Control using CO2 sensors
Demand control based on CO2 content is an excellent method for managing the indoor air quality in places of learning, such as schools, academies and universities. An indoor environment with good air quality could be expected to have a CO2 concentration of between 850 and 1250 ppm, which poses no issues to mental function. Studies have shown that high levels of CO2 have an adverse effect on students’ ability to concentrate and take decisions (eg, Fisk & Mendell, Berkeley Lab – Univ of California, 2012). Beyond 2,000 ppm students can expect to experience lethargy, drowsiness and lack of concentration.
CO2 sensors for AirMaster air handling units with demand controlled ventilation are compact and can be wall-mounted or integrated within the casing. Their operating range is between 0 – 2,000 ppm. Crucially, they can be used for modulating control of the heat recovery ventilation system. Typical set values for a CO2 sensor are Minimum CO2 = 500 ppm (for the initiating signal) and Maximum CO2 = 900 ppm (for full throughput).
Demand controlled ventilation using carbon dioxide (CO2) sensors is essentially a combination of two technologies: sensors which monitor CO2 levels in the air inside a building and an air handling unit (AHU) that uses data from the sensors to regulate the amount of ventilation air admitted.
CO2 sensors can be paired with a ‘base’ level of direct ventilation. In this mode, the heat recovery ventilation unit is programmed to run continuously at low load, until a signal from the sensor indicates that air quality has begun to change. As long as the CO2 level remains above the minimum threshold, the AirMaster heat recovery air handling unit continues under modulating control.
To prevent stop/start cycling around the CO2 entry level, the start point is automatically programmed to ‘lower limit + 10%’. The stop signal is generated at the lower limit exactly.
From an energy saving perspective, CO2 control makes a great contribution by preventing losses incurred through over-ventilation, without compromising on consistency of good indoor air quality.
The potential of CO2 sensors and demand controlled ventilation for operational energy savings is considerable. The highest benefit can be expected in school classrooms, seminar rooms, meeting rooms and other spaces in which occupancy is variable.
The higher the prevailing utility rates, the higher the benefit.
Passive Infrared (PIR) motion sensors provide signals which can be used for start / stop control of heat recovery ventilation units. These sensors respond to minute increases of infra-red radiation emitted from within their field of vision, which are then interpreted in terms of human movement. Sensors have uniform detection sensitivity within a range of 0.5 – 12m. Their circuitry includes a true motion algorithm, which allows each sensor to distinguish between (a) the motion of a room occupant and (b) other factors arising from infra-red sources, but without any corresponding displacement.
PIR devices can be programmed to operate with an ‘over-run’ timer. This instructs the heat recovery ventilation unit to continue in operation for an over-run period (say, 30 minutes) after the last detected movement and then switch itself off. This feature of PIR control contributes to reducing energy usage, by ceasing direct ventilation when not required.
Alternatively, it may be desirable to have a programmed level of ‘base’ ventilation. The PIR signal (to confirm that people have entered the room) is then used to switch the heat recovery ventilation unit up to full throughput. Once the last person has left, the over-run period counts off, followed by reversion to base load.
AirMaster heat recovery ventilation units can be used to monitor and control relative humidity (RH). This can be achieved with AirMaster hydrostats, which have a humidity measuring element of stabilised synthetic gauze. The measured variable is fibre length, which is proportional to the surrounding RH. Hydrostats can be used with either minimum or maximum settings, with accurate operation within the range 25 – 95% RH.
Hydrostats are suited to systems designed to have a base level of ventilation, with the ‘dehumidify’ signal being used to take the heat recovery ventilation unit to full power. This combination works well in facilities such as sports and leisure centres.
For heat recovery ventilation to operate with the lowest energy usage, the choice of sensor is significant. Whether the installation is for a classroom, meeting room, reception area, individual office or treatment room, typical usage patterns can be anticipated and sensor selection made accordingly. SAV are happy to advise the type and positioning of sensors for any type of project. Our objective is to hold ventilation flow only to what is required and thereby minimise carbon footprint.