Understanding Sensors and Actuators

Learning Objectives

By the end of this module, learners will be able to:

• Understand the role of sensors and actuators within a Building Management System (BMS).

• Identify common sensor types used in HVAC and building automation applications.

• Explain how sensors convert physical conditions into usable control signals.

• Understand the purpose and operation of different actuator technologies.

• Recognise common control signals used between controllers, sensors, and actuators.

• Apply sensor and actuator knowledge when commissioning, fault finding, and maintaining BMS installations.

Approximate Reading Time

10-15 Minutes

Category

BMS Fundamentals

Knowledge Level

Beginner

Introduction

Sensors and actuators provide the link between the physical building environment and the Building Management System (BMS). They allow the controller to monitor real-world conditions and make adjustments to maintain comfort, efficiency, and safe operation.

Sensors act as the sensory inputs to the BMS, detecting conditions such as temperature, pressure, humidity, carbon dioxide levels, and airflow. They convert physical measurements into electrical signals that the controller can interpret and use for decision-making.

Actuators provide the physical outputs of the BMS. They receive commands from the controller and carry out actions in the field, such as opening and closing valves, positioning dampers, adjusting fan speed, or controlling water flow.

Together, sensors and actuators create the feedback loop that enables a BMS to continuously monitor building conditions and automatically respond to changing demands.

Understanding Inputs and Outputs

Every point within a Building Management System can be considered either an Input or an Output.

Think of the BMS controller as the brain of the system.

• Inputs provide information to the controller.

• Outputs allow the controller to influence building services equipment.

Inputs

Inputs answer the question:

"What is happening right now?"

Examples include:

• Room temperature

• Duct pressure

• Humidity level

• CO₂ concentration

• Water flow status

• Pump fault indication

The controller monitors these points but cannot directly alter them.

Outputs

Outputs answer the question:

"What action should be taken?"

Examples include:

• Opening a heating valve

• Starting a fan

• Enabling a boiler

• Positioning a damper

• Adjusting fan speed via a VSD

Outputs are controlled by the BMS and are used to maintain desired operating conditions.

Without inputs, the controller cannot make informed decisions.

Without outputs, the controller cannot influence the building environment.

What is a Sensor?

A sensor is a device that detects or measures a physical condition and converts it into an electrical signal. The BMS controller continuously reads these signals and uses them to make control decisions.

Common Applications

• Measuring room temperature

• Monitoring duct pressure

• Detecting occupancy

• Measuring humidity levels

• Monitoring water flow

• Detecting carbon dioxide levels

• Monitoring plant performance

Typical Sensor Locations

• Plant rooms

• Air Handling Units (AHUs)

• Fan Coil Units (FCUs)

• Heating and cooling pipework

• Occupied spaces

• Duct systems

• Roof plant installations

Choosing the Right Sensor

The performance of a control system is heavily dependant on the quality and suitability of the sensors installed.

When selecting sensors, engineers should consider:

• Accuracy requirements

• Measuring range

• Environmental conditions

• Installation location

• Maintenance requirements

• Signal type or communication protocol

For example, a room temperature sensor may not be suitable for installation outdoors, while a pressure sensor selected for water applications may not be suitable for air measurement.

Selecting the wrong sensor can result in poor control performance, increased energy consumption, inaccurate monitoring, and unnecessary maintenance visits.

Common Sensor Types

Temperature Sensors

Temperature sensors are the most common sensors found in a BMS.

They are used to monitor:

• Space temperature

• Duct temperature

• Outside air temperature

• Flow water temperature

• Return water temperature

• Frost protection conditions

Common Technologies

• Thermistors

• RTDs (Resistance Temperature Detectors)

• PT100

• PT1000

• Semiconductor sensors

Common Signal Types

• Resistance

• 0-10 VDC

• 4-20 mA

• BACnet

• Modbus

Example

A room temperature sensor measures 22°C and sends this information to the BMS controller. If the room temperature rises above the setpoint, the controller may reduce heating output or increase cooling demand.

Pressure Sensors

Pressure sensors monitor air, water, or gas pressure.

Common Applications

• Duct static pressure control

• Filter monitoring

• Chilled water differential pressure

• Heating water differential pressure

• Pump control

Example

If duct pressure falls below the required level, the controller may increase fan speed through a Variable Speed Drive (VSD).

Humidity Sensors

Humidity sensors measure the amount of moisture present within the air.

Common Applications

• Office buildings

• Healthcare facilities

• Laboratories

• Pharmaceutical environments

• Museums and archives

Example

If relative humidity exceeds a defined threshold, the BMS may increase ventilation rates or activate dehumidification processes.

Carbon Dioxide (CO₂) Sensors

Carbon dioxide sensors are widely regarded as one of the most effective indicators of occupancy-related Indoor Air Quality (IAQ).

