Demand controlled ventilation is only as effective as the information behind its decisions.
CO2, relative humidity, temperature and occupancy sensors can help a ventilation system respond to changing conditions-but each signal describes a different aspect of the indoor environment.
For distributors, installers, building-services engineers and specifiers, the important question is not simply how many sensors a system includes. It is whether the measurements are relevant to the building, whether the sensors are positioned appropriately, and how the control system uses the information.
Understanding these differences helps professionals assess ventilation systems more accurately, set realistic expectations and specify solutions that match the needs of each building.
1. CO₂ sensors: an indicator of occupancy-related ventilation needs
Carbon dioxide (CO₂) is a useful indicator of occupancy-related indoor air conditions in many residential and non-residential spaces. People exhale CO₂, so concentrations tend to rise when a room is occupied and outdoor-air supply is insufficient relative to the number of people present.
A CO₂ sensor can therefore help a demand-controlled ventilation system adjust airflow as occupancy-related loads change.
However, CO₂ is not a universal measure of indoor air quality. It does not directly measure particles, volatile organic compounds (VOCs), radon or moisture. A low CO₂ reading does not guarantee that every aspect of indoor air quality is satisfactory.
How should CO₂ readings be interpreted?
A single number should not be treated as a universal pass-or-fail threshold.
Indoor CO₂ concentrations depend on outdoor levels, occupancy, activity, room volume, ventilation rate and the time elapsed since people entered the space. The difference between indoor and outdoor CO₂ is often more informative for ventilation assessment than an indoor reading considered in isolation.
EN 16798-1 provides indoor environmental categories and associated design approaches. In the CO₂ method commonly referenced for these categories, the values are expressed as concentrations above outdoor CO₂, rather than as fixed indoor concentrations. The relevant category and design criteria should be selected based on the specific project requirements.
The often-quoted figure of 1,000 ppm should therefore not be presented as a universal threshold established by EN 16798-1.
Outdoor CO₂ also changes over time and by location. Where a comparison is needed, use a representative outdoor measurement or a clearly identified, current reference rather than assuming a fixed outdoor concentration.
What specifiers should check:
- Measurement method: Is the sensor measuring CO₂ directly, or estimating an equivalent CO₂ value from other signals?
- Accuracy and calibration: Are the accuracy specifications, calibration requirements and expected operating conditions documented?
- Placement: Does the sensor represent the occupied space, or is it located where readings may not reflect the room’s conditions?
- Control response: How does the ventilation unit translate the reading into airflow changes, and can that response be configured for the application?
Direct CO₂ sensors commonly use non-dispersive infrared (NDIR) technology, while other direct measurement technologies also exist. An estimated CO₂ value derived from a VOC sensor is not equivalent to a direct CO₂ measurement. The appropriate choice depends on the application, documented performance and compatibility with the control system.
2. Relative humidity: identifying moisture loads, not diagnosing mould
Relative humidity (RH) describes how close the air is to saturation at a given temperature. In homes, it can help identify moisture loads associated with activities such as showering, cooking and drying laundry.
When humidity rises, a demand controlled ventilation system with suitable control logic can increase airflow to help remove moisture from the indoor air.
But relative humidity must be interpreted in context. It changes with temperature, so the same amount of water vapour can produce different RH readings at different temperatures.
Does a humidity sensor detect mould risk?
Not by itself.
Mould risk depends on several factors, including moisture availability, temperature, the duration of damp conditions, material properties and the conditions at a surface. A room-air RH measurement does not directly establish the humidity conditions at a cold wall, window reveal or thermal bridge.
For example, indoor air may show a moderate RH while a poorly insulated surface is cold enough for local surface humidity to become significantly higher. Conversely, a brief rise in room-air RH does not automatically mean that mould will develop.
ISO 13788 provides methods for assessing critical surface humidity and interstitial condensation under specified assumptions. It should not be reduced to a universal room-air RH threshold.
For ventilation control, RH is a useful signal of changing moisture loads. For building-envelope assessment, surface temperature, thermal bridging, moisture transport and the duration of conditions may also need to be considered.
What specifiers should check
- Application: Is the sensor intended to respond to short moisture peaks, provide general monitoring, or support another control function?
- Location: Does the measurement represent the space or moisture source that matters?
- Control settings: How quickly does the system respond to a rise in RH, and what conditions trigger a return to normal operation?
