BAS Sensors: 7 Hidden Lies Wrecking Your Comfort

The most expensive controls system in the world will do nothing useful if the sensors lie. And sensors lie all the time. Usually it’s because they were spec’d wrong, installed wrong, or never recalibrated.

Here’s the short tour. We’ll cover what’s on a modern BAS sensor list. We’ll also cover what each one does and where each one fails.

Temperature sensors

These are the workhorse. Most BAS temperature sensors today use 10kΩ Type II or Type III thermistors. RTDs (1kΩ platinum is common) also show up.

They’re accurate enough for HVAC control. Typical accuracy is ±0.4°F. However, they drift over time. They drift especially fast in poor locations.

For example, a return-air sensor mounted next to a variable frequency drive reads 78°F. The system thinks the zone is warm. As a result, cooling stays on. Meanwhile, the actual space is 70°F. So always check sensor placement against the original sequence of operations.

Humidity sensors

These cost more and drift faster. Capacitive RH sensors are standard. New ones spec at ±2–3% RH. After 3–5 years they routinely drift to ±5–8% RH.

For applications where humidity actually matters, annual calibration is mandatory. Examples include hospitals, labs, museums, and data centers. For typical commercial offices, drift to ±10% RH usually doesn’t change comfort much. Our healthcare controls post covers the clinical-space requirements in more detail.

CO₂ sensors

You use these for demand-controlled ventilation (DCV). DCV modulates outside air based on occupancy. NDIR is the standard technology.

Accuracy is around ±50 ppm or ±5% of reading. However, the sensors need an auto-calibration routine. They need to see fresh outdoor air sometimes.

If you install a CO₂ sensor in a sealed space, calibration drifts. As a result, DCV stops working correctly. Title 24 has specific DCV requirements for densely occupied spaces. An uncalibrated CO₂ sensor will fail acceptance testing.

Occupancy sensors

PIR (passive infrared) detects motion. Ultrasonic detects displacement. Dual-tech combines both to reduce false positives.

Newer optical (camera-based) counting is more accurate. However, it costs more and raises privacy questions. For BAS purposes, the real question is rarely “is the room occupied.” Instead, it’s “is the room occupied enough to justify conditioning it.”

Indoor air quality (IAQ) sensors

The post-pandemic shift here is real. PM2.5, TVOC, and formaldehyde sensors are now affordable at the zone level. They’re useful for occupant dashboards and IAQ-driven ventilation strategies.

However, they’re less accurate than lab-grade instruments. A $200 PM2.5 sensor isn’t a regulatory tool. So use them for trends, not for compliance.

Pressure sensors

These are mission-critical in hospitals, labs, and clean rooms. ASHRAE 170 and CDC guidance specify pressure differentials for isolation rooms, ORs, and pharmacies.

Differential pressure transducers are standard. Ranges are typically 0.01″ to 1.0″ w.c. for HVAC. These need annual calibration in any critical-pressurization space. The certificates are part of the regulatory audit trail.

The takeaway

Spec the right sensor. Install it in the right place. Calibrate it on a known cadence. Then your control logic actually does what you wrote it to do. Skip any of those steps, and you’re tuning an instrument that’s lying to you.

Talk to Signet Controls. Planning a BAS install, retrofit, integration, or service contract in California? We work across Los Angeles, Orange County, the Inland Empire, the Central Coast, and Kern County. Reach our team at info@signetcontrols.com or call (877) 874-4638.

References[1] ASHRAE — Health Care Facilities Resources (Standard 170) — https://www.ashrae.org/technical-resources/bookstore/health-care-facilities-resources