Quote Originally Posted by chuckcrj View Post
Honeywell makes a CO2 sensor that uses contact closure. This could be used to control any ventilation system including ERV/HRV.

Very reasonably priced.
That's what I was thinking. With an HRV/ERV you could use the CO2 sensor relay to cycle the unit. Since you are recovering most of the heat, the ventilation temperature limits wouldn't really be needed. You still might want to use the ventilation dew point limit though, depending on the local climate. I can see why most people wouldn't bother with the extra cost of a CO2 sensor. If you are getting 70-80% energy recovery, it doesn't matter as much if you are over-ventilating, and the ROI on the CO2 sensor is long.

With a simple supply-side fresh air intake, you could use the CO2 sensor's proportional 10V voltage output to control an actuator on a damper to let in just the right amount of air. Since there is no heat recovery, you would also probably want to use both the ventilation temperature limits and the dew point limit.