CO2 Monitoring Calculator — Sensor Placement & Ventilation

CIBSE Guide A treats around 1000 ppm as the upper limit for an acceptably ventilated occupied space — roughly 8 L/s/person of fresh air above a 400 ppm outdoor background. The HSE does not set a legal CO2 limit, but its ventilation guidance uses CO2 as a proxy: below 800 ppm indicates a well-ventilated space, while a persistent average above 1500 ppm signals poor ventilation that needs action. Setting a 1000 ppm target and triggering ventilation boost at around 800 ppm keeps a comfortable margin, which is what this tool recommends for an office.

Approved Document F 2021 requires a CO2 monitor in each occupiable room of a new office between 50 m² and 320 m² floor area (125-800 m³ volume); it must be a non-dispersive infrared (NDIR) type, mains powered, mounted at breathing height, at least 500 mm from people, and away from windows, doors and ventilation openings. AD F sets the monitor type and placement but not a per-area count, so this tool uses a practical rule of thumb of about one sensor per 100 m² of open-plan area, with at least one per separately controlled ventilation zone. Larger rooms or tall, stratifying spaces may need more.

DCV uses CO2 sensors to modulate ventilation rates in real time based on occupancy. When CO2 rises (indicating more people), fan speeds or damper positions increase to provide more fresh air; when the space is lightly occupied, ventilation reduces toward the design minimum, saving fan energy. Reported savings vary widely with climate, building type and system, but CO2-DCV field and simulation studies (for example ASHRAE RP-1747 and RP-1819) typically show fan-energy reductions of roughly 10-30% versus a fixed-rate system. DCV must never drop supply below the Approved Document F design rate while the space is occupied.

CO2 data logging is recommended for Building Regulations compliance evidence, TM44 air-conditioning inspections, and WELL Building Standard certification. Logged data at 5-15 minute intervals helps identify ventilation issues and demonstrate ongoing compliance. Many modern sensors include cloud-based logging.

A practical approach is to trigger ventilation boost at around 800 ppm (80% of the 1000 ppm target) so the system has time to respond before levels approach the limit. This is a sensible control set-point rather than a value fixed by any single standard. A higher alarm — 1500 ppm in this tool — flags persistently poor ventilation that needs investigation, in line with HSE guidance that a sustained average above 1500 ppm indicates inadequate ventilation.