co2 and oa dampers 2

[semperfi1]

I have a return air co2 sensor that opens outside air dampers. The outside air exceeds the coiling coil design capacity and raises the discharge air temperature above the 55 degree design set point. This condition also raises humidity levels in the space. At these conditions, the comfort levels in the space are no longer maintained. Should there be a limit on how far the outside air dampers should open?

 

[walkes]

The outside air dampers should be at minimum position when mechanical cooling is enabled. Your minimum outside airflow rate should match the design load for the building. The CO2 sensor should decrease the OA volume when not required. Sounds like the coil may be undersized or you are experiencing conditions that exceed the design.

 

[ChasBean1]

The return air CO2 sensor should NOT affect the OA volume in any way when below a certain setting (usually about 800 ppm).

The percentage of outside air (outside, return, and relief damper positions) should control via a normal economizer cycle; preferably an enthalpy economizer.

If the return air CO2 concentration exceeds 800 ppm, the enthalpy economizer cycle should be SLOWLY overridden. This control should be a proportional control relative to the CO2 concentration. Here’s an example: if the return CO2 concentration is 800 ppm, the dampers should remain at minimum, or at their current value. If the return CO2 concentration rises to 900 ppm, the economizer dampers should be at 50% outdoor air. If the return CO2 rises to 1,000 ppm, the economizer dampers should be at 100% outdoor air. Once return air CO2 drops below 775 ppm (adj.), the CO2 control is disabled.

The CO2 economizer control shouldn't exceed five minutes from minimum to full outside air. IF the designer uses CO2 control as a method of overriding the enthalpy economizer, the cooling and heating coils need to be sized for full outdoor design conditions for that area. They should also have the ability to operate well at part-load conditions.

-CB