38
Normal speed = 70%
Low speed = 50%
Measured CO₂ rate
(ppm)
1000
800
14.5. Ventilation setpoint adjustment and CO₂ management
The principle of CO₂ regulation is to control the speed of the fans (and therefore the airflow) according to the level of
CO₂ in the air in the room. The higher the CO₂ level, and therefore the higher the occupancy rate, the faster the fans
turn, and therefore the higher the airflow (air change). Comfort and air quality are maintained at all times.
The adjustment of the ventilation setpoints involves associating a minimum speed setpoint ("low speed" setpoint
parameter) with a minimum (measured) CO₂ level, and a maximum speed setpoint ("normal speed" setpoint parameter)
with a maximum (measured) CO₂ level. These two pairs allow us to construct the following regulation law:
When the measured CO₂ level is:
■
Below 800 ppm (adjustable) the fan speed setpoint is equal to the minimum setpoint (=low speed)
■
Above 1000 ppm (adjustable) the fan speed setpoint is equal to the maximum setpoint (=normal speed)
■
Between 800 and 1000 ppm, the fan speed setpoint is calculated in relation to the straight line that passes through the
2 points.
Recommendation for setting the lower CO₂ threshold:
Ambient air contains a variable amount of CO₂ depending on the location. This concentration is generally above 300
ppm for a non-urban location and can exceed 700 ppm for a very dense urban location. Increasing the fan speed and
thus the airflow rate will not lower the CO₂ concentration in the room below this "base" value.
It is therefore advisable not to set the CO₂ threshold for the low speed too far below the basic CO₂ concentration of the
outside air in order not to generate unnecessary air exchange, which leads to energy consumption and noise pollution.
Recommendation for setting the upper CO₂ threshold:
The level of CO₂ in the air we breathe has a direct impact on the comfort and well-being of the occupants. Repeated and
prolonged exposure to CO₂ concentration values above 1000 ppm can have adverse effects on the human body. This
value of 1000 ppm will therefore be kept as a reference for the CO₂ threshold setting for normal speed.
If the ventilation unit is equipped with a heating coil to meet total heating needs, it may be recommended to increase the
ventilation setpoints of low speed (in particular) and normal speed to maintain the room temperature if the heat losses of
the room require it.
When the high CO₂ threshold (1000 ppm) cannot be maintained, especially when the room occupancy is high, it may be
necessary to increase the normal speed setpoint.
If the unit is not equipped with a heating coil, the low speed setpoint can be reduced, but not below 30 %, to ensure:
Zehnder Eversky
