How to select the right sensors in HVAC systems

How to select the right sensors in HVAC systems


Oxygen (O2) and Carbon Dioxide (CO2) sensors are two of the most common gas sensors used in Heating, Ventilation, and Air Conditioning (HVAC) systems. HVAC systems are deployed globally to monitor and regulate environmental conditions in nearly every dwelling, building and vehicle. In particular, this article will focus on the Ventilation aspect of HVAC and its relation to gas sensors.

Ventilation units can be used to control the particulate concentration and gaseous composition of indoor air based on measurements received from gas sensors. Ambient air is comprised of approximately 78 % Nitrogen, 21 % Oxygen and 0.04 % Carbon Dioxide. A lack of indoor air circulation can dramatically change the gas composition towards O2 depletion and CO2 enrichment.

HVAC systems require a strategic placement of sensors and switches to help ensure operational efficiency, comfort, and occupant safety. Particularly in Ventilation (HVAC), the selection and placement of sensors can help reduce risks and increase efficiency.


But before discussing the importance of sensor selection, it’s vital in the application of HVAC to understand the difference between Carbon Monoxide and Carbon Dioxide. They have many properties in common but are often confused as both are thought of as a “silent killer”. They cannot be smelled or seen, but both are extremely dangerous.

Carbon Dioxide is produced naturally on Earth and is released when organisms breathe (respiration taking place at the cellular level). In addition to humans, all animals and plants return CO2 and H2O vapor into the atmosphere. Also, there are natural sources of atmospheric carbon dioxide, such as outgassing from the ocean, decomposing vegetation and other biomass, venting volcanos, and naturally occurring wildfires.

Carbon Monoxide (CO) is largely man-made; natural sources can include volcanos, wildfire and forest fires, and other forms of combustion such as fossil fuels. It is one of the major pollutants regulated by the United States. CO is also produced by the incomplete burning of carbon-containing fuels, including coal, oil, charcoal, natural gas, propane, wood, and kerosene.


In HVAC systems, there is ample opportunity for CO2 and CO (both dangerous gases) to enter the ambient air. Awareness is key to reducing safety hazards and preventing accidents. For this reason, it’s critical for HVAC technicians, home inspectors and others to be aware of CO2 and CO exposure and poisoning.

Carbon Dioxide

Many office buildings, schools, restaurants and more use CO2 detectors to monitor and control indoor air quality. These range from portable devices to detectors installed throughout an HVAC system. With the COVID-19 pandemic, the on-going focus on proper air quality will not be reduced. Additional guidelines are (and have been) issued to enhance ventilation in order to assist in mitigating the transmission of many airborne illnesses, as well as improve overall indoor air quality.

There are multi-levels of research that have concluded high concentrations of CO2 gas and contaminants create health problems for occupants, ranging from airborne illnesses to sick-building syndrome. As defined by the National Health Institute, sick-building syndrome is the name for symptoms one has when in a particular building; it usually happens in an office but can happen in any building. Sick-building syndrome can be caused by poor ventilation or poorly maintained systems, bright or flickering lights, dust, smoke, fumes, or even airborne fabric fibers. 

To combat these, HVAC designers have developed systems to provide a constant flow of fresh outdoor air into buildings and also added carbon dioxide sensors to determine when the buildings/rooms/floors were occupied. These sensors work in concert with the HVAC system. As the CO2 levels begin to rise from exhaled breath, the HVAC system is triggered to pull in fresh outdoor air. Conversely, when occupants leave buildings/rooms/floors and the CO2 levels drop, the HVAC system closes the fresh air dampers. The positioning of the CO2 monitors is important to ensure the most accurate readings. In most situations, sensors are added at head height, and kept away from windows, doors, and air conditioners.

Carbon Monoxide

The Environmental Protection Agency points out there are many sources of carbon monoxide (CO) that can affect HVAC technicians and other individuals in the home or business1. These include:

•      Unvented kerosene and gas space heaters

•      Leaking chimneys and furnaces

•      Back-drafting from gas water heaters, furnaces, wood stoves and fireplaces

•      Generators

•      Incomplete oxidation during combustion in unvented heaters

•      Damaged, worn, or poorly adjusted or maintained combustion devices

These applications have the potential to produce dangerous levels of CO if improperly installed, maintained or inspected. To help reduce the danger from elevated levels of CO, portable and fixed carbon monoxide detectors can be used throughout indoor spaces. Experts believe detectors should be placed inside each room that receives forced-air heat powered by a gas or propane furnace. Carbon monoxide monitors should not be placed above or within 15 feet of potential sources of carbon monoxide (i.e., fireplaces, stoves, etc.), in or near humid areas, or next to windows.


To excel in monitoring CO2 or CO, gas sensors must be precise, highly sensitive, rugged, and have an extended life.


CRIR Series Gas Sensor (E1 & M1)

ECOSURE X Gas Sensor

Gas measured

Carbon dioxide

Carbon monoxide

Operating principle

Single channel, non-dispersive infrared (NDIR)

Two-electrode electrochemical

Measurement range

Analog: 400 ppm to 2000 ppm

Digital: 400 ppm to 10,000 ppm

4 ppm CO to 500 ppm CO


33 mm W x 22 mm L x 11,2 mm H (tolerance ±0,5 mm)

Ø19,3 mm x 16,6 mm H

Expected operating life*

10 years

> 10 years in normal use from date of manufacture


• Single channel, non-dispersive infrared technology

• Automatic Baseline Correction

• Temperature compensation

• UART digital interface

• Long life

• Stable performance

• UL certified – UL 2075

• Six Sigma output performance

* Specifications are valid at 20°C, 50 %RH, and 1013 mBar using Honeywell recommended circuitry. Performance characteristics outline the performance of sensors supplied within the first three months. Output signal can drift below the lower limit over time.


Honeywell CRIR Series Gas Sensors are single channel, non-dispersive infrared (NDIR) sensors. Within the CRIR Sensor is a sensing chamber with an infrared source at one end and a detector fitted with an optical filter at the other end. The source emits radiation at wavelengths which include the absorption band of CO2. The filter blocks wavelengths which are not sensitive to the presence of CO2, thereby increasing selectivity and sensitivity. As the light passes through the sensing chamber, a fraction is absorbed if CO2 is present. The difference between the light emitted by the source and received by the detector can then be converted to a CO2 concentration reading.

Honeywell ECOSURE X Sensor is an electrochemical gas sensor. Carbon monoxide diffusing into the sensor reacts at the surface of the sensing electrode by oxidation, causing a current to flow between the electrodes through the external circuit. The current is proportional to the concentration of gas and can be measured across a load resistor in the external circuit.

ECOSURE X Sensor has been designed for unattended operation and an expected operating life of 10 years in normal use (please see above for use specifications). Additionally, it is UL Certified - UL2075.

Honeywell CRIR and ECOSURE sensors are well engineered for design in HVAC systems and their placement can vary depending upon the system’s structure. Start a conversation with Honeywell today to learn more.

1 https://www.epa.gov/indoor-air-quality-iaq/carbon-monoxides-impact-indoor-air-quality