Duct CO₂ Sensor vs Wall-Mount CO₂ Sensor: Which Should You Buy?

Indoor air quality soaks up an estimated 168 billion dollars each year in lost U. S. economic productivity, with poor ventilation being the culprit. CO sensors are the top priority,  fueling the DCV systems that optimize comfort while slashing energy bills.  Duct CO₂ sensors or wall-mounted CO sensors which do you need?

This sensor guide outlines precisely the operation of each sensor type,  the best application, ASHRAE 62.1 specifications, and which sensor is best for your building, system type, and budget.

What Are a Duct CO₂Sensor?

The Duct CO₂ sensor (also named the duct-mounted CO transmitter) is a sensor mounted into the sheet-metal ductwork of the HVAC system, where it usually inserted into the return-air duct through a small hole and a probe protrudesthrough into the air stream. The sensor then reads the air passing over it and transmits the CO reading to the BAS or HVAC system.

Since the return air consists of exhaust from a mixture of all the spaces connected to a given duct run,  one duct sensor provides a reading equivalent to an average CO in all zones served by that duct.

How Duct CO₂ sensors are installed:

1. Use the 3/8 inch drill to make a 1-inch opening through the duct wall at least 3 duct diameters from any elbow, damper or obstruction.
2. Insert aspiration probe ensures that the flow of air passes through the head of the probe.
3. Mount the enclosure to the duct with the fasteners provided making sure the foam backing seals all air gaps.
4. Connect the sensor output (0–10 VDC or 4–20 m A) to your BAS or AHU control,
5. Make sure all conduit or wiring penetrations are air-sealed. Otherwise, fresh outdoor air will flood the box and distort the readings.

Duct CO₂sensors are supplied by manufacturers such as Honeywell, Veris Industries, Dwyer Omega and ACI; the commercial grade of their NDIR CO sensors range is approximately $350–$550.

What Is a wall-mount CO₂Sensor?

wall-mount CO₂sensor (sometimes called a room or space CO transmitter) is located within the space that is being monitored, often on an interior wall at a height comparable to the location of building occupants’ heads. It measures the temperature and level of CO in the surrounding area (room or zone) and sends this information to the control system.

Since the sensor is physically installed inside the space it monitors, it detects the actual level of CO in the airspace rather than a ventilation corrected mixture of CO measured in the duct.

How Wall-mount CO₂Sensors are Installed:

1. Mount the backplate to an interior wall 4 to 5 ft above finished floor(inside the ASHRAE defined breathing zone of 3 6 ft).
2. Position the sensor so it is not near doors or windows, supply air diffusers,  or exterior walls that may lead to local dilution or concentration.
3. Connect AWG signal conductors to the screw terminals, using the back plate.
4. If running through a hollow wall or conduit,  seal any openings with a foam plug to avoid draughts of clean-air ruining records. 4.
5. Setup the output signal and control the onboard display preferences.

There are also wall-mount CO₂ sensors from all three of the above-mentioned brands at similar price ranges ($350–$550 for standard NDIR).

Key Differences: Duct vs wall-mount CO₂ Sensors

Measurement Position

Duct CO₂₂ sensor: standard installation in the air conditioning system back to the main pipeline, branch return air ducts, generally inserted into the mainstream area of the air duct installation, relying on the flow of the duct back to the air to complete the gas sampling, only to detect the pipeline mixed with the return air flow of the CO₂ concentration, is not the indoor concentration of the air in situ.

Wall-mounted CO₂ sensor: installed in accordance with HVAC standards on the wall of the indoor personnel activity area, the conventional installation height of 1.2-1.5m (the human body’s standard breathing height), direct convection collection of indoor static air, accurate detection of personnel breathing activities in the region of the in-situ concentration of CO₂, the value of the human body in accordance with the actual air quality environment.

Data Characteristics

Duct CO₂₂ sensor: adopts indirect sampling method to monitor the average CO₂ concentration of the mixed return air in multiple areas, unable to recognize the concentration fluctuation in a single room, and is only applicable to the whole area of the unit for unified ventilation control.

Wall-mounted CO₂ sensor: directly collects real-time CO₂ concentration in situ in independent space, the sampling point is close to the human respiratory zone, not interfered with by airflow in other areas, the data accurately reflect the air quality around the indoor personnel, and can provide effective detection data for independent ventilation regulation and control in a single room.

