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CO2 Demand-Controlled Ventilation: ASHRAE 62.1 Calculation and Multi-Level Automation for Thai Homes

CO2 Demand-Controlled Ventilation: คำนวณ ASHRAE 62.1 และ Automation หลายระดับสำหรับบ้านไทย

May 13, 2026 · 1 min read
CO2 Demand-Controlled Ventilation: ASHRAE 62.1 Calculation and Multi-Level Automation for Thai Homes

Why Fixed-Speed Ventilation Falls Short

Fixed-speed ventilation runs at the same speed continuously regardless of room occupancy. Problems that result:

  • 40-60% excess energy consumption when rooms are unoccupied
  • Constant noise even when unnecessary
  • Filter loading faster than needed — more frequent replacements
  • Excessive dryness during lower-humidity periods

Demand-Controlled Ventilation (DCV) solves this by using CO2 as a proxy for human presence in the room.

Calculating Minimum Ventilation Rate per ASHRAE 62.1

ASHRAE 62.1 is the international ventilation standard used worldwide. Minimum fresh air formula:

Ventilation Rate = (People Component) + (Area Component)

  • People component: number of occupants × 7.5 L/s/person (or 15 CFM/person)
  • Area component: room area (m²) × 0.3 L/s/m² (or 0.06 CFM/sq.ft)

Example — 20 m² bedroom (2 occupants):

  • People: 2 × 7.5 = 15 L/s
  • Area: 20 × 0.3 = 6 L/s
  • Total: 21 L/s = 44.5 CFM minimum
  • Converted to ACH: 44.5 CFM × 60 ÷ (20 m² × 2.7m × 35.3) = ~1.4 ACH

Living room 30 m² (4 occupants):

  • People: 4 × 7.5 = 30 L/s
  • Area: 30 × 0.3 = 9 L/s
  • Total: 39 L/s = 82.7 CFM minimum

CO2 Thresholds for DCV in Thai Homes

CO2 Level Meaning Response
< 600 ppm Empty or excellent ventilation Fan 20% low speed
600-800 ppm Normal, few occupants Fan 40% medium
800-1,000 ppm High occupancy or activity Fan 70% high
1,000-1,200 ppm Air quality declining Fan 100% max + alert
> 1,200 ppm Health risk Critical alert + auto open window

Thai bedroom target: < 900 ppm throughout the night (Harvard T.H. Chan School research: CO2 > 1,000 ppm reduces sleep quality by 10-15%)

Multi-Level DCV Automation in Home Assistant

yaml
alias: "Bedroom CO2 DCV - Multi Level"
trigger:
  - platform: numeric_state
    entity_id: sensor.bedroom_co2
    above: 800
  - platform: numeric_state
    entity_id: sensor.bedroom_co2
    above: 1000
  - platform: numeric_state
    entity_id: sensor.bedroom_co2
    below: 600
action:
  - choose:
      - conditions:
          - condition: numeric_state
            entity_id: sensor.bedroom_co2
            above: 1000
        sequence:
          - service: fan.set_percentage
            target:
              entity_id: fan.bedroom_erv
            data:
              percentage: 100
          - service: notify.mobile_app_owner
            data:
              title: "⚠️ CO2 High"
              message: "Bedroom CO2: {{ states('sensor.bedroom_co2') }} ppm"
      - conditions:
          - condition: numeric_state
            entity_id: sensor.bedroom_co2
            above: 800
            below: 1001
        sequence:
          - service: fan.set_percentage
            target:
              entity_id: fan.bedroom_erv
            data:
              percentage: 70
      - conditions:
          - condition: numeric_state
            entity_id: sensor.bedroom_co2
            below: 600
        sequence:
          - service: fan.set_percentage
            target:
              entity_id: fan.bedroom_erv
            data:
              percentage: 20

VOC vs CO2 Sensors: What to Use Where

Sensor Measures Best for Price (THB)
CO2 (NDIR) Direct CO2 Bedrooms, living rooms 1,500-3,000
eCO2 (MOX) Estimated CO2 from VOC Budget option, less accurate 500-1,000
TVOC Total volatile organics New furniture, paint, adhesives 800-1,500
PM2.5 Fine particulate matter Entry areas, reception rooms 1,000-2,500

Recommendations for Thai homes:

  • Bedrooms: CO2 NDIR (Sensirion SCD40 or SCD41)
  • Rooms with new furniture or recent renovation: add TVOC sensor
  • Home entry points: PM2.5 to monitor outdoor air infiltration

Questions & answers

What is CO2 Demand-Controlled Ventilation?
DCV adjusts fan speed based on room CO2 level instead of running at a fixed speed continuously. High CO2 means more occupants, so fresh air increases. Low CO2 means the room is empty, so fan speed drops. Energy savings of 40-60% in unoccupied periods.
What is the CO2 target for a Thai bedroom?
Below 900 ppm throughout the night. Harvard T.H. Chan School research shows CO2 above 1,000 ppm reduces sleep quality by 10-15%. At above 1,200 ppm, opening windows or activating ERV at full speed is recommended immediately.
How does ASHRAE 62.1 calculate room ventilation requirements?
Formula: (occupants × 7.5 L/s) + (room area m² × 0.3 L/s). A 20 m² bedroom with 2 occupants = (2×7.5) + (20×0.3) = 21 L/s = 44.5 CFM minimum. DCV increases airflow above this minimum as actual CO2 rises.
Should I use CO2 or VOC sensors for demand-controlled ventilation?
CO2 NDIR sensors (such as the SCD40) are the most accurate for DCV because they measure CO2 directly — the best proxy for human occupancy. Add TVOC sensors in rooms with new furniture or fresh paint. Avoid eCO2 (MOX) sensors for DCV as they are less accurate.

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