Because people exhale CO₂, concentration levels naturally increase as occupancy rises. This makes CO₂ an excellent indicator for demand-controlled ventilation strategies.

Benefits

• Improved Indoor Air Quality (IAQ)

• Better occupant comfort

• Reduced risk of stale air conditions

• Reduced energy consumption

• Ventilation rates matched to occupancy levels

Example

As a meeting room fills with occupants, CO₂ levels rise. When concentrations exceed a predefined threshold, the BMS increases fresh air intake by opening dampers or increasing supply fan speed.

Training Note

CO₂ sensors are one of the best indicators of ventilation effectiveness but do not directly measure all air contaminants.

VOC Sensors

Volatile Organic Compound (VOC) sensors complement CO₂ sensors by detecting airborne chemical contaminants.

Common Sources of VOCs

• Paints and coatings

• Cleaning products

• Carpets and furniture

• Adhesives and sealants

• Office equipment

• Cooking activities

What VOC Sensors Tell Us

While CO₂ sensors indicate occupancy and ventilation effectiveness, VOC sensors indicate the presence of chemical pollutants and odours.

Why Use CO₂ and VOC Sensors Together?

Using both technologies provides a more complete assessment of Indoor Air Quality.

Examples include:

• High CO₂ and low VOCs in a crowded meeting room.

• Low CO₂ and high VOCs in a recently decorated office.

• High CO₂ and high VOCs within healthcare or education environments.

Modern BMS systems may use both measurements to optimise ventilation rates while minimising energy consumption.

Flow Sensors

Flow sensors measure the movement of water or air through a system.

Common Applications

• Chilled water circuits

• Heating water circuits

• Airflow monitoring

• Pump proving

• Equipment protection

Example

A flow switch can confirm water movement through a chilled water circuit before enabling a chiller.

Occupancy Sensors

Occupancy sensors detect whether a space is being used.

Common Technologies

• Passive Infrared (PIR)

• Ultrasonic

• Dual-Technology Sensors

Example

When a room becomes unoccupied, the BMS may reduce ventilation rates, lower lighting levels, or switch equipment into setback mode.

Installing Sensors Correctly

Even the most accurate sensor will produce poor control performance if installed in the wrong location.

Examples include:

• Room sensors should be installed approximately 1.5 m above floor level.

• Avoid mounting near radiators, printers, photocopiers, windows, doors, or direct sunlight.

• Outdoor sensors should be shielded from solar radiation and mounted where air circulates freely.

• Duct sensors should be positioned away from turbulence caused by fans and dampers.

• Pressure sensors should include test points to allow calibration.

Why it matters: Incorrect sensor placement can cause unnecessary energy use, comfort complaints, and unstable control loops.

Practical Installation Considerations

When installing sensors:

• Leave sufficient slack in cables for future maintenance.

• Use drip loops on external cable entries.

• Label sensor locations clearly.

• Ensure immersion probes reach the centre of the measured medium.

• Protect pressure tubing from condensation.

• Allow safe access for future replacement and calibration.

These seemingly small details greatly reduce future maintenance costs and troubleshooting time.

Common Temperature Sensor Formats

Different applications require different sensor construction types:

This would help junior engineers recognise what they're actually looking at on site.

Calibration and Verification

All sensors should be periodically checked against a known reference value.

Common reasons for recalibration include:

• Sensor ageing

• Contamination

• Environmental exposure

• Mechanical damage

Engineers should verify sensor accuracy during commissioning and planned maintenance visits.

A sensor that reads only 2°C incorrectly can dramatically affect energy consumption and occupant comfort.

This is very important from a maintenance perspective.

What is an Actuator?

An actuator is a device that converts a command issued by the BMS controller into physical movement or action.

If sensors are the sensory inputs of the system, actuators are the physical outputs.

The controller cannot directly open a valve or move a damper. Instead, it sends commands to actuators that perform these actions on its behalf.

Actuators allow the BMS to influence:

• Temperature

• Airflow

• Water flow

• Indoor Air Quality

• Energy consumption

• Occupant comfort

Common Actuator Types

Valve Actuators

Valve actuators regulate water flow through heating and cooling systems.

Common Applications

• AHU heating coils

• AHU cooling coils

• Fan coil units

• Chilled water systems

• LTHW circuits

• Variable temperature circuits

• Zone isolation valves

Typical Control Signals

• 0-10 VDC

• 2-10 VDC

• Floating control

• BACnet

• Modbus

Example

If a room temperature falls below the desired setpoint, the controller may command a valve actuator to open to 75%, increasing the flow of hot water through a heating coil.

Damper Actuators

Damper actuators control airflow throughout ventilation systems.

Common Applications

• Fresh air dampers

• Return air dampers

• Extract systems

• Smoke control systems

• VAV terminals

• Mixing boxes

Common Positions

Unlike digital outputs which are simply ON or OFF, many damper actuators can be positioned precisely.