Monitoring Coverage

Ducted CO₂ sensor: a single unit can cover multiple functional areas, obtaining comprehensive air values for the whole area; it is unable to screen the difference in concentration between individual rooms and partitions, and has no partition identification capability.

Wall-mounted CO₂ sensor: a single unit is limited to serve a single room or independent functional partition, can realize one-to-one independent monitoring, support for single-room air volume independent regulation, partition control ability is very strong.

Construction and Installation Difficulty

Duct CO₂₂ sensor: complicated construction process, need to open the duct, sealing and fixing, pipeline wiring multi-operation, large amount of overhead work, high construction difficulty, later maintenance, dismantling and operation and maintenance is more inconvenient.

Wall-mounted CO₂ sensor: installation with the wall thermostat, only need to punch holes in the wall, wiring can be completed, construction is convenient and efficient, short construction period, maintenance and replacement of the latter is simple and convenient.

Visibility of Indoor Appearance

Duct CO₂₂ sensor: hidden installation in the ceiling duct, no exposed components, excellent concealment, does not affect the integrity of the interior decoration, suitable for all types of well-decorated architectural space.

Wall-mounted CO₂ sensors: Exposed and mounted on indoor visible walls, most models come with a display screen, which can be used to view real-time CO₂, temperature and humidity values in situ, making it easy for personnel to monitor indoor air quality visually.

Detection Response Speed

Ducted CO₂ sensor: airflow transmission path is long, there is a delay in air return sampling, lagging concentration feedback, slow linkage adjustment of air volume, difficult to adapt to the instantaneous flow of personnel ventilation control needs.

Wall-mounted CO₂ sensor: in-situ direct collection of indoor air in the personnel activity area, air transmission path is extremely short, no pipeline airflow delay loss, indoor CO₂ concentration rise and fall changes in response to the sensitive, can be quickly linked to the air-conditioning damper to regulate the air volume, ventilation linkage regulation and control of instantaneous and efficient, adapted to the refinement of the ventilation self-control.

Applicable System Scenarios

Duct CO₂₂ sensor: suitable for integrated central air conditioning, fixed air volume fresh air system, applicable to the working conditions of the unified, without the need for a single room to adjust the fresh air independently of the hotel, supermarkets, halls and other public spaces.

Wall-mounted CO₂ sensor: suitable for VAV variable air volume air conditioning, independent fresh air self-control system, suitable for offices, classrooms, conference rooms and other spaces with large fluctuations in the flow of people, requiring a single room independent air volume control.

Industry specifications

1. Single-zone complete ventilation and air-conditioning system: Ducted CO₂ sensor parameter compliance, can meet the DCV demand control ventilation national standards and industry norms;
2. Modular multi-area, sub-household controllable ventilation systems: ASHRAE 62.1 standard prioritizes the use of wall-mounted CO₂ sensors to ensure accurate ventilation air volume ratios and project compliance and acceptance.

Equipment Cost Reference

Two types of sensors purchasing unit price is similar, the market mainstream commercial brand models cost 350 ~ 550 U.S. dollars / unit, the equipment price difference is mainly determined by the detection accuracy, communication system, LCD configuration, and the duct, wall mounted two types of installation form has nothing to do.

Accuracy and Measurement Quality: What the Data Shows

This is probably where the two sensors differ to the greatest extent.

Wall-mount sensors reflect physical and thermal conditions experienced by building occupants.  Since the sensor is located in the breathing zone (3 to 6 ft above the floor) as specified in ASHRAE 62.1, the sensor reads CO₂ concentrations for each occupant when they breathe, and not the weighted average through the entire space; therefore,  a wall-mount sensor detects higher CO concentrations for 20 people occupying a conference room the moment they enter the space and close the door, and quickly ramps outdoor airflow to the added load.

Duct sensors measure a mixture, not a point. So if you take the return air from a 10-person conference room and it mixes in with the return air from a very empty open office before it arrives at the duct sensor,  then the increased CO₂ from the conference room office is diluted. The sensor reads an average, and vents more slowly until the busy office gets too hot to remain comfortable. This is a known phenomenon called“averaging,“and is well documented in the training records of HVAC engineers everywhere.

