Smart Ventilation: The Role of Humidity and CO2 Sensors in Automated Air Quality Management

Modern residential DCV systems monitor three principal air quality indicators.

Relative Humidity (RH)

Relative humidity is the most direct indicator of condensation risk and mould growth potential. At RH above 70%, mould spores begin to germinate on porous building surfaces within 24–48 hours. At RH above 80%, surface condensation becomes probable on materials with a surface temperature below the dew point.

Capacitive humidity sensors embedded in extract terminals or duct sections measure RH continuously and signal the air-handling unit or extract fan controller to increase airflow rate when RH exceeds a user-defined setpoint—typically 65–70% RH. When humidity falls, the system returns to background ventilation rate, conserving energy and reducing cold-air infiltration in winter.

CO2 Concentration

Carbon dioxide is the primary bio-effluent marker for human occupancy. Outdoor air contains approximately 420 ppm CO2 (current atmospheric level). In an occupied room with inadequate ventilation, CO2 rises progressively: above 800 ppm, occupants report reduced concentration; above 1,200 ppm, fatigue and headaches become common; above 2,000 ppm, cognitive performance measurably declines.

Building Regulations Part F uses CO2 concentration as an indicative metric for ventilation adequacy. A correctly sized and commissioned ventilation system should maintain bedroom CO2 below 1,000 ppm during overnight occupation. Non-dispersive infrared (NDIR) CO2 sensors—the technology standard for reliable measurement—are now compact and cost-effective enough for residential MVHR integration.

VOC/Air Quality Index Sensors

Metal oxide semiconductor (MOS) sensors respond to a broad range of volatile organic compounds, cooking odours, and combustion products. They are less precise than CO2 sensors but provide a fast response to sudden pollutant loading events—an ideal trigger for kitchen boost ventilation, for example. Many current MVHR controllers incorporate combined RH/VOC sensors as standard; CO2 sensors are typically specified as an additional component for bedroom circuits in premium systems.

In an MVHR System

The air-handling unit operates continuously at background ventilation rate—typically 50–70% of design airflow. Sensor signals from individual rooms trigger zone-level or whole-house boost modes. In a properly zoned system:

• A high-humidity signal from the bathroom extract terminal raises the extract rate for that zone, drawing more fresh air through the house
• A CO2 exceedance signal from a bedroom CO2 sensor raises supply airflow to that room independently
• A VOC spike in the kitchen triggers a temporary kitchen boost without disturbing bedroom ventilation

Modern EC-motor MVHR units modulate fan speed continuously across a wide range—from 40% to 100% of design airflow—through 0–10 V or BACnet control interfaces, enabling smooth, stepless DCV response.

In Standalone Smart Extract Fans

For properties without whole-house MVHR, humidity-controlled extract fans provide room-level DCV. These units contain an integral RH sensor and operate on a trickle-boost-overrun logic: continuous low-speed extraction at background rate; automatic boost when RH exceeds the setpoint; post-use overrun at boost speed to clear residual moisture before returning to background. Flexivent specifies this approach as standard for bathroom installations in MEV and hybrid systems.

Energy optimisation: DCV systems consistently use less fan energy than fixed-rate systems operated at design airflow. An MVHR system spending 60% of its operating hours at 55% speed runs its fans at approximately 17% of design power consumption during that period (fan power scales with the cube of speed), delivering substantial electricity savings over the system's lifespan.

Extended component life: Fans operating at reduced speeds for extended periods run cooler and experience less mechanical wear. For EC-motor units with rated lifespans of 40,000–60,000 hours, this translates to extended service intervals and lower lifecycle costs.

Occupant comfort: DCV systems respond to actual conditions rather than timer schedules. Ventilation increases when needed and reduces when not—eliminating the cold-draught complaints common in oversized fixed-rate systems.

Regulatory alignment: Part F Approved Document 2021 explicitly references humidity-based controls as an appropriate demand-responsive strategy for residential extract systems, and CIBSE Guide A supports CO2-based control in occupied spaces.

Current-generation MVHR controllers from Zehnder (ComfoAir), Brink (Renovent), and Nuaire (drimaster) offer connectivity via Modbus, KNX, or proprietary app interfaces. Integration with Apple HomeKit, Google Home, and Amazon Alexa ecosystems allows occupants to view real-time IAQ data, adjust setpoints, and receive maintenance alerts from smartphones. Flexivent commissions and configures these integrations as part of our standard handover process.

Demand-controlled ventilation is now the appropriate engineering baseline for new residential MVHR installations—not a premium option. Flexivent designs and commissions DCV systems as standard, with sensor locations and control setpoints specified to the occupancy profile and layout of each property.

Contact Flexivent today to discuss integrating smart humidity and CO2 control into your ventilation system. The result is a system that is quieter when you are out, more responsive when you are in, and demonstrably better for both your health and your energy bills.