In the CO2 graph shown below, then the value is above 400ppm, it is because I am int he room. What I have trouble understanding is the 3 straight up jumps that happen over a 2 minute period (which may be the update rate I use). For the first 2, I was the only one at home, and for the 3rd, nobody was at home. The HVAC systems are set to circulate air 50% of the time 24/7 so that should not be a factor.
The CO2 sensor is the default S8 sensor inside the AirGradient Air Quality Sensor that I flashed with ESPHome.
That should not produce CO2, but the more lethal CO (Carbon monoxide)
This looks like a possible self-calibration to me, as CO2 levels these days could/should not actually reach any value below 400 ppm - the outdoor concentration. Strange that it does not calibrate and stick to the 400 pp level though, as my CO2 sensor regularly does after airing the home extensively.
While CO is a product from incomplete burning, CO2 is a product of more complete burning along with water vapor…
Please look it up if you do not believe me.
Water heater, furnace, stove can all increase levels of either of them.
The sensor should only detect CO2 (Carbon Dioxide) and not CO (Carbon Monoxide) that is released with combustion, but maybe I should check to be sure.
Anyhow, the only combustion appliances in the stand alone home are the 2 HVAC furnaces (in the attic) and the cooktop. The HVAC systems are set to cooling and therefore the furnaces are disabled. I was also not using the cooktop anywhere close to those times and the kitchen in on the opposite side of the house on another floor.
However, I do see particulate rise when I cook throughout the house so that likely applies to CO spreading too… but I can’t think of any source with an empty home. The nighttime CO of the few plants I have is unlikely to be detectable.
Great point… I recall putting the sensor outside to calibrate it but I can’t recall what I had to do on the sw side, if anything. There shouldn’t be anything in the code that triggers calibration as the sensor is indoors and it would not make sense for it to be calibrated inside a house.
My sensors says that it autmaitcally calibrates itself regularly whenever the value goes as low as possible, usually when I do major airing sessions then it sets the lowest point to the minimum 400 ppm.
Maybe have a look if some similar statement is your the manual for your device.
I found that “The Senseair S8 sensor incorporates an ABC algorithm that continuously monitors the lowest CO2 reading over a period and slowly adjusts for any long-term drift compared to the expected fresh air value of 400 ppm (or 0.04% vol) CO2.”
I have recently set the blowers in my HVAC systems to run 50% of the time in an effort to filter all home air through the 3 MERV11 filters in the return grilles. My son is suffering from asthma and I am hoping this will act as a giant air purifier. This may be pulling down the normal CO2 concentration in my office upstairs (I spend a lot of time in there) causing the sensor to recalibrate.
Indeed each spike pulls the value back up to 400ppm and then it start dropping until the next recalibration. I wonder if the sensor is bad…
PS: Looking at historical data the issue indeed started after I set the blowers to run 50% of the time. Before that the CO2 in my office was always much higher than it is now.
Something does not look right, as even if you continuously brought in fresh air, the value should never dip below 400 ppm, especially not so frequently, even with any HVAC settings.
The only way this could happen is if you added substantial amounts of pure oxygen for example, which I doubt your or your HVAC does.
It might be worth getting a second CO2 monitoring device for comparison and possible replacement.
If your son is suffering from asthma, you might want to get a device which also monitors VOC and Particulate Matters like PM2.5 and PM10, which affect asthma sufferers a lot more than CO2 levels.