Examples include:

• 25% Open

• 50% Open

• 75% Open

• 100% Open

Example

If CO₂ levels rise within a space, the BMS may command the fresh air damper to open further to improve ventilation.

Motorised Equipment and Variable Speed Drives

Not all actuators directly move valves or dampers.

Many BMS outputs control equipment such as:

• Pumps

• Fans

• Boilers

• Chillers

• Heat pumps

This may be achieved through:

• Digital outputs

• Motor starters

• Contactors

• Relays

• Variable Speed Drives (VSDs)

Example

A pressure sensor detects a reduction in system pressure. The BMS increases its output signal to a VSD, causing the pump to run faster and restore the required pressure.

Smart Sensors and Intelligent Actuators

Modern BMS installations are increasingly utilising intelligent field devices.

Smart Sensors

Examples include:

• BACnet room sensors

• Modbus sensors

• Wireless sensors

• LoRaWAN sensors

Benefits

• Reduced wiring

• Enhanced diagnostics

• Easier installation

• Improved analytics

Intelligent Actuators

Modern actuators may provide:

• Position feedback

• Run-time monitoring

• Fault diagnostics

• BACnet communication

• Modbus communication

This allows engineers to monitor not only the commanded position, but also the actual status of the device.

Common BMS Signal Types

Sensors and actuators communicate with controllers using several standard signal types.

Analogue Inputs (AI)

Used by sensors providing continuously variable values.

Examples

• Temperature

• Pressure

• Humidity

• CO₂

Common Signal Types

• 0-10V

• 4-20mA

• Resistance

Analogue Outputs (AO)

Used when proportional control is required.

Examples

• Valve position

• Damper position

• Fan speed reference

Common Signal Types

• 0-10V

• 2-10V

• 4-20mA

Digital Inputs (DI)

Used for status monitoring.

Examples

• Flow switches

• Pump run status

• Smoke detectors

• Fault contacts

Digital Outputs (DO)

Used for simple ON/OFF control.

Examples

• Start pump

• Enable boiler

• Start fan

• Operate relay

The Complete BMS Control Loop

Every automatic control strategy follows the same basic process.

Step 1: Measure

A sensor measures a physical condition such as temperature, pressure, humidity, or CO₂.

Step 2: Compare

The controller compares the measured value with the desired setpoint.

Step 3: Decide

The control strategy determines the required response.

Step 4: Act

An actuator receives the command and performs a physical action.

Step 5: Verify

The sensor measures the result and provides updated feedback to the controller.

Example Control Loop

Room Temperature Sensor → Controller → Heating Valve Actuator → Room Warms Up → Room Temperature Sensor

This continuous process is known as a closed-loop control system and forms the basis of modern building automation.

Common Faults and Troubleshooting

Sensor Faults

Common issues include:

• Failed sensing element

• Incorrect wiring

• Calibration drift

• Damaged cables

• Poor installation location

Typical Symptoms

• Unrealistic readings

• Intermittent values

• Constant maximum or minimum readings

• Comfort complaints

• Poor plant performance

Actuator Faults

Common issues include:

• Mechanical seizure

• Failed motor

• Incorrect control signal

• Broken linkage

• Loss of power supply

Typical Symptoms

• Valve not moving

• Damper not responding

• Poor temperature control

• Increased energy consumption

• Intermittent plant operation

Why Sensors and Actuators Matter

Sensors and actuators are among the most important components within a Building Management System.

Sensors provide the information needed for intelligent decision-making, while actuators allow those decisions to be translated into real-world actions.

Poor sensor performance can lead to:

• Occupant discomfort

• Increased energy consumption

• Equipment damage

• Unstable control loops

Failed actuators can prevent the system from responding correctly, reducing performance, increasing running costs, and generating unnecessary maintenance call-outs.

Understanding how sensors, actuators, inputs, and outputs work together is fundamental to the design, commissioning, operation, and maintenance of any BMS installation.

Key Takeaways

✅ Sensors are the sensory inputs of a BMS and provide information about building conditions.

✅ Actuators are the physical outputs of a BMS and perform actions on behalf of the controller.

✅ Every BMS point can be classified as either an Input or an Output.

✅ Common sensors include temperature, pressure, humidity, CO₂, VOC, flow, and occupancy sensors.

✅ Common actuators include valve actuators, damper actuators, relays, motor starters, and VSD-controlled equipment.

✅ Modern BMS systems increasingly use smart sensors and intelligent actuators with integrated communications and diagnostics.

✅ Sensors and actuators work together within a closed-loop control system to maintain comfort, safety, air quality, and energy efficiency.

✅ Understanding these devices is fundamental to commissioning, troubleshooting, and maintaining Building Management Systems.

If you need more help:

Then why not contact our sales or support team.

sales@forestrock.co.uk or support@forestrock.co.uk

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