I have a CO2 monitor and I don't understand one thing - it seems that CO2 increases more quickly during summer than during winter. If I close my windows it takes longer to reach 1000 ppm during winter than it does during the summer.

I didn't gather concrete data on this but this is just what I eyeballed over the last few years. Does anyone know why could this be the case?

Do you have a forced-air heating system?