Over part of Christmas I was away for a few days. As you can see from the plot, this was long enough for the CO2 concentration inside my flat to relax to the atmospheric value (or close to that) of a bit over 400 ppm. The plot has time as the x-axis, starting at midnight of the day I left. I left at about midday of the first day and the CO2 concentration then relaxed back to about 450 ppm, and stayed there. The relaxation is well fit by an exponential function with a time constant of close to ten hours. The fit is the dashed cyan curve.
This implies that with all windows closed (I closed them before I left), the air turns over* about once per ten hours. The total volume of my flat is a bit less than 100 m3, so this means a turnover rate of about 10 m3/hour or about three litres per second**.
With one person in the flat with the windows closed, they are breathing out about 0.5 m3/hour of breath containing 40,000 ppm of CO2. With all windows closed and a turnover of 10 m3/hour, then as turnover is 20 times the volume breathed out by a person, the CO2 should increase to a plateau of 40,000/20 + 400 = 2,400 ppm CO2. This is really rather high, and of course would be twice as high if two people lived here.
I don’t know how typical my flat is, some of you may live in more airtight, some in less airtight, homes. I would guess my 1970s built flat is probably not too far off being typical of that era. This turnover is good in the sense of contributing to keeping the air fresher, but there is a cost. Heating all the air in my flat by 10 C costs maybe 3 p, at current prices***.
Assuming the flat is heated to keep air indoors 10 C warmer, and with two turnovers per day, then this turnover means during the day the air turns over and needs to be heated, at a cost of 6 p. This looks pretty small, I assume that the dominant heat loss mechanisms are elsewhere, house radiating heat and losing it into the ground, etc.
* This assumes that all the CO2 leaves via air exchange between in and outside, i.e., that a negligible CO2 is adsorbed. This seems reasonable but am not expert here.
**This is consistent with the variation of CO2 in my bedroom overnight when I closed the window, see earlier blog post.
*** For details see previous post.
Chance & Necessity 