[stevehull]

AC,

I can guarantee that CO2 is not building up. Rather, "stale air" is out gassing of construction components, the infamous body odor, cooking smells, but by far the humidity is the dominant issue.

Our respiratory control system is such that even tiny increases in the ppm of CO2 (2-3 ppm) will cause us to increase ventilation rate. It is almost impossible to withstand an environment with 1000 ppm of CO2.

Let's see what your house has in it. One way to test this is to hook up the CO2 sensor and record a week's data. All houses have air leakage. I believe the very best that I know of had one complete air exchange every 48 hours. I suspect your tight house has an exchange every 30 hours or so (very, very tight construction).

I have a lot of experience with CO2 monitoring and can help you with hooking up the analog output of the CO2 sensor to the A/D port. Biu while you are there, hook up a relative humidity detector as well.

Most homes do very well by using a constant 10-20 CFM HRV (or ERV) exchange volume all day long. The reason to kick it up is shower/bathroom use.

I used a DPDT wall switch to both control the 120 V light above the shower and to increase the air CFM to 50-60 CFM with a 555 timer "one shot" to keep the HRV fan on for an additional 15 minutes after the light went off.

The wall switch just to the bathroom turned also turned on the HRV fan to high for only 5 minutes.

Measure your CO2 and humidity - let's see what the house has in it now.

Steve