Overview
In a large commercial building, HVAC systems account for 40–60% of total energy consumption. Even a 1% improvement in system efficiency generates significant operating cost savings over a building's lifetime. Sensor accuracy is one of the highest-leverage, lowest-cost levers available to engineers designing or upgrading HVAC control systems.
Quantified Impact of Temperature Sensor Accuracy on Chiller COP
A water-cooled centrifugal chiller's Coefficient of Performance (COP) is approximately:
Chiller COP Relationship
COP = Q_evap / W_compressor ≈ T_evap / (T_cond − T_evap) × η_Carnot. For T_evap = 7°C (280K) and T_cond = 35°C (308K): Carnot COP = 280 / (308 − 280) = 10.0. Real chiller COP ≈ 6.0–7.5 with η ≈ 0.65. A 1°C error in the supply chilled water temperature setpoint causes the chiller to operate at a non-optimal lift, reducing COP by approximately 2–3%.
Temperature Sensor Error Sources in HVAC
For a chilled water supply temperature sensor, total measurement error includes:
- Sensor calibration error: ±0.2°C (precision NTC) to ±1.0°C (low-cost bimetal)
- Sensor placement error: ±1–5°C if positioned in a low-velocity zone or near a stratification layer in the pipe
- Electrical interference: ±0.5°C if thermocouple signal runs parallel to high-current cables without shielding
- Sensor drift: ±0.1–0.5°C per year (NTC without periodic recalibration)
Total error can easily exceed ±2°C in a poorly designed installation — equivalent to 4–6% chiller efficiency loss.
Differential Pressure Sensor Accuracy and Fan Energy
VAV (Variable Air Volume) box pressure sensors control discharge air damper position to maintain set air volume. A ±5 Pa error in a 50 Pa setpoint (10% relative error) results in 10% over-ventilation of that zone — wasting fan energy and thermal conditioning. At building scale (500 VAV boxes), this represents 5–10% excess fan energy consumption.
ASHRAE 90.1 Sensor Accuracy Requirements
| ASHRAE 90.1 Requirement | Minimum Sensor Accuracy |
|---|---|
| Supply and return temperature monitoring | ±0.5°C |
| Outdoor air temperature | ±0.5°C |
| Differential pressure (duct static) | ±25 Pa or ±5% of reading (whichever is larger) |
| Chilled water ΔT (reset strategy) | ±0.2°C for temperature differential calculation |
| Humidity (demand-controlled ventilation) | ±5% RH |
CO₂ Sensor Integration for Demand-Controlled Ventilation
Demand-controlled ventilation (DCV) uses CO₂ concentration as a proxy for occupancy. Reducing outdoor air intake when a space is unoccupied can save 20–40% of ventilation energy. CO₂ sensors must be specified carefully:
- NDIR (Non-Dispersive Infrared): ±50 ppm accuracy, stable long-term — preferred for DCV
- Electrochemical: lower cost, but ±200 ppm and 3–5 year sensor replacement — not recommended for DCV accuracy requirements
- Placement: mount at 1.1–1.7 m height, away from supply air diffusers (dilution effect) and returns (concentration effect)
- ASHRAE 62.1 threshold: 700 ppm above outdoor baseline CO₂ (~1100 ppm total) indicates full occupancy design condition
Improve Your HVAC System Efficiency with Precision Sensors
Our engineering team can help you select sensors that meet ASHRAE 90.1 requirements and maximise your building's energy performance.
People Also Read
Sensor Accuracy and Energy Efficiency
How sensor accuracy reduces industrial energy waste across VSD pumps and compressors.
10 min read
Read MoreIndustrial HVAC Sensors
Pressure and temperature sensors for industrial HVAC and building management.
10 min read
Read MoreAutomotive HVAC Temperature Sensors
NTC, RTD, and thermopile sensors for automotive climate control.
12 min read
Read More