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Air Quality - Study on HVAC Systems -

Heating Ventilation and Air Condition system, often referred to as HVAC, is a system for conditioning the air quality of an interior environment. “Air conditioning for health” is a Japanese term for the A/C planning for people’s health and comfort. COVID-19 revealed the importance of air quality control and air ventilation by the HVAC system, which was out from the main interest in MEP design such as heating and cooling air. Pandemic approximately has occurred every 10 years as it mentioned in second article of series (CASE STUDY02).
This issue introduces the new basic specification of HVAC for offices and public spaces where people spend many hours.

1. Study on utilization of HVAC system in response to the COVID-19
The Ministry of Health, Labour and Welfare*1 , and several organizations such as the Architectural Institute of Japan*2, The society of Heating, Air-Conditioning and Sanitary Engineers of Japan*3, and the Japanese Association of Building Mechanical and Electrical Engineers*4 have published countermeasure guidelines on the HVAC systems for non-healthcare facilities. The following are excerpts of outlines.

(1)“Air ventilation” is the introduction of outdoor air into an indoor space. “Ventilation rate” indicates the amount of outside air entering per hour relative to a room volume. Recirculating indoor air circulation is not considered as ventilation.
(2)Military of Health, Labour and Welfare of Japan has announced “Three Cs,(Closed spaces, Crowded places and Close-contact settings)” to prevent COVID-19 outbreak.
Closed space with poor ventilation which is one of the “Three Cs,”, does not apply to a condition that meets the standard stated in the Building Health Act: 30 cubic meters of ventilated air per person, per hour. Although this standard is reasonable, it is not a perfect prevention measure for the spread of COVID-19 as its transmission mechanism has not been fully understood.
(3)Recommended use of HVAC system (for a typical central air conditioning system)
・Minimize the volume of recirculated indoor air, and maximize the outdoor air exchange volume for air ventilation.
・Increase the air exchange volume of outdoor air by using energy-saving technologies. (e.g., Use the lowest setting on an outdoor air inflow control system that responds to the level of CO2 of indoor space and pause the function. For the A/C with a control mode that allows the use of midseason’s cooled outdoor air, moderate its parameters to increase the volume of outdoor air intake. For a total heat exchanger unit which recycle the heat of the indoor air, use low exchange mode and utilize a bypass circuit that lowers air resistance to increases outdoor air intake.)
・Set up with mechanical ventilation when using open air supply port. And make sure of enough air exhaust capacity. (The same level of air quality can be achieved by increasing the volume of outdoor air intake by three times the room volume, which replaces 95% of the air in the space.)

There are many parts of the COVID-19 countermeasures to consider in terms of outdoor air intake systems. Both quantity and quality play key roles, including “ More air exchange volume,” “appropriate humidity level,” and “optimized air cleanliness.”

2. Study on methods for outdoor air intake into indoor spaces
At Nihon Sekkei, methods to supply outdoor air to indoor spaces are critical consideration points of facility design in the post-COVID-19 era. The unique characteristics of the Japanese climate require the HVAC system to functionally adjust to large temperature and humidity differences between seasons. In summer, a de-humidifying function is required, as opposed to winter, when a humidifying function is necessary. To increase the intake volume of outdoor air as currently desired, improvements will need to be implemented more than ever before. Here are some past examples of improvements from Nihon Sekkei, as a clue towards future changes to come.
In a typical office environment, an A/C unit generally treats a mixture of outdoor and indoor air to supply comfort air. However, in the Akasaka Intercity AIR project, outdoor air condition unit (A/C unit that treats only the air taken from outside) were added to its HVAC system to supply conditioned outdoor air directly to offices. This method supplies necessary amount of outdoor air into and through a closed space with a high occupancy rate, such as a meeting room where closed from other space with poor air ventilation to.
These outdoor air conditioners have built-in total heat exchanger units, which efficiently recover the energy contained in indoor air and maintains relative humidity in indoor spaces by utilizing middle-temperature cooling water. Such an outdoor air supply system is energy-saving method and simultaneously contribute to fulfilling No.3, No.7 and No.11 out of Sustainable Development Goals.

3. Study on Air Quality

The performance of a ventilation system is measured by its ventilation rate and efficiency. Even if an abundant amount of fresh air is introduced, the system’s performance rate would be considered low if it is not circulating the air within an indoor space effectively. As a countermeasure to the spread of COVID-19, it is recommended to locate an exhaust port near areas where indoor air pollutants might proliferate, and separate air intake openings and outlets with proper distance. Fresh outdoor air is known to influence people’s physical performance and intellectual productivity.
Supplying clean outdoor air directly into a person’s breathing zone and unmix with polluted indoor air,  can lower the risk of contracting the virus in closed environments. Personalized air conditioning serves this purpose. The figure below suggests that placing the air supply close to an occupant’s work station yielded greater efficiency in delivering clean air to the person’s breathing zone. Although research related to personalized air conditioning has been conducted since 1970, the number of scientific reports and applications of their findings surged during the 1997 avian influenza outbreak  and the 2001–02 Influenza A(H1N2) outbreak. It has drawn much attention due to its high impact on ventilation.
The water radiator  system is another healthy radiation/convection HVAC technology. Heated and cool water rely on pipes of water pass through a heat exchanger beam installed on the ceiling to control air temperature without air circulation, and it can keep the targeted environment free of draft noise. This system can become more practical not only for its potential of increased comfort, but also for infection prevention.
COVID-19 forced to reconsider the HVAC system as a countermeasure for infection prevention, considering of health and hygiene from the value of air quality. HVAC system design can greatly benefit these aspects of a built environment and Nihon Sekkei will keep contributing to explore the methods to improve air quality.


*1 Improving the air quality of a “closed space with poor ventilation” in commercial buildings. (March 30, 2020)
*2 COVID-19 countermeasures on “ventilation.” Q&A on “ventilation.” (March 30, 2020)
*3 Use of HVAC system as a prevention of the COVID-19 transmission. (April 8, 2020)
*4 JABMEE’s proposal on the COVID-19 countermeasure (March 26, 2020)
*5 Arsen K. Melikov, Radim Cermak, Milan Majer : Personalized Ventilation :evaluation of different air terminal devices, Energy and Buildings, pp. 829-836, 2002.
*6 Radim Cermak, Milan Majer, Arsen K. Melikov : Measurements and Prediction of Inhaled Air Quality With Personalized Ventilation, Indoor Air, 2002.
*7 T. Akimoto, S. Tanabe, T. Yanai, and M. Sasaki,Thermal Comfort and Productivity — Evaluation of Workplace Environment in a Task Conditioned Office,Building and Environment, Vol.45, Issue 1, pp.45-50, 2010.1

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