SystemAir HHFlex C, Operating Instructions Manual, Page: 34 / 47

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5.15.5 Modulated recirculation using CO2 sensor
This control mode uses the logic of the heatcool modulation corrected by the requested fresh air value.
Minimum fresh air threshold
The minimum fresh air threshold value is now calculated using PI loop, based on co2PID parameters used for
CO2 fancontrol, controlling the recirculation ratio with the CO2/IAQ sensor feedback.
The heatcool mix request is calculated as in the pevious paragraph.
The Mix command is then evaluated as below:
Mix_req = MIN(hc_mix_req, 100 - fresh_air_req)
5.15.6 Modulated recirculation as heating/cooling capacity with fresh air calculation
In some condition, sucha as duct pressure airflow control, the airflow may vary in great proportion.
Indeed, the amount of fresh air cannot be fixed considering only the recirculation damper opening.
Thus, the fresh air rate can be estimated using Mixing Air Temperature method. To do this an additional
temperature sensor is needed after the mixing section and before the heating/cooling stage. (for the supply
only units, it is the intake sensor).
Then we have:
%fresh_air = (EAT – IAT) / (EAT – OAT)
This method cannot be used when EAT = OAT, but in these conditions, recirculation is not needed.
Then the minimum fresh air threshold can be driven using a PID, using %fresh_air as control input value,
%min_fresh as setpoint value, and mix_cmd as control output value.