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Sensors for demand based ventilation systems

04/01/2023 Yves Vinck

Which sensors should I use in my building? This is the question that is regularly asked these days with most people spending up to 90 % of their time indoors. Due to Corona and the ever-improving insulation of our buildings, the importance of good ventilation is becoming increasingly clear. Along with this advancing insight, additional questions arise.

Sensors for demand based ventilation systems

The importance of indoor air quality
In these modern times we spend more and more time indoors. Some studies indicate that we spend an average of 90% of our time indoors! Homes and buildings are better insulated to save energy. Better insulation and airtightness of homes creates the need to ventilate them better. After all, ventilation is necessary to keep indoor air quality under control. The indoor air we breathe is not only important for the comfort and concentration of residents. It also has a direct impact on our health. Certainly in the long term. Typical complaints caused by long exposure to poor air quality range from headaches, irritation of the eyes, nose and throat to serious conditions such as respiratory disease, heart disease and cancer. The importance of monitoring and optimizing indoor air quality should therefore not be underestimated. A well maintained ventilation system removes harmful substances from the indoor air and replaces them with filtered, fresh outdoor air.

Excessive ventilation has no negative impact on indoor air quality. The disadvantage of too much ventilation is unnecessary energy consumption. This energy consumption consists on the one hand of electrical energy and on the other hand of thermal energy. The higher the fan speed, the more electrical energy it consumes. Most fans have a quadratic torque curve. This means that even a small reduction in fan speed can yield significant energy savings.
In addition, there is also the thermal energy. When cold outside air is brought into a house and the warm, used air is removed from the house, a loss of heat (thermal energy) occurs. Thanks to modern ventilation systems with high-efficient heat exchangers, these losses are negligible. Further optimization is possible by controlling the air volume flows (controlling fan speed). HVAC sensors monitor indoor air quality. Based on these measurements, fan speed can be optimised. In this way, the supply of fresh air can be controlled demand based and good indoor air quality can be combined with energy efficiency. There are many different options for measuring indoor air quality. The nature of the indoor space often determines the type of sensor that is required to keep the air quality optimal.

Temperature and humidity are the basic parameters
Temperature and humidity have a direct influence on our feeling of comfort. Neither a cold, damp environment nor a dry, warm room make us feel comfortable. Depending on our activity, we will feel most comfortable in a room with a temperature between 20 and 25 °C and a relative humidity between 35 and 60 %. Through our daily activities such as cooking, showering, drying the laundry, etc., we bring a lot of moisture into the home. When it is well insulated and airtight, it is difficult for this moisture to escape. Too much moisture in a building is not only a problem for our sense of comfort. It is harmful to the structure of the building and increases the risk of mould formation. Mould formation is detrimental to the health of residents. Inhalation of mould spores increases the risk of the above-mentioned conditions, especially in the long term.

Relative humidity is the ratio of the amount of water vapour in the air to the maximum amount of water vapour that can be present in the air. This maximum value is determined by the temperature. Relative humidity is expressed in %. The warmer the air, the more water vapour the air can absorb. When warm (indoor) air comes into contact with a cold surface – for example a window – condensation occurs. The temperature at which condensation occurs is the dew point temperature or dew point (expressed in °C). A ventilation system must therefore ensure that the relative humidity remains within comfortable limits. Typically this is between 35 and 60%. In addition, care must be taken that the indoor temperature is always higher than the dew point. When the indoor temperature is lower than the dew point, condensation will occur with the risk of mould formation.

Temperature, relative humidity and dew point are the most essential parameters for the comfort of the residents. These parameters are usually taken into account when controlling a ventilation system. For that reason, most professional HVAC sensors can measure these parameters. These basic HVAC sensors prove their usefulness, especially in humid areas such as bathrooms and kitchens.

CO2 as an indicator of indoor human activity
Good ventilation not only keeps the humidity in balance, it also prevents harmful substances and gases from accumulating in the indoor air. One of those gases is CO2 or carbon dioxide. CO2 is not harmful to humans in normal concentrations. It is even one of the 5 main components of our atmosphere, after nitrogen, oxygen, water vapour and argon. Plants cannot grow without CO2. CO2 is less harmless at higher concentrations. When the concentration of CO2 in indoor air becomes too high, complaints of drowsiness, loss of concentration and subsequently headaches occur.
Without a ventilation system, CO2 concentrations can increase very quickly in a closed space. The more people are present and the more physical activity there is, the faster the CO2 concentration will rise. In our body, food containing carbon is 'burned' and converted into energy. This metabolic combustion process releases CO2. We then exhale this carbon dioxide. Measuring the CO2 concentration in the indoor air therefore provides relevant information about the occupancy rate of a room and about the need for extra supply of fresh air.

The CO2 concentration in an enclosed space also gives an indication of the risk of the amount of aerosols in the air. Aerosols can spread viruses. They are miniscule droplets that are released when coughing, sneezing or talking. When other people inhale these droplets or get them into their mouth, nose or eyes through their hands, they can become infected with the virus. To make the residents feel comfortable and to prevent drowsiness and loss of concentration, it is recommended to keep the CO2 level below 800 ppm through a sufficient supply of fresh air.

CO2 sensors give a good idea of the occupancy rate of a room because the CO2 concentration correlates with human activity. This type of sensors is therefore mainly used in rooms with strongly fluctuating occupancy rates. The higher the detected CO2 concentration, the higher the human activity and the more ventilation is required. Not only the metabolism of humans and animals is responsible for the production of CO2. In addition to human activity, there are many other sources of CO2 production. CO2 is also created during the (complete) combustion of fossil fuels. The CO2 concentration in the outside air therefore depends on the region. It will be higher in an urban environment than in a rural environment. A typical CO2 concentration in outdoor air is about 450 ppm.

