Product review (5 stars): Accurate and sensitive CO2 monitoring
When I've received this item, the first thing I've done is to take out the CO2 measure outside. The reading was 397 PPM, wich is accurate enough, since it varies mostly from 350 to 450 CO2 year round. Regarding inside readings, again, I think it's very accurate and sensitive. When the HRV is fully founctioning (no defreezing cycle), the readings inside varies from 600 to 700 ppm, wich is what we should expect. When it's very cold outside, the HRV is often on the defreezing cycle, so it takes less fresh air from outside. The readings are then near 450 ppm when we come from work, and approximately 1000 ppm next morning. So again, that's consequent with the fresh air ventilation rate. At Christmas, we were 40 people at home and it was cold. At the end of the night, the CO2 reading was at 4000 ppm (!). So I took it outside to see. Within few seconds, the reading was at 450 ppm. It's very sensitive.

That being said, I can't tell that relative humidity monitoring (and so dew point measurement) is accurate. I have some reliable HR sensors in the house and they all tell the same with 1 to 3% diffences. This monitor is 6% below all of them. That's too much of a difference. If you are looking to buy a cheap and reliable CO2 sensor, I think that it is a very good buy. If you're looking for a relative humidity sensor, there is more reliable and cheaper options out there.