A real-world example complicates the matter: a convention center installed CO₂ sensors at ceiling height and in return ducts and labeled system “acceptable”. However,  measurements taken at human level in an occupied space revealed dangerously high levels the duct reading was a combination of the two plumes and hid the problem.

In the end when accurate (per zone) ventilation control is important, like in schools,  hospital wards, conference rooms, gyms etc., the best measurement is from wall-mount sensors. When TLA is used in large open spaces with fairly static occupancy schedules, duct sensors can be acceptable.

ASHRAE 62.1 Compliance: What the Standard Actually Requires

ASHRAE 62.1 is the standard reference in the United States for designing a ventilation system, and it provides recommendations on the type, installation, and accuracy of CO sensors for a demand-controlled ventilation system.

• l Sensor accuracy requirements (Addendum ab, 2023): For the purposes of DCV, CO₂ sensors shall be manufacturer-certified to be accurate to+/-75 ppm at concentrations of at least 600 ppm and 1,000 ppm at a Sea Level pressure (29.92 inches of Hg) and 77 degreesF (25 degreesC). CO₂ sensors shall be factory calibrated and shall not require recalibration for greater than 5 years.  Upon sensor malfunction detection, the system shall default back to providing the required minimum outdoor air to the breathing zone.
• l Placement requirement: As approved in ASHRAE 62.1, page 87-88, Section 6.2.6.1.3.1,  CO₂ sensors shall be installed in the space 3 feet to 6 feet above the finished floor a clear preference for a wall-mounted sensor in the occupied zone,  rather than a duct-mounted sensor.
• l Multiple-zone systems: In multi-space applications, ASHRA‘s advice aligns with independently-engineered recommendations: a single duct sensor in the common return is never sufficient, as it will provide only an averaged reading of CO₂ levels within the entire space, and some areas may not be getting proper ventilation while others are over-ventilated. Individual space-mounted sensors should be provided either at each zone, or at least at “critical” zones, as determined by occupancy density and activity type.
• l Single-zone exception: A duct sensor is justifiable in a single-zone operation, or where a return airstream can serve multiple zones which are occupied at a similar ratio at the same time. However, most engineers would still use space-mounted sensors instead of duct-mounted.

Energy Savings: Do Both Types of Sensor Provide?

Duct CO₂sensor and wall mount CO₂ sensor can be used to support DCV, and DCV provides actual energy savings. Analyzes of commercial building DCV installations have shown average energy cost reductions of 38% for all commercial building types combined when demand-controlled ventilation is substituted for fixed-schedule outdoor air delivery.

The sensor type affects how well those savings are realized:

Wall-mounted sensors allow discretely and accurately modulating ventilation at the zone level.  If a zone is genuinely unoccupied, the VAV box supplying fresh air to it can dial down outdoor airflow (as required by code) to avoid conditioning needlessly delivered outside air. When the occupancy rapidly increases, the sensor detects this fact and reacts accordingly.

Duct sensors respond more to the entire building and are therefore relatively more conservative.  Since the signal is averaged, at low occupancy zones will be over-ventilated (again by busy zones using the same return duct) while the need for increased ventilation at high occupancy zones will be delayed.

In the most common commercial building utilization varying occupancy in multiple zones wall-mounted sensors offer greater Potential for energy savings and are also more effective in reducing daily ventilation heat load.

When to Choose a Duct CO₂ Sensor

A Duct CO₂ sensor is the right choice when:

• l You have a single-Zon e HVAC system that is drawing all return air from one space (i.e.,  warehouse, large open manufacturing floor, single-tenant retail space).
• l All the areas serviced by the system have comparable occupancy patterns – they load and unload at the same time with the same load densities.
• l This system is left on all the time, so return airstream is constantly moving and duct sensor has a fresh sample of air.
• l You want to use fewer sensors with less installation cost on a large,  homogeneous building where individual zone accuracy is less important than the integrated building operation.
• l You are measuring intake air CO₂ to compare outdoor ambient levels against return-air levels employing a differential DCV strategy.

Duct sensors work best in single tenant large retail stores, warehouse type facilities or manufacturing environments where the occupancy is predictable and relatively constant throughout the day.

