Carbon dioxide concentration is the most practical, real-time indicator of ventilation quality in an occupied commercial building. When CO₂ levels rise, it means occupants are breathing air that has already been exhaled — air that is depleted of oxygen, loaded with moisture, and potentially carrying airborne pathogens. The fix is almost always an HVAC maintenance issue.

This guide covers what the numbers mean, what utility companies and standards bodies recommend, and — most importantly — what maintenance tasks actually move the needle.

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The Numbers: What CO₂ Levels Tell You

Outdoor ambient CO₂: approximately 420 ppm (parts per million) as of 2026

ASHRAE 62.1 threshold: ASHRAE Standard 62.1-2022 does not set a direct CO₂ limit, but a differential of 700 ppm above outdoor levels (approximately 1,120 ppm total) is widely used as the threshold indicating inadequate ventilation for the occupancy level.

Utility company recommendations: Pacific Gas & Electric (PG&E), Southern California Edison (SCE), and Consolidated Edison (Con Edison) all recommend maintaining CO₂ below 1,000 ppm in occupied commercial spaces as part of their demand-controlled ventilation (DCV) rebate programmes.

OSHA PEL: 5,000 ppm (8-hour time-weighted average) — this is a safety limit, not a comfort threshold. Occupant complaints about stuffiness, headaches, and difficulty concentrating typically begin around 1,000–1,200 ppm, well below the OSHA limit.

LEED v4.1 requirement: LEED EQ Credit 1 requires CO₂ monitoring with alarms set at no more than 10% above the ASHRAE 62.1 design concentration — typically 900–1,000 ppm for office occupancies.

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What Causes High CO₂ in Commercial Buildings

High CO₂ is almost always a ventilation problem. The most common causes:

1. Insufficient outdoor air intake The most common cause. The HVAC system is not bringing in enough outdoor air to dilute the CO₂ produced by occupants. This can be caused by:

• Economiser dampers stuck closed or set to minimum position
• Outdoor air dampers undersized for the actual occupancy
• Demand-controlled ventilation (DCV) system not functioning correctly
• Building operating at higher occupancy than the HVAC system was designed for

2. Short-circuiting Outdoor air is being introduced but is short-circuiting directly to the return air without mixing with the occupied space. This is a duct design issue but can be worsened by supply diffuser damage or blocked return grilles.

3. Recirculation without adequate outdoor air The HVAC system is recirculating return air without introducing sufficient outdoor air. This is the most energy-efficient mode of operation but requires careful balancing against ventilation requirements.

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Recommended CO₂ Monitors

For spot checks and portable monitoring:

• Aranet4 — NDIR sensor, ±50 ppm accuracy, Bluetooth data logging, 7-day battery life. The most widely used portable CO₂ monitor for commercial IAQ assessments. Approximately $250.
• Govee H5106 — budget option for spot checks, ±50 ppm accuracy, no data logging. Approximately $50.

For fixed installation with data logging:

• Onset HOBO MX1102A — NDIR sensor, ±50 ppm accuracy, Bluetooth data logging, 1-year battery life. Ideal for monitoring individual zones without wiring. Approximately $200.
• Vaisala GMP252 — industrial-grade NDIR sensor, ±20 ppm accuracy, 4–20 mA or Modbus output for BAS integration. The standard for permanent installation in commercial buildings. Approximately $600.

For BAS integration and DCV control:

• Vaisala GM10 — wall-mount CO₂ transmitter, BACnet/Modbus/analog output, ±30 ppm accuracy. Designed for integration with building automation systems for demand-controlled ventilation. Approximately $400.
• Johnson Controls CO₂ Sensor Series — compatible with Metasys BAS, ±50 ppm accuracy, wall-mount or duct-mount configurations.

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The HVAC Maintenance Tasks That Control CO₂

High CO₂ readings are a symptom. The root cause is almost always one of these maintenance issues:

Economiser Damper Inspection and Repair

The economiser damper controls how much outdoor air the HVAC system introduces. A stuck-closed economiser means the system runs on minimum outdoor air regardless of outdoor conditions — the most common cause of elevated CO₂ in commercial buildings.

Inspection procedure:

1. Set the thermostat to cooling mode with the outdoor temperature below the economiser changeover setpoint (typically 55–65°F)
2. Observe the outdoor air damper — it should open to a position greater than minimum
3. If the damper does not open, check the economiser controller, actuator, and linkage
4. Verify the damper closes fully in heating mode — a stuck-open damper wastes heating energy

Inspection frequency: Quarterly per ASHRAE 180 and most manufacturer IOM manuals

Outdoor Air Damper Minimum Position Verification

Even when the economiser is working correctly, the minimum outdoor air position may be set incorrectly — either too low (causing high CO₂) or too high (wasting energy).

Verification procedure:

1. Measure the outdoor air CFM with a flow hood or by calculating from duct velocity measurements
2. Compare to the ASHRAE 62.1 design outdoor air rate for the space (typically 0.06 CFM/sq ft + 5 CFM/person for offices)
3. Adjust the minimum damper position if the measured outdoor air rate is below the design value

DCV System Calibration

If your building uses demand-controlled ventilation (DCV), the CO₂ sensors that control the outdoor air damper must be calibrated annually. A sensor that reads 100 ppm low will allow CO₂ to rise 100 ppm above the setpoint before triggering additional outdoor air.

Calibration procedure:

• Use a certified calibration gas (typically 1,000 ppm CO₂ in nitrogen) and a two-point calibration procedure per the sensor manufacturer's instructions
• Log the calibration date, pre-calibration reading, and post-calibration reading in the equipment service record

Filter Replacement

A heavily loaded filter restricts airflow through the HVAC system, reducing the volume of outdoor air delivered to occupied spaces even when the damper is in the correct position.

Connection to CO₂: If CO₂ levels are elevated and the economiser is functioning correctly, check the filter pressure drop. A filter at or above the replacement threshold can reduce system airflow by 15–25%, directly reducing outdoor air delivery.

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Building a CO₂ Monitoring Programme

Step 1: Establish baseline readings Install monitors in representative zones — at minimum, one per floor and one in any high-occupancy space (conference rooms, break rooms, open-plan areas). Record readings during occupied hours for two weeks to establish a baseline.

Step 2: Identify problem zones Any zone consistently above 1,000 ppm during occupied hours is a ventilation problem that needs investigation.

Step 3: Investigate and repair For each problem zone, work through the maintenance checklist above: economiser operation, minimum outdoor air position, DCV calibration, filter condition.

Step 4: Verify improvement After completing repairs, monitor CO₂ for two weeks and verify readings have dropped below 1,000 ppm.

Step 5: Log everything Log all CO₂ readings, investigations, and repairs in your Maintain America equipment records. This documentation supports LEED certification, ASHRAE 62.1 compliance, and utility rebate applications for DCV upgrades.

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Ready to Simplify Your Maintenance Program?

Maintain America gives facilities teams one place to track HVAC equipment, log service history, store CO₂ monitoring records, and set maintenance reminders — across every system in your building portfolio.

Start tracking your maintenance on Maintain America — or explore the member portal if you already have an account.

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Related Resources

• HVAC Filter MERV Ratings Explained: MERV 10 vs. MERV 11 vs. MERV 13
• MERV 13 Filters, California Title 24 Compliance & Commercial HVAC Maintenance
• Fall HVAC Maintenance Checklist: Manufacturer-Recommended Steps Before the Cold Hits
• How to Claim Utility Rebates for HVAC Maintenance: A State-by-State Guide