How can the CO2 level remain more or less constant when people and animals have been walking around for centuries that produce CO2? Nature itself ensures that CO2 is removed from the atmosphere. Trees and plants convert CO2 into carbon and oxygen during the photosynthesis process. The carbon is used by plants to grow. Trees and plants themselves consist largely of carbon. The oxygen is released by the trees and plants into the atmosphere. Oceans also absorb CO2 from the air. The carbon dioxide is first absorbed in the upper layers of the ocean and then sinks to greater depths, where krill, plankton and seaweed convert it back into carbon and oxygen. However, these processes take a long time. The combination of global population growth and ever-increasing industrialization disrupts this natural balance. Human activity emits much more CO2 than the maximum absorption capacity of nature. The extra CO2 molecules that linger in the atmosphere absorb infrared radiation – also known as heat radiation – and send some of it back to Earth. As a result, the earth is slowly warming up more and more.

VOC as a measure of indoor air quality
VOC or Volatile Organic Compounds is a collective name for a group of chemicals that can be present in a residential environment. They are volatile or rapidly evaporating products containing one or more carbon atoms (organic substances). Typical examples are benzene, ethylene glycol, formaldehyde, methylene chloride, tetrachloroethylene, toluene, xylene and butadiene. These chemicals can be found in household environments in cleaning products, perfumes, solvents in paints and propellant for hair spray cans. VOCs are also found in fragrance fresheners, building materials and cigarette smoke. The typical smell of new furniture or a new car can give a pleasant feeling. In reality it is a mixture of volatile organic compounds. In the open air, VOC concentrations are normally quite low. On busy roads and in cities, a higher VOC concentration can be measured, usually as a result of exhaust gases. The effect and harmfulness of these substances is very diverse.

Sometimes you can smell the presence of high concentrations of VOC (e.g. smell of paint), but harmful concentrations can also be present without you noticing. The impact on the health of residents depends on the nature of the VOC, the amount of VOC inhaled and the duration of exposure. Short exposure to a high VOC concentration, such as during painting or when using cleaning agents, can cause dizziness, nausea, concentration problems and irritation of the eyes and respiratory tract. These effects are temporary. OPS or Organo Psycho Syndrome is a known consequence of prolonged or repeated exposure to high VOC concentrations among professional painters. This manifests itself in all kinds of mental problems and memory problems. The damage caused in this way is permanent. At typical VOC concentrations in a residential environment, the effects are less obvious. Often there are no complaints in the short term and you do not smell the VOCs.

VOCs are volatile, so the concentration will decrease over time. This period depends on the source and the VOC concentration. New construction and renovation work, but also a carpet or a new sofa usually temporarily cause higher VOC concentrations in the indoor air. Extra ventilation is then recommended during the first months. The use of VOCs indoors should be limited as much as possible, given their negative impact on indoor air quality. At higher VOC concentrations, extra ventilation is the solution. In principle, VOC sensors can be used in all rooms. Especially in storage areas for detergents and in bathrooms, a VOC sensor is the obvious choice.

Detect toxic gases via CO and NO2 sensors
Carbon monoxide (CO) is a colourless, odourless and tasteless gas. It is an extremely dangerous gas. CO is created when fossil fuels (coal, gas, fuel oil, wood, pellets, petroleum, etc.) are burned incompletely or poorly. CO can therefore only be formed where there are flames and in the room where the heating appliance is located. CO is slightly lighter than air, but the difference is so small that in practice CO usually mixes completely with normal air in enclosed spaces. It is therefore sometimes called the silent killer. The World Health Organization (WHO) applies a maximum limit of 6 ppm for continuous exposure. Increasing to a maximum of 26 ppm with an exposure of 1 hour per day.

In humans, haemoglobin, the dye in red blood cells, carries oxygen from the lungs to the cells. The affinity of CO for haemoglobin is 210 to 260 times higher than that of oxygen. Even in the presence of low concentrations, CO will attach to haemoglobin instead of oxygen. This disrupts the transport of oxygen to the cells and causes an oxygen deficiency. An exposure to low CO concentrations will initially be recognizable as symptoms of nausea, dizziness and headache. The victim feels weak and is easily short of breath with moderate exertion. Over time, the victim will lose consciousness and - if no help arrives - die. It goes without saying that toxic gases such as carbon monoxide must be removed from the building as quickly as possible. As soon as this gas is detected, sufficient fresh air must be supplied.

The same applies to other toxic gases such as nitrogen oxides (NOx). NOx is the collective name for NO and NO2. NOx is created during combustion processes at high temperatures and can be found in exhaust gases from cars with a combustion engine. In addition, NO2 - just like CO - is also created during incomplete combustion processes. NO2 is also a poisonous gas that is harmful to health. People with lung complaints and asthma suffer from it in particular.

CO and NO2 sensors are therefore mainly used in parking garages or in technical areas where heating appliances are installed. As soon as toxic gases are detected, sufficient ventilation must be provided to restore safe indoor air quality as quickly as possible.

The advantage of demand-controlled ventilation
Each room in a building has a specific purpose. Therefore, a room is rarely used continuously usually not always with the same intensity. The bathroom for example is typically used in the morning and evening. Bedrooms during the night. Each room in a building has its own specific usage and occupancy pattern. A ventilation system is usually calculated with an overcapacity so that it can supply sufficient fresh air during peak times. Typically, these peak moments only represent a limited part of the total cycle. Most of the time, the ventilation system can operate at low speed. By applying the right sensors in each room and controlling the ventilation system based on these measurements, the indoor air quality can be optimized and at the same time significant energy savings can be achieved. An additional advantage is that a ventilation system produces less noise when it works at low speed.
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