When to Choose a wall-mount CO₂ Sensor

A wall-mount CO₂ sensor is the right choice when:

• l You operate a multi-zone VAV system with independently filling and emptying zones, perhaps the most typical application in office,  school, healthcare and hospitality buildings.
• l For example, standard compliance with ASHRAE 62.1 or LEED is needed,  especially for DCV in densely occupied spaces (defined as greater than 25/1,000 sq ft).
• l Important areas for zone-specific ventilation control, such as conference rooms,  school rooms, gyms,  auditorium or other intermittently congested spaces.
• l Response time is important too you want the system to respond immediately when a room fills, not send the CO₂ to take a ride from the room to a remote duct sensor.
• l California Title 24,  or other applicable state energy codes,  including individual space sensors for medium-density occupied zones:
• l Wall-mount sensors are the norm for K-12 schools (where post-COVID IAQ upgrades have led to huge investments), hospitals, conference centers, and projects striving for LEED or WELL certification.

Installation Pitfalls That Undermine Either Sensor Type

Incorrect CO₂ readings are the prevalent result of installation error, without regard to how good the sensor hardware is.  Below are the critical areas to watch out for in both.

For Duct CO₂ sensors:

1. Do not install within 3 duct diameters of an elbow, damper or mixing box air is stratified near these fittings and CO readings will not show an accurate average.
2. Make sure that all conduit entries into the duct are air sealed. A draft of clean outside air permeating into the sensor enclosure through conduit or hollow walls will depress CO readings and result in a chronic underventilation condition.
3. Place probe on center of the duct wall away from corners where flow would be slowest and stratification most at its maximum.
4. Plan an access port for future calibration. -Removing the probe from a fully commissioning duct systemcan be problematic without an access port.

For wall-mount CO₂ sensors:

1. Never at a supply air diffuser,  door,  window, or operable skylight local dilution will produce artificially low readings and under-ventilation.
2. No install over printers, copiers, or kitchen equipment producing CO₂ imass exhaust (elevated readings will initiate overly air-conditioned areas).
3. Mount in the original orientation as specified by the manufacturer.  Sensors electronics produce a small amount of heat but the wrong angle of mounting could distort the measurement cell in the long term.
4. Check conduit or back entry wiring gaps, and fill with foam as required to prevent infiltration of the wall cavity by clean-air.

Cost Analysis: Total Cost of Ownership

Per sensor, duct and wall-mount CO₂sensors are similarly priced at $350–$550 for a commercial NDIR product of one of the larger manufacturers. The difference is in total sensors needed and installation labor.

• l Cost savings of a duct sensor: One duct sensor can control a multi-zone return duct run. You save on the number of units. Installation is simpler with one duct penetration (minor sheet metal work) and no interior wall work or long wiring runs to individual rooms.
• l Wall-mount sensor total cost: One sensor per zone,  and wire runs from each sensor to BAS or zone controller. In a building with 20 zones independently controlled, that is 20 sensors versus possibly one or two duct sensors — a substantial hardware and labor difference.
• l Counterpoint: industry professionals and ASHRAE guidance point out that CO₂ sensor prices have tumbled in recent years,  so the higher cost of space-mounted deployments is a small fraction of total project cost. The life-cycle energy savings associated with proper per-zone DCV control normally outweigh the added cost of the sensor within one to three HVAC seasons,  providing the longer-term financial incentive for wall-mount sensors.

Conclusion

Most commercial buildings require wall-mount CO₂sensors. They measure CO₂ at the breath level of building occupants,  provide more rapid responses to sudden occupancy fluctuations,  meet ASHRAE 62.1 and LEED multi-zone DCV standards for vent control, and provide the detailed per-zone control necessary for maximum energy savings. The added cost over duct sensors is small and is repaid in energy savings in short order.

When scoping a new DCV installation or retrofitting an existing one, it is best to collaborate with a controls integrator who can diagram sensor location to your zone layout prior to the hardware specification. Sensor type is the first consideration proper location is what makes a DCV system work correctly.

Prepared to specify CO₂ sensors for your next project? Download our CO₂ Sensor Selection Checklist or consult a controls specialist to determine the appropriate sensor type for your building‘s ventilation system.