Basic requirements for HVAC systems for healthy indoor air quality
On average, we spend about 90% of our lives indoors and breathe in around 12,000 litres of air every day. We assume the air in buildings is "clean" and will not harm our health. Therefore, it is astonishing how little users and operators actually know about indoor air quality at their premises. Essential variables such as air humidity, CO2 content or VOC concentration are hardly ever measured and even less often displayed.
Belimo interviewed consulting engineers and experts in the ventilation sector around the world to identify the priorities when aspiring to create a healthy indoor air environment in a building. In the process, seven essential factors for ensuring healthy indoor air in non-residential buildings have emerged.
- Continuous and reliable measurement, display and monitoring of indoor air quality
- Accurate amount of air to the zone and controlled removal of contaminated air
- Well designed air dilution and airflow pattern
- Active pressurisation of envelope and spaces
- Correct temperature and humidity conditioning
- Effective filtration
- Proper amount of outdoor air
1. Continuous and reliable measurement, display and monitoring of indoor air quality
Ideally, air humidity, CO2 content or VOC concentration are measured by sensors for the monitoring of air quality. This is because only measured variables can be controlled. From today's point of view, both the measurement and the display of these values should represent the minimum standard for indoor air quality measurement.
It is important that relative humidity indoors is held between 40-60%. Dry air droplets from an infected person speaking or sneezing easily evaporate and the contained virus travels further in the room as a light aerosol. If the humidity is higher, droplets do not evaporate as quickly and fall to the ground in a shorter distance. Thus, many bacteria and viruses are considerably more contagious in dry air conditions, which can dehydrate mucous membranes and weaken the immune system.
CO2 concentration of more than 1000 ppm (parts per million) decreases the brain's ability to concentrate; starting at 2000 ppm and higher, it can lead to fatigue or even headaches. CO2 levels in indoor air is an excellent indicator of potential bio-contamination, an example is the COVID-19 viruses. If the CO2 value is high due to increased occupancy and limited air exchange, high potential risk from infectious aerosols could result.
Volatile Organic Compounds (VOCs) are organic compounds that originate from many different sources, including perfume, paint, printers, carpeting and building materials. Even low concentrations of VOCs can irritate the eyes, nose, or throat and indicate insufficient fresh air intake.
It is essential to measure these variables using suitable sensors so that appropriate measures can be implemented, for example, ventilation, air purification or humidification.
3. Well designed air dilution and airflow pattern
An important factor is the way in which the air introduced into a room flows through that room and then exits it again. Ideally, fresh air flows undiluted from the bottom up past a person and is then extracted directly from the room. It must be ensured that indoor air does not "swirl" around the room several times or become trapped in certain zones of the room. Modern airflow simulations enable typical flow patterns in a room to be studied in detail. The correct design, placement and orientation of air outlets can help prevent major healthy air errors.
4. Active pressurisation of envelope and spaces
Air hygiene in a room is negatively affected by unwanted air currents entering a zone from outside (e.g., a busy road) or from other rooms (e.g., cafeteria). This typically occurs when air-pressure ratios are not properly balanced. Especially in connection with the spread of COVID-19 aerosols in buildings, there has been much discussion about "cross-contamination" between different rooms. The use of VAV controllers in the supply air and extract air of rooms and the use of differential pressure sensors and controllers between zones can prevent such undesired airflow.
5. Correct temperature and humidity conditioning
In a central ventilation system, the supply air can be conditioned relatively precisely to the desired temperature in the air handling unit by heating or cooling coils. High-quality control components at the coils such as the Belimo Energy Valve™ ensure that this is not only done with high precision but also in an energy-efficient manner.
In addition to temperature, humidity is also crucial for healthy indoor air. If aerosols or viruses present in a room encounter dried-out mucous membranes, the risk of infection increases considerably. It has also been shown that viruses on dry surfaces survive longer than under more humid conditions. Proper humidification of the room air (40-60% relative humidity) is therefore an essential factor for safe indoor air.
7. Proper amount of outdoor air
Today, a large proportion of smaller and medium-sized non-residential buildings do not have an automated, mechanical fresh air supply. It is often assumed that from time to time, users ventilate by opening a window. If this does not occur, the concentration of infectious aerosols can greatly increase. A ventilation system with central air conditioning is therefore part of the minimum standard equipment when planning a new building or renovation. Many countries have issued recommended or mandated standards on mechanical ventilation in commercial buildings and required minimum air-exchange rates, depending on the type of building and the number of occupants (for example ASHRAE 62.1 Ventilation requirements). Other considerations focus on poor inner-city air quality in many countries. Ideally, outside air variables are measured before air is mechanically introduced into a building. An automated system can supply more outside air when pollution levels from traffic and industry are low and return to the minimum required ventilation rates when pollution levels increase.
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