These recommendations were last updated on July 31, 2020.
As experts in MEP engineering, our clients trust us to provide guidance and insight on the best air cleaning systems. Our team of engineers has reviewed a number of technologies and our findings are below.
Air filters are used in air handling equipment to remove particles and contaminants from the airstream to keep these units clean internally. A secondary purpose is to remove particulate to improve indoor air quality and to remove limited amounts of bacteria and viruses, more predominately in medical facility applications.
Filters come in a variety of types and efficiencies. Viruses range in size from as small as 0.005 up to 0.30 microns, or slightly larger, depending on the source. Filters are generally rated in regard to efficiency to remove 0.30 micron and larger particulate.
- MERV 8 filters are commonly used in standard commercial designs and generally remove particles that are 3 to 10 microns in size so they do very little to remove viruses from the air stream.
- MERV 11 filters are a reasonable upgrade in filtration and are twice as efficient as MERV 8 filters, while maintaining a comparable pressure drop (slightly higher). MERV 11 filters can generally remove particles that are 1 to 10 microns in size so they have limited, if any, impact on removing viruses from the air stream. There may be limitations on using MERV 11 filters if existing equipment is being retrofitted which needs to be assessed on a case by case basis.
- MERV 13 filters can remove particles that are 0.3 to 10 microns and are effective at removing larger viruses only from the air stream but are not a complete solution.
- MERV 16 filters are at least 95% efficient at removing particles that are 0.3-10 microns but again only removes the larger viruses from the air stream.
To implement the use of MERV 13 or higher efficiency filters, the design engineer should be notified at the start of design. If MERV 13 or higher efficient filters are installed in existing equipment that was not designed for it, the increase in air pressure drop can negatively impact the amount of air the unit can supply to the space and affect the unit’s ability to properly condition the space.
As such, we recommend using the highest MERV rating possible and which is reasonable for the equipment in which it is installed. We believe that MERV 13 filtration is the highest rated filter that is reasonable and which should be considered in new equipment only. In most general air conditioning system applications we recommend a minimum of MERV 11 filters. Other components or systems should be used in addition to filtration to combat viruses as filters are only one part of an overall risk mitigation plan related to virus control.
We recommend using the highest MERV rating possible and which is reasonable for the equipment in which it is installed
Bipolar ionization works by creating positive and negative ions that attach themselves to pathogens, neutralizing them. This also makes the pathogens larger, so they are more easily caught in filters. Bipolar ionizers can be placed within an air handling unit or ductwork and are sized based on the airflow amount of each unit.
There are two types of ionizers: corona discharge and needlepoint.
- Corona discharge, or tube type, ionizers have a higher ion output which lasts longer than needlepoint ionizers. However, most corona discharge ionizers produce small levels of ozone, whereas needlepoint ionizers do not. Corona discharge ionizers must have their bulbs replaced every two years.
- Needlepoint ionizers will last 10-15 years but require cleaning every 1-2 years. This kind of disinfection only works when the unit is running. However, if the bi-polar ionizer is located properly, the ions it produces will travel through the ductwork into the space and actively clean the space as long as the unit is running.
Bi-polar ionizers do not remove all contaminants from the air in a single air pass, but work over time to reduce most of the contaminants in the air.
Ultraviolet (UV) lights deactivate contaminants by scrambling the DNA of microorganisms to prevent reproduction. While all UV light can deactivate microorganisms, UV-C energy is the most effective type of light. UV bulbs typically last 1-2 years.
UV lights can be placed in mechanical equipment or within a straight run of ductwork. We suggest a minimum exposure time of 0.25 seconds where possible. When this exposure time is not available, the intensity of the UV lights can be increased by adding additional lights in order to maintain the same effectiveness (in air handlers). UV lights placed at the coil provide longer exposure times and give the added benefit of coil and drain pan disinfection.
This kind of disinfection can be sized to whatever the customer desires (for air handlers), including a 99.9% effectiveness on a single air pass, however it is a passive system and only cleans the air when the unit is running.
UV lights can be installed in Roof-Top AC units (RTUs) as well but there are not as many options on intensity or type and typically require 2-3 passes to fully kill a pathogen.
There are also room UV light emitters designed to treat a space directly as opposed to installing these in air handling equipment. These appear to have the same benefits and can be used independently or in lieu of the air handling or duct mounted devices, however they require good room air mixing since they are typically installed at the ceiling level to minimize potential of direct line of sight. In high volume spaces space mounted UV light emitters would require additional circulation fans to circulate the room air up to these emitters.
Image sourced from Synexis.
Dry hydrogen peroxide (DHP) systems spread gaseous hydrogen peroxide to all parts of the space in which it is located, where it attaches to microorganisms and causes them to decompose. DHP systems are considered safe and create water vapor and oxygen as by-products.
DHP can be placed in ductwork or can be a stand-alone system in a room. If placed in the ductwork, disinfection will only occur up to one hour after the unit turns off. The ductwork application of a DHP system provides approximately 1,000 square feet of coverage. This device is not intended to be installed in the main duct of large units, but is instead intended to provide coverage for a single classroom or several offices.
There are also options that allow for the DHP system to be installed in the space, and require a device in every space to provide coverage. To go along with these systems, the manufacturer requires a maintenance agreement to provide replacement filters and catalysts.
Currently, there is only one manufacturer of these replacement parts. This system does provide the benefit that it actively cleans the space and always removes contaminants regardless of if the HVAC system is operating or not. In general, there are three types of DHP systems, one sized for ductwork, and two other devices intended for small and large spaces.
There are many other emerging or existing technologies such as Photocatalytic Oxidation (PCO), activated or charged charcoal filtration, and space