Sensors & Physical Point Recognition

Sensor in BAS

A device that tells the controller what is happening in the real system.

Sensor vs Actuator

A sensor reports information to the controller. An actuator physically moves or controls something.

Why sensors are inputs

Sensors are inputs because the controller reads them.

Analog Sensor

A sensor that reports a changing value like temperature, pressure, humidity, or CO2.

Digital Sensor or Switch

A device that reports only normal/alarm, open/closed, or on/off.

Most common analog inputs in HVAC

Temperature, pressure, humidity, CO2, airflow, water temperature.

Most common digital inputs in HVAC

Fan status, smoke detector, freeze stat, flow switch, safeties, alarms.

2-wire temperature sensor

Usually a thermistor or RTD wired to an Analog Input.

3-wire sensor

Usually has power, common, and signal wires.

0-10V sensor signal

An analog signal where voltage represents a measured value.

4-20mA sensor signal

An analog current signal commonly used for pressure, flow, or industrial sensors.

Thermistor

A temperature sensor whose resistance changes with temperature.

RTD

A temperature sensor whose resistance changes predictably with temperature, often used in more precise applications.

Open sensor circuit symptom

The controller may read an extremely high, low, or invalid value.

Shorted sensor circuit symptom

The controller may read an unrealistic extreme value.

Bad sensor vs bad wiring

A bad reading could come from the sensor, wiring, controller input, scaling, or configuration.

Supply Air Temperature Sensor (SAT)

Type: AI. Measures air leaving the unit. Usually located in the supply air stream after the coil.

Discharge Air Temperature Sensor (DAT)

Type: AI. Measures air discharged from equipment. Often similar to SAT.

Return Air Temperature Sensor (RAT)

Type: AI. Measures air returning from the building to the AHU or RTU.

Mixed Air Temperature Sensor (MAT)

Type: AI. Measures air after outside air and return air mix.

Outside Air Temperature Sensor (OAT)

Type: AI. Measures outdoor ambient temperature. Usually mounted outside or on the RTU exterior.

Space Temperature Sensor

Type: AI. Measures room or zone temperature.

Zone Temperature Sensor

Type: AI. Same general idea as space temperature; used for VAV zones or rooms.

Duct Temperature Sensor Appearance

Usually a small probe inserted into the duct with a small sensor head mounted outside the duct.

Averaging Temperature Sensor

A longer flexible sensor used across a large duct or coil face to measure average air temperature.

Strap-On Water Temperature Sensor

A sensor strapped to the outside of a pipe to estimate water temperature.

Immersion Temperature Sensor

A sensor inserted into a water pipe through a temperature well.

Temperature Well

A metal pocket in a pipe that lets a temperature sensor measure water temperature without touching the water directly.

Humidity Sensor

Type: AI. Measures relative humidity in a space, duct, or outside air stream.

CO2 Sensor

Type: AI. Measures carbon dioxide level, often used for ventilation control.

CO2 Sensor Purpose

Helps the controller decide how much outside air ventilation is needed.

Duct Static Pressure Sensor

Type: AI. Measures air pressure in the supply duct. Often connected with small plastic tubing.

Duct Static Pressure Transmitter Appearance

Small rectangular device with wires and small pressure tubing connected to the duct.

Filter Differential Pressure Sensor

Type: AI or DI. Measures pressure difference across filters to detect dirty filters.

Filter DP Switch

Type: DI. Trips when filter pressure drop exceeds a set limit.

Filter DP Transmitter

Type: AI. Reports the actual pressure difference across filters.

Airflow Sensor

Type: AI. Measures air volume, often in CFM.

VAV Airflow Sensor

Type: AI. Measures airflow through a VAV box using pressure pickup tubes or a flow ring.

VAV Flow Pickup

The small tubes or cross-shaped pickup inside a VAV inlet that senses airflow pressure.

Fan Status

Type: DI. Confirms whether a fan is actually running.

Fan Command vs Fan Status

Command is what the controller wants. Status is proof of what actually happened.

Current Switch

Type: DI. Detects motor current to prove a fan or pump is running.

Current Transducer

Type: AI. Measures actual electrical current draw.

Auxiliary Contact

Type: DI. A contact on a starter, contactor, or VFD that proves equipment status.

Smoke Detector

Type: DI. Safety device that shuts down or alarms HVAC equipment if smoke is detected.

Duct Smoke Detector Location

Usually mounted in supply or return ductwork near an AHU or RTU.

Freeze Stat

Type: DI. Safety device that trips if coil temperature gets too low.

Freeze Stat Appearance

Often a long capillary tube stretched across a coil.

Freeze Stat Purpose

Prevents coils from freezing and bursting.

High Pressure Switch

Type: DI. Refrigeration safety that trips if refrigerant pressure is too high.