- System scope: Is ventilation alone an appropriate response, or does the building also require investigation of leaks, thermal bridges, water ingress or other moisture sources?
Depending on the application, dew point or absolute humidity may provide additional useful information. They are complementary measures, not universal replacements for relative humidity.
3. Occupancy sensors: detecting presence is not the same as measuring demand
Occupancy sensors can help ventilation systems respond when a space is in use. Depending on the technology, they may detect movement, presence or another indicator of occupancy.
This can be useful in rooms where occupancy changes substantially throughout the day. A system may, for example, use an occupancy signal to initiate a temporary airflow increase or to switch between predefined operating modes.
However, the meaning of the signal depends on the sensor and its location. A motion detector may not reliably detect a person who remains still for an extended period. Other presence-detection technologies may behave differently.
Occupancy detection also does not directly quantify the ventilation load. Two occupied rooms can have different numbers of people, activity levels, room volumes and air-quality conditions.
How does occupancy control compare with CO₂ control?
The signals serve different purposes:
- Occupancy detection can indicate that a space is in use or that presence has changed.
- CO₂ measurement can provide an indication of occupancy-related bioeffluent accumulation relative to the outdoor-air supply.
- Humidity measurement can help identify moisture loads, including those caused by activities that may occur independently of occupancy detection.
These signals can complement one another, but they are not interchangeable. Whether a system needs one, several or other types of sensors depends on the building, ventilation strategy and intended control functions.
What specifiers should check
- What type of occupancy or presence detection is used?
- Which rooms and zones does it cover?
- What happens when the sensor detects no movement but a person remains in the room?
- Does occupancy trigger a fixed operating mode, a temporary boost or a response combined with other sensor readings?
- How does the system return to its normal operating state?
A sensor is useful only when its signal leads to an appropriate and reliable control response.
4. Temperature sensors: supporting stable and appropriate operation
Temperature measurements help a ventilation system respond to thermal conditions and manage functions such as supply-air temperature control, heating and heat-exchanger operation, depending on the system design.
In heat-recovery ventilation, temperature readings may contribute to control decisions involving the heat exchanger, bypass and post-heating functions. The exact sensors and control sequence vary by unit.
Temperature sensors should therefore be assessed in relation to the specific functions they support—not simply counted as another feature in a product specification.
For example, a sensor used to monitor extract-air temperature has a different role from one used to monitor supply-air temperature. Sensor position, measurement accuracy and the control logic all influence how the reading is used.
Temperature sensors in Ensy AHU units serve different operational functions depending on their position in the unit.
They are used to measure extract-air and fresh-air temperatures, monitor operating temperature and overheating, support defrost control, and protect the rotor through a dedicated rotor guard sensor. The sensor type, length and application may vary for every AHU model, so the correct function should be verified against Ensy’s technical documentation.
5. How should ventilation control combine different sensor signals?
There is no single sensor hierarchy that is correct for every building. The control strategy should reflect the ventilation unit’s capabilities, the building layout, the distribution of airflow, the expected loads and the required operating conditions.
A system may use temperature and heat-exchanger protection functions alongside CO₂, humidity, occupancy signals, schedules or manual boost commands. These inputs can have different priorities, limits and time responses.
The key question is not whether a system follows a consistent sequence. It is whether its control logic is documented and suitable for the intended application.
For example, a humidity increase may call for a temporary boost after showering, while elevated CO₂ may indicate that additional outdoor air is needed during a period of higher occupancy. A temperature-related limit may constrain how the unit operates under certain conditions.
These decisions should be coordinated rather than treated as independent sensor features.
Sensor placement and zoning matter
A sensor only measures conditions at its measurement location. A reading from a hallway or extract-air duct may not represent the conditions in every occupied room.
Whether individual room sensors are necessary depends on the distribution system, room usage, zoning and control design. In some applications, a central measurement may be suitable; in others, additional sensors may provide more representative information.
Before specifying a system, establish:
- Which spaces are measured and which are not.
- Whether sensor locations reflect the intended control objective.
- How readings from different locations influence airflow.
- Whether the system supports the required zones and operating modes.
- How the installation will be commissioned and verified.
Ensy’s guide to how demand-controlled ventilation works provides an overview of the sensors and control options used in ventilation applications.
