Your CO2 Reading is Misleading

Hello fellow air nerds. I just stumbled across this sub and I figured I’d throw my two cents in.

I see a lot of posts referencing indoor CO2 sensor readings and making/requesting recommendations based on the measured PPM.

It’s worth noting that nearly every reading you see is likely inaccurate to a large enough degree that a single indoor CO2 measurement alone is not sufficient to make ventilation recommendations.

All CO2 sensors from consumer grade to commercial grade are prone to significant sensor drift and are difficult/expensive to calibrate properly even for an experienced HVAC/BMS tech.

I referenced this Lawrence Berkeley National Laboratories study frequently when consulting building managers, owners and engineers on IAQ questions. I thought you guys might find this interesting.

https://indoor.lbl.gov/publications/accuracy-co2-sensors-commercial

TLDR: LBNL tested 44 of their CO2 sensors being used for demand control ventilation, only 25% of them were within +- 20% accuracy required for DCV. No matter how old or what brand they were, the data is practically useless.

I’d recommend taking your CO2 sensors outside to see what they read out there first, then take them inside and measure the increase. The difference between indoor and outdoor CO2 levels will be accurate if read on the same exact sensor. An increase of 400-600 PPM means you’re likely getting proper ventilation. Any more than that, open a window (or OA damper).

Stay fresh!