Temperature Sensor Specification for Air Handling Units
Specifying the right temperature sensors for an air handling unit (AHU) is more than selecting a sensor type — it requires careful attention to placement, accuracy class, time constant, and environmental protection. A poorly placed or incorrectly specified sensor can undermine the most sophisticated control sequence.
### Essential Sensor Positions
Every AHU requires at minimum four temperature measurement points. The **outdoor air temperature sensor** must be an NTC thermistor or Pt1000 RTD rated for -40°C to 60°C, mounted in a naturally aspirated or fan-aspirated solar radiation shield on the north-facing building elevation. Without a shield, direct solar radiation can introduce errors of 5–8°C, completely corrupting economiser changeover decisions and free cooling availability calculations.
The **mixed air temperature sensor** must be an averaging type — a flexible capillary or multiple-point sensor spanning the full duct cross-section downstream of the mixing box. Stratification in the mixed air plenum is common, especially when outdoor and return air temperatures differ by more than 10°C. A single-point sensor in a stratified mixed air stream can read 3–5°C above or below the true average, leading to incorrect coil staging and potential freeze risk.
The **supply air temperature sensor** is the most critical for control. It must be positioned at least 3–4 duct diameters downstream of the last coil to ensure complete mixing, and installed in a thermowell if the airstream may carry condensate or moisture. Platinum RTDs (Pt1000, Class A, ±0.15°C at 0°C) are preferred over thermistors for supply air because they maintain calibration across the full operating range without drift — essential when the sensor drives economiser, cooling, and heating sequences.
The **return air temperature sensor** can be a standard room or duct-mount sensor, typically located in the main return duct or ceiling plenum before the return fan. In buildings with multiple return paths, an averaging sensor array may be warranted.
### Accuracy and Time Constant
Per ASHRAE Guideline 36, all AHU temperature sensors should have a time constant of less than 30 seconds to ensure the control loop can respond to real process changes rather than lagged readings. Sensor accuracy should be ±0.3°C or better at the control setpoint — typically 12–13°C for supply air in VAV systems. Regular calibration verification against a certified reference thermometer should be part of the annual preventative maintenance schedule.
AHU Temperature Sensor Specification Guide
Recommended sensor types, accuracy, and placement requirements for each AHU temperature measurement location.
| Sensor Location | Recommended Type | Accuracy Requirement | Mounting Requirement | Additional Protection |
|---|---|---|---|---|
| Outdoor Air | Pt1000 RTD or NTC thermistor | ±0.3°C | Solar radiation shield, north-facing | Weatherproof enclosure IP65 minimum |
| Mixed Air | Averaging capillary (6–12 m) | ±0.5°C | Full duct cross-section coverage | None — internal to AHU |
| Supply Air | Pt1000 RTD Class A | ±0.15°C at 0°C | Thermowell, 3–4 duct diameters downstream of coil | Condensate-proof connection head |
| Return Air | Pt1000 or NTC 10k Type II | ±0.3°C | Return duct or ceiling plenum | Perforated guard if in occupied space |
| Preheat Discharge (optional) | Pt1000 RTD | ±0.2°C | Immediately downstream of preheat coil | High-temperature rated to 80°C |
🔑 Key Takeaways
- ✓Outdoor air sensors require aspirated solar radiation shields mounted on north-facing elevations; unshielded sensors can read 5–8°C high in direct sun.
- ✓Mixed air sensors must be averaging types spanning the full duct cross-section — single-point sensors cannot detect stratification.
- ✓Supply air sensors should be Pt1000 RTDs with ≤30-second time constant, installed 3–4 diameters downstream of the last coil in a thermowell.
- ✓Sensor accuracy of ±0.3°C or better at the control setpoint is essential for stable economiser and coil staging sequences per Guideline 36.
- ✓Annual calibration verification against a certified reference should be part of the preventative maintenance programme.