CO2, Enthalpy & Occupancy Sensors Explained: Demand-Controlled Ventilation
Bringing in more outdoor air than a space actually needs wastes energy conditioning it; bringing in too little hurts indoor air quality. CO2, enthalpy, and occupancy sensors exist to solve that balance — feeding real-time data into demand-controlled ventilation strategies instead of running ventilation on a fixed schedule regardless of actual need. Jaideo Automation supplies the CO2 Transmitter, CO2 & Air Quality Sensors, Enthalpy Sensors, and IAQPoint2 Indoor Air Quality Monitor.
CO2 Sensors and Demand-Controlled Ventilation
CO2 concentration in occupied space rises with occupancy — people exhale it. A CO2 sensor measures that concentration and feeds it to the BMS, which can increase outdoor air ventilation only when CO2 levels indicate the space actually needs it, rather than ventilating at a fixed rate regardless of how many people are present. This is the core principle behind demand-controlled ventilation (DCV).
Enthalpy Sensors
Enthalpy measures the total heat content of air — both temperature and humidity combined — which matters for economizer control. A pure temperature-based economizer might bring in outdoor air that’s cool but very humid, adding a dehumidification load even while “saving” on cooling. An enthalpy sensor lets the economizer make a better decision by weighing both factors together.
Occupancy Sensors
Occupancy sensors (including PIR — passive infrared — and dual-technology types) detect whether a space is occupied at all, enabling ventilation, lighting, and temperature setback strategies that respond to actual use rather than a fixed schedule.
Air Quality Sensors: Beyond CO2
CO2 is the most common proxy for air quality because it correlates well with occupancy, but it isn’t the only pollutant worth monitoring. Broader air quality sensors, like the IAQPoint2, can also track VOCs (volatile organic compounds), particulates, and other contaminants that CO2 alone won’t catch — relevant in spaces with off-gassing materials, nearby combustion sources, or process emissions rather than just occupant-driven CO2 buildup. Where a project needs to demonstrate broader indoor air quality compliance, not just occupancy-driven ventilation, a combined air quality monitor is the more complete choice over a CO2-only sensor.
Contact Sensors
A contact sensor is a simple two-part magnetic switch — one half on the door or window frame, one on the moving part — that reports open or closed status to the BMS. In HVAC terms, contact sensors are most commonly used to interlock conditioning with openings: suspending heating or cooling to a zone while a window is open, or triggering an alarm if an exterior door is left open longer than expected. They’re inexpensive, require no calibration, and are often the simplest fix for spaces where occupants overriding the HVAC by opening a window is a recurring energy problem.
How These Work Together
| Sensor | What it measures | Drives |
|---|---|---|
| CO2 | Carbon dioxide concentration | Demand-controlled ventilation rate |
| Enthalpy | Total heat content (temp + humidity) | Economizer decision-making |
| Occupancy (PIR) | Presence/absence of people | Ventilation, lighting, setback scheduling |
| Contact | Door/window open or closed | HVAC interlock with openings |
Why This Matters for Energy Performance
Demand-controlled ventilation strategies built on these sensors are commonly credited toward green building certification points, and directly reduce the energy spent conditioning outdoor air that isn’t actually needed.
Where This Fits
These sensors typically work alongside air velocity sensors and temperature sensors in a complete BMS ventilation control strategy. See our HVAC Sensor & Flow Switch Selection Guide for the full picture.
Frequently Asked Questions
What is demand-controlled ventilation?
Demand-controlled ventilation adjusts outdoor air ventilation rates based on real-time occupancy or CO2 levels, rather than ventilating at a fixed rate regardless of actual need, saving energy.
Why do economizers need enthalpy sensors instead of just temperature sensors?
Temperature alone doesn’t capture humidity, so a temperature-only economizer could bring in air that’s cool but very humid, adding dehumidification load. Enthalpy sensors account for both temperature and humidity together.
What is a PIR occupancy sensor used for in HVAC?
A PIR occupancy sensor detects whether a space is occupied, enabling ventilation, lighting, and temperature setback strategies that respond to actual occupancy rather than a fixed schedule.