Demand-controlled ventilation with TVOC sensor


Airmaster has added a TVOC sensor to the already known demand-control with CO2 sensor, adding more parameters for controlling the ventilation

CO2 concentration has long been an indicator of human activity, but research shows that this is not the only substance capable of causing concentration difficulties and similar effects. Tiny concentrations of VOC – volatile organic compounds – are found in indoor environments and are also crucial to how humans perceive air quality and our wellbeing.

VOCs can be emitted from solids and liquids such as hand sanitiser, cleaning agents, building materials, furniture, carpets, work processes, cosmetics and human bodily processes. VOCs include substances such as formaldehyde, acetone, methanol, acetic acid and acetaldehyde.

Multiple parameters for controlling indoor air

CO2 and TVOC levels don’t always correspond. This explains why measuring VOC separately is relevant. Controlling air ventilation based on multiple relevant parameters benefits the occupants of indoor spaces.

When TVOC and CO2 sensors work in combination, the most critical signal will determine the need for air replacement. The demand control emits a modulating operating signal to ensure that the requisite volume of air is added to the indoor space to maintain a healthy indoor environment.

Good indoor air quality is not just a low CO2 level

The new Demand-control with TVOC sensor has given more parameters to control the ventilation.

In this video TVOC is explained and examples from a school are presented. One school even changed their cleaning routines due to the TVOC measurements.

Displaying TVOC levels

The TVOC concentration can be read on both the Orbit panel and Airlinq Online whenever a TVOC sensor is operating.

To simplify the interpretation of TVOC measurements, we have chosen five colour-coded levels. For recognisability’s sake, we chose a similar five-segment scale for CO2.

For instance, CO2 and TVOC levels in dark green indicate excellent indoor air quality.

The TVOC sensor measures TVOC concentrations roughly 1,000 times smaller than CO2 measurements.

TVOC and CO2 sensor control

TVOC, CO2 and air volume are viewable in Airlinq Online.

demand controlled ventilation with CO2 and TVOC

The TVOC (Total VOC) sensor registers all VOC substances.

When is TVOC relevant?

TVOC control is relevant in situations where human occupancy is not the only cause of poor air quality.

Other possible sources:

  • In rooms with widespread presence of hobby products, toys, pillows and the like in places such as institutions and the like
  • Wherever chemicals, hand sanitiser, perfume, cleaning agents, etc., are used, or when gluing, painting or similar activities are taking place.
  • In printer rooms and similar spaces.
  • Building materials and equipment can emit VOCs, which can be important in spaces filled with VOC-emitting solids or liquids but with low levels of human occupancy.

Examples of TVOC and CO2 measurements taken at a school

An Airmaster air handling unit with both CO2 and TVOC sensors is installed in different types of spaces at the school.

In this view, we show some examples of everyday activities that indicate the presence of VOCs and explain why Airmaster focuses on demand-controls that include TVOC sensors.

In most of the examples, we know the source of the emissions, which enables us to explain the fluctuating level of TVOC. This will hardly explain emissions everywhere, however. The TVOC sensor can indicate the presence of ‘something’ and whether this is a ‘little’ or a ‘lot’ but it cannot specify the type of TVOC.

CO2 ppm TVOC ppb

Because we are familiar with the premises, we know that the TVOC concentration at the library was caused by a printer.

Library, 2 september

With CO2 and TVOC demand-control

The graph shows a low level of CO2 from early morning that fluctuates concurrent with the number of visitors in the library.

The CO2 concentration declines in late morning, but the use of a printer at 11 am causes the level of TVOC to rise sharply.

Not until the CO2 and TVOC levels decline does the air handling unit gradually slow down.

Library, 7 september

TVOC control disengaged

This shows the controls after the TVOC demand-control was disengaged, so controls were based solely on the level of CO2.

The space had few occupants, so the CO2 level activated only limited flow in the air handling unit.

Compared to the graph from 2 September, it shows how a lower flow causes TVOC to accumulate.

bibliotek behovstyrd ventilation

TVOC control disengaged

ftx ventilation

With CO2 and TVOC controls

Classroom, 27 august

With CO2 and TVOC controls

The events of 2020 have taught us to use hand sanitiser quite often.

The graph shows measurements taken in a classroom where hand sanitiser is used twice a day: once in the morning and once in the afternoon. On both occasions, all the occupants use hand sanitiser and the air handling unit reacts by rising to 100% operation.

In the interim, the air handling unit reacts to both CO2 and TVOC levels and controls the climate according to the sensor with the highest fluctuation.

Classroom, 11 september

TVOC control disengaged

This shows the controls after the TVOC-sensor was disengaged, so controls were based solely on the CO2 concentration.

The air handling unit reacts in textbook fashion with demand-control triggered by the level of CO2.

The TVOC sensor continues to measure VOCs, even if the air handling unit is not using this to control airflow. Hand sanitiser is used in the room, causing a sharp rise in the level of TVOC.

The air handling unit does not react to this, however, as the TVOC-based demand control is switched off.

behovsstyrd ventilation

TVOC control disengaged

Behovsstyrt inomhusklimat

With CO2 and TVOC demand-control

Classroom, evening meeting, 3 september

With CO2 and TVOC demand-control

A parents’ meeting was held at the school, starting at 7 pm. The teacher arrived a little earlier, which is apparent from the curves, showing signs of activity around 6.30 pm. Quite a number of people attended the meeting, as indicated by the CO2 level.

We were told that hand sanitiser was used at the start of the meeting, which is also indicated by the TVOC level. The high TVOC level causes to the air handling unit to promptly reach 100% operating capacity to reduce the TVOC concentration.

The air handling unit quickly brings down the high TVOC level caused by the use of hand sanitiser and slows down a little before hand sanitiser is used once again at 8.40 pm. The air handling unit accelerates to maximum operation again where it remains until everyone leaves the classroom. The air handling unit does not gradually slow down again until around 9.10 pm after CO2 and TVOC levels gradually drop.

Office, 28 august

TVOC control disengaged

The ventilation of this space is controlled solely by CO2 measurements. The air quality seems fine if we solely consider the CO2 level. If we include the TVOC measurement, the curve becomes more uneven.

The level of TVOC steadily rises during the day with many peaks, which we know is caused by the use of hand sanitiser whenever visitors enter the office.

CO2 koncentration

TVOC control disengaged

Ready for a breath of fresh air

At Airmaster, we are “decentral” through and through. We are unrivalled when it comes to creating the best indoor climate and optimal well-being because we take each room and its occupants into account – and because we constantly strive to improve on what we do.