Reducing Spread of COVID-19 HVAC System Retrofit

Since many States are opening, reducing pathogens transmission is becoming more critical. There is great concern about the possibility of transmission through the air of various pathogens, especially COVID-19, among staff and administration in healthcare facilities, workers in office environments, staff and patrons in retail settings, workers in manufacturing, residents in private and public facilities. The pathogens that cause infectious diseases are spread from a primary host to secondary hosts via several different routes. The challenge for reducing the spread of COVID-19 has two categories. One of them is our daily behavior which includes social distancing, wearing masks, cleaning, sanitation etc. recommended by CDC. The other category is office lay out, occupant flow and HVAC improvements. We have seen lot of opinions about how HVAC systems can reduce COVID-19 spread in an office and occupied space. New York City has many different type existing HVAC systems. Office spaces and mechanical rooms are limited and congested. Since the City has a unique condition, in this article we will focus on retrofitting existing HVAC system to reduce pathogens spread and constructability of the provided options.

We have to understand, there is no quick fix and complete elimination of the pathogens from the regular office environment. It is not feasible to convert an entire existing office floor to a clean room or isolation room, completely exhaust it or create positive pressurization. There will be pathogens introduced to the environment through breathing and coughing and environment will be contaminated immediately even though the HVAC system is providing cleaner or 100% outside air. But certain HVAC upgrades will help to reduce spread.

For the past 30 or more years there are known proven methods for cleaning the supply air. There is difference between providing cleaner air to the space and creating isolation, eliminating the pathogens from the contaminated environment. This article is going to focus on reducing the pathogens from the air supply through the HVAC system thus reducing the spread.

Retrofit Options and Upgrades for Existing HVAC System
Focusing on the renovations, tenant fit outs or retrofit type projects which are typical office buildings in New York City. The office buildings have different types of HVAC systems.

• Central air handling unit with ducted air distribution and radiators by the window for heating. This mechanical system is the most common modern-day office system.
• Individual heat pump or fan coil units serving each office. The units are usually located under each window. This mechanical system relies on natural ventilation and seen mostly older or residential buildings.
• Window units located above the window or in the window. This type of systems is very limited and seen mostly very old building with limited air conditioning systems.

If your office has the 1st category type of system, retrofitting is available and can be accomplished. Other categories will require more substantial renovation to accomplish recommendations.

In this article, we will focus on 1st category, central air handling unit system upgrades. We have identified the following methods and technologies per ASHRAE guidelines.

Increasing ventilation for an existing central air handling unit: The air handling units are designed and specified by engineers for specific ventilation air flow according to site conditions. New York City design conditions in summer is 95 degF and humidity level can be above 60%RH. Increasing the ventilation rate randomly will result humidity and temperature problems in the space. Interestingly, uncontrolled humidity and temperature can increase the pathogens growth. it remains unclear how much infection disease particles must be reduced with increased ventilation to achieve measurable reduction in disease transmission. For an existing HVAC system, increasing the ventilation and diluting the air option is very limited. Mechanical engineers must review and decide how much the ventilation rate can be increased for an existing unit. Increasing ventilation approach cannot be the only action item to reduce the spread.

Exhaust strategies for designated areas: Common office central unit recirculates air and supply it back to the space. Portion of the return air is exhausted based on ventilation rates. The exhausted air is called pressurization relief. The pressurization relief usually accomplished by the return system for many of the office buildings in New York City. Identifying the highly occupied areas or high traffic areas for strategic exhaust system installation may help reducing the spread. After high density and high traffic areas are identified, air balance shall be performed based on unit capacity and ventilation. Room pressure differences compare to surrounding area will reduce the cross contamination.

Filtration systems: highly efficient particle filtration in centralized HVAC systems reduces the airborne load of infectious particles. This strategy reduces the transport of infectious agents from one area to another when these areas share the same central HVAC system through supply of recirculated air. Usually adding high efficiency filtration require large filter section for air handling units. High efficiency filters are widely available, but they are costly. Additionally, adding filter system will increase the static pressure which many of the existing units cannot handle. As we mentioned, the mechanical rooms in New York City are very tight and will not allow or will not be feasible for adding new air handling unit filter sections. This approach will not be very feasible for retrofit projects. The filter addition shall be reviewed by engineer if the mechanical room is large enough.

UV Lights: UV-C light spectrum can kill or deactivate the pathogens on the surfaces, and it is well known technology. Since it will require high density, it may damage eye or outer layer of human skin, ASHRAE recommends protective gear for the exposure for UV lights. Engineers usually use UV light technology which helps to kill pathogens on the air handling units cooling coil. Additionally, UV light may not be as effective for moving air through the coil. As a result, this method may be used and can supplement other methods.

Bipolar Ionization: This method has been used for clean rooms and health care facilities for many years. It is relatively simple concept and can be applied to traditional ducted systems. Producing a natural bio-climate rich in positive and negative oxygen ions will help neutralizing the particles. The negative ions contain an extra electron while the positive ions are missing an electron resulting in an unstable condition. In an effort to restabilize, these bipolar ions seek out atoms and molecules in the air to trade electrons with, effectively neutralizing particulate matter, bacteria and virus cells, odorous gases and aerosols, and VOCs.

1. Airborne particles are charged by the ions causing them to cluster and be caught in filters

2. As they divide to reproduce, bacteria and virus cells bond with oxygen ions and are destroyed

3. Odorous gases and aerosols oxidize on contact with oxygen ions and are neutralized.

4. Oxygen ions cause a chemical reaction with VOCs breaking down their molecular structure

This approach for capturing and eliminating the pathogens will require slip in sections installed into the ductwork. It is very feasible and easily constructible for New York City office buildings with central air handling unit systems. Engineers must review the existing system and locate ionization system in the ductwork.

Solution

Although this is challenging times, the approaches we discussed above will help the conditions in office spaces in New York City. Even though, there is no quick fix and complete elimination, engineers can easily analyze and apply these approaches to reduce transmission. Considering the constructability and New York City conditions, we recommend following in order, for ducted central air handling unit systems,

• Concentrated exhaust system
• Bipolar Ionization
• Engineered ventilation increase.
• Optional: UV light inside air handling unit

If you want to reduce the pathogen spread with modifying existing ducted HVAC system, contact us. We will analyze and provide you options.

YEC Engineering has designed many offices, medical facilities, clean room projects in NYC and other states.
Questions about MEP engineering and our services. Contact us

Baltu Yorkos, PE
byorkos@yecengineering.com
(646) 248 6788
www.yecengineering.com