6. What about winter operation and indoor humidity?
In cold climates, ventilation must balance indoor air quality, moisture removal, energy use and thermal comfort.
Cold outdoor air generally contains less water vapour than warm indoor air. When that outdoor air is heated indoors without adding moisture, its relative humidity decreases. This is one reason indoor air can feel dry during the heating season.
Reducing unnecessary ventilation when a space is unoccupied may reduce the amount of outdoor air that needs to be heated. However, the appropriate airflow must still meet the building’s ventilation requirements and account for actual occupancy and moisture loads.
Heat recovery helps transfer energy from extract air to incoming outdoor air. Rotary heat exchangers can also transfer some moisture under certain conditions, depending on the rotor design and operating conditions. Their performance should be assessed using the manufacturer’s documented data rather than assuming that all rotary exchangers provide the same level of moisture transfer or frost protection.
No sensor or control strategy should be expected to solve every cause of dry air, condensation or mould. Ventilation is one part of the building’s moisture and indoor-climate strategy; the building envelope, heating, occupant activities and other moisture sources also matter.
7. A practical checklist for distributors, installers and managers
When comparing smart ventilation systems, use the following questions to look beyond the number of sensors listed in a product brochure.
Measurement
- What does each sensor measure directly?
- Are any values estimated rather than directly measured?
- Are accuracy, calibration and operating limitations documented?
Coverage and placement
- Which rooms or areas are represented by the measurements?
- Do sensor locations match the control objective?
- Could the measured conditions differ significantly from those in occupied spaces?
Control logic
- What conditions trigger an airflow change?
- Are thresholds and response times configurable where required?
- How do CO₂, RH, occupancy and temperature signals interact?
- What happens when a sensor fails or provides implausible readings?
Commissioning and verification
- Can installers verify the sensor readings and control response?
- Is the airflow response documented across relevant operating modes?
- Are maintenance requirements and replacement procedures clear?
Application fit
- Does the control strategy match the building’s occupancy patterns and moisture loads?
- Is the zoning appropriate for the layout?
- Are the system’s documented capabilities suitable for the project requirements?
A well-specified system is not necessarily the one with the longest sensor list. It is the one whose measurements, control logic and airflow response are appropriate for the building—and can be verified during commissioning.
For a broader introduction to the principles and benefits of demand-controlled ventilation, read Ensy’s article, Why demand-controlled ventilation?. For an overview of Ensy’s approach and related resources, visit the demand-controlled ventilation section.
Frequently asked questions:
What does a CO₂ sensor tell a ventilation system?
A CO₂ sensor measures the concentration of carbon dioxide at its location. In occupied spaces, this can help indicate occupancy-related ventilation needs. It does not directly measure every pollutant or provide a complete assessment of indoor air quality.
Can a humidity sensor detect mould risk?
No. An RH sensor measures relative humidity at its location. Mould risk also depends on surface temperature, moisture availability, materials and the duration of damp conditions. Room-air RH alone cannot establish whether a particular surface is at risk.
Can occupancy sensors replace CO₂ monitoring?
Not in every application. Occupancy sensors indicate presence according to their technology, while CO₂ measurements provide a different signal related to occupancy and outdoor-air supply. Whether one or both are needed depends on the building and control strategy.
Do all smart ventilation systems use the same sensors?
No. Sensor selection depends on the unit, application, control functions, zoning and project requirements. Specifiers should assess what each sensor measures and how the control system uses that information.
What should specifiers check before choosing a ventilation control system?
Check sensor measurement methods, accuracy, placement, calibration, control logic, zoning, commissioning procedures and the documented airflow response. The goal is to verify that the system’s capabilities match the building’s actual requirements.
Bibliography:
CO₂ and indoor air quality: ASHRAE — Position Document on Indoor Carbon Dioxide
The 1,000 ppm misconception: NIST — Quit Blaming ASHRAE Standard 62.1 for 1,000 ppm CO₂
CO₂ monitoring and indoor air quality: REHVA Journal
Outdoor CO₂ trends: NOAA Global Monitoring Laboratory
Surface humidity and condensation assessment: ISO 13788:2012 — Hygrothermal performance of building components and building elements
Smart ventilation evidence and energy considerations: Lawrence Berkeley National Laboratory — Smart Ventilation: Energy and Indoor Air Quality Performance