Low Pressure Switch

Type: DI. Refrigeration safety that trips if refrigerant pressure is too low.

Compressor Safety Switches

Type: DI. Safety inputs that must be normal before compressor operation is allowed.

Condensate Float Switch

Type: DI. Trips if condensate water level gets too high.

Water Flow Switch

Type: DI. Proves that water is flowing through a pipe.

Pump Status

Type: DI. Confirms the pump is actually running.

Pump Differential Pressure Sensor

Type: AI. Measures pressure difference across a pump or piping system.

Water Pressure Transducer

Type: AI. Measures water pressure in a pipe.

Chilled Water Supply Temperature

Type: AI. Measures temperature of water leaving the chiller or going to coils.

Chilled Water Return Temperature

Type: AI. Measures temperature of water returning from the building.

Hot Water Supply Temperature

Type: AI. Measures heating water supplied to coils or zones.

Hot Water Return Temperature

Type: AI. Measures heating water returning to the boiler plant.

Condenser Water Supply Temperature

Type: AI. Measures water from the cooling tower to the chiller.

Condenser Water Return Temperature

Type: AI. Measures warmer water returning from the chiller to the cooling tower.

Cooling Tower Basin Level Sensor

Type: AI or DI. Measures or detects water level in the cooling tower basin.

Cooling Tower Fan Status

Type: DI. Confirms the tower fan is running.

Boiler Water Temperature Sensor

Type: AI. Measures boiler water temperature.

Boiler Flame Sensor

Type: DI or internal safety. Proves flame exists during burner operation.

Boiler Safety Inputs

Safeties that must be normal before burner operation is allowed.

Chiller Leaving Water Temperature

Type: AI. Measures chilled water leaving the chiller.

Chiller Entering Water Temperature

Type: AI. Measures water returning to the chiller.

Chiller Flow Proof

Type: DI. Confirms water flow before chiller operation.

RTU Sensor Template

Typical RTU sensors include SAT, RAT, MAT, OAT, space temp, smoke detector, filter DP, fan status, high pressure switch, low pressure switch, and freeze stat.

AHU Sensor Template

Typical AHU sensors include SAT, RAT, MAT, OAT, duct static pressure, filter DP, fan status, smoke detector, and sometimes humidity or CO2.

VAV Sensor Template

Typical VAV sensors include space temp, airflow, and discharge air temp.

Pump Sensor Template

Typical pump points include pump status, flow switch, differential pressure, and sometimes current.

Boiler Sensor Template

Typical boiler points include water temperature, pressure, flow proof, flame status, and safety inputs.

Chiller Sensor Template

Typical chiller points include entering water temp, leaving water temp, flow proof, pressures, alarms, and status.

Cooling Tower Sensor Template

Typical tower points include basin level, condenser water temperature, fan status, and sometimes vibration or flow.

Physical path of a temperature input

Sensor → Field wire → Terminal block → Controller AI → Niagara point.

Physical path of a digital safety input

Safety switch → Field wire → Terminal block → Controller DI → Niagara alarm or status.

Physical path of fan status

Motor/VFD/contactor proof → Field wire → Controller DI → Niagara fan status.

Physical path of duct static pressure

Pressure pickup tube → Pressure transmitter → Signal wire → Controller AI → Niagara pressure point.

Why pressure sensors have tubing

The tubing carries air pressure from the duct to the pressure sensor.

Why temperature sensors are usually small

They only need to sense air or water temperature, not control power.

Why sensors are hard to notice in equipment

They are small compared to fans, coils, compressors, and wiring.

Best way to find a sensor physically

Start at the Niagara point or controller terminal, then trace the field wiring.

Why wire color is not enough

BAS wire colors are not universally standardized.

What matters more than wire color

Point name, terminal number, wire number, and wiring diagram.

What to ask when seeing any sensor

What does this measure, where does it send the signal, and what logic uses it?

What to ask when seeing a weird sensor value

Is the value physically possible?

If SAT reads 140°F during cooling

Suspect bad sensor, bad wiring, wrong scaling, failed cooling, or sensor in wrong location.

If OAT reads -40°F

Suspect open circuit, failed sensor, or configuration/scaling issue.

If duct static pressure reads zero while fan is running

Suspect tubing issue, failed sensor, wrong location, or actual no airflow.

If fan command is ON but fan status is OFF

The controller wants the fan running but does not have proof.

If smoke detector input is active

The unit may shut down for life safety.

If freeze stat is tripped

Cooling or fan operation may be locked out until reset.

If high pressure switch is tripped

Compressor operation may be disabled for safety.

If low pressure switch is tripped

Compressor operation may be disabled for safety.

Why safeties are usually inputs

They tell the controller whether equipment is allowed to run.

Why safeties can block outputs

The controller may refuse to start equipment if a safety input is abnormal.