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The original was posted on /r/homeassistant by /u/Ok_Combination_895 on 2025-08-15 07:19:04+00:00.

A couple of nights ago, I noticed something strange with CO2 levels at home. Usually, it goes up in the bedroom at night, but this time I saw it rising in the living room too. I have an automation that increases the speed of my supply & exhaust ventilation (HRV), it triggered, but CO2 didn’t drop.

It’s summer here - hot during the day (+28°C), but the last few nights it dropped to +8°C. The ventilation unit switched on heat recovery. I even started thinking maybe the rotor is leaking exhaust air back into supply.

Last night CO2 started rising again. I set the ventilation to full speed and turned on an extra fan to move air around the house, but levels didn’t drop at all. Checked my supply air with a sensor - ~700 ppm. Took it outside - also ~700 ppm (I regularly calibrate my sensors).

This morning I checked again outside - 420 ppm. So it seems CO2 is genuinely higher outdoors at night/evening. I live near a forest, no factories nearby, few neighbors (nobody is heating right now), and the weather has been completely still with no wind.

Question: is this just a temporary thing during calm weather and temperature inversion, or is outdoor CO2 always higher at night? Why does it happen? Do you take this into account in your ventilation automations? Anyone here tracking outdoor CO2?