There is a basic principle that everyone in controls work understands but that rarely makes it into the conversation with building owners: your BAS is only as good as the data it receives. You can have the most sophisticated control sequences, the best supervisory platform, and the most carefully tuned PID loops in the industry. If the sensors feeding that system are reading inaccurately, you are making expensive decisions based on bad information. We see this every week on service calls, and the cost to building owners is real.
The Sensor Types You Are Working With
Commercial building automation systems use several categories of sensors, each with its own failure modes.
Temperature sensors are the most common. The two types you will encounter most often are RTDs (resistance temperature detectors, usually Pt1000 elements) and thermistors (10K ohm elements are standard in building controls). Both work on the principle that electrical resistance changes with temperature, and both are generally reliable. The failure mode is usually drift, not sudden failure, which makes it harder to catch.
Pressure transducers for duct static pressure, building static pressure, and differential pressure across filters and coils typically output a 4-20mA signal proportional to a calibrated pressure range. A transducer that has zeroed out will read near the bottom of its range regardless of actual conditions. One that has shifted its calibration will read consistently high or low, causing the BAS to modulate fans incorrectly.
CO2 sensors for demand-controlled ventilation are among the most commonly neglected sensors in commercial buildings. A good CO2 sensor needs periodic calibration against a known reference gas. Most manufacturers specify annual calibration. Most buildings never do it. We have found CO2 sensors reading 600 ppm in an occupied conference room when the actual concentration was over 1,500 ppm, which means the ventilation dampers were throttling back on a space that was severely under-ventilated.
Humidity sensors are particularly relevant in our climate. Capacitive humidity sensors drift over time and are sensitive to contamination. A humidity sensor that reads 5-10% RH low will cause a dehumidification sequence to underperform, with mold risk as the consequence in Louisiana's climate.
What Drift Actually Costs You
The scenario we see most often is a mixed-air temperature sensor, a discharge air temperature sensor, or a zone temperature sensor that has drifted 3-5 degrees from its actual value. That range is meaningful because it typically falls within the range where the system continues to operate without generating a fault alarm, so nobody catches it. The BAS just compensates, usually by running equipment harder than it needs to.
A zone temperature sensor reading 3 degrees low means the BAS thinks the space is 68 degrees when it is actually 71 degrees. The system keeps cooling. In a 50-zone building in the middle of a Louisiana summer, that kind of systematic error across multiple sensors translates to a measurable increase in compressor runtime. Run the numbers on three additional hours of chiller operation per day over a cooling season, and a fifty-dollar sensor problem costs you thousands.
This is exactly why sensor calibration is a core part of every commissioning walkthrough we do, as I described in our post from a few weeks ago covering the full commissioning process. You cannot skip that step and trust that your sensors are reading correctly out of the box or after years of field use.
Common Failures and How We Find Them
Drifted temperature sensors are found by comparing the sensor reading in the BAS to a calibrated reference thermometer placed at the sensor location. A deviation of more than half a degree on a properly calibrated sensor indicates a problem. We find sensors three to five degrees off with some regularity on buildings that have not had a commissioning visit in several years.
Stuck pressure sensors are usually caught by checking the reading against known conditions. A duct static pressure sensor reading 0.8 inches water column in a fan system that is known to be off is obviously wrong. But a sensor reading 0.45 inches when the actual pressure is 0.65 inches will not alarm, and the fan will speed up unnecessarily trying to maintain setpoint.
Uncalibrated CO2 sensors are identified by comparing the sensor output to a portable reference meter. The gap between an unserviced sensor and actual conditions is often surprisingly large, enough to cause meaningful ventilation errors.
Sensor Placement Is Half the Battle
Even a perfectly calibrated sensor gives bad data if it is in the wrong location. We have seen zone temperature sensors mounted directly across from HVAC supply diffusers, which means they are reading supply air temperature rather than room temperature. We have found outdoor air temperature sensors in locations that receive direct afternoon sun, reading 10-15 degrees above actual ambient temperature and causing the system to think it is much hotter outside than it is.
Proper placement follows a few basic rules. Space temperature sensors belong on interior walls, away from windows, supply air diffusers, and heat-generating equipment. Outdoor air sensors need proper sun shields and should be located where they see representative ambient conditions, not the exhaust side of a condenser. Duct-mounted sensors need to be far enough downstream from mixing points that they are seeing well-mixed air rather than stratified layers.
Mixed-air temperature sensors, measuring the blend of return air and outdoor air entering the air handler, require special attention. Stratification in the mixing plenum is common, and a single sensor may not represent the average. When we see systems where the BAS consistently reads a mixed-air temperature that does not match what the supply temperature and load would suggest, poor sensor placement is often the explanation.
A BAS with good sequences and bad sensors is going to perform poorly in ways that are very difficult to diagnose without systematically checking the instrumentation. We always start with the sensors when something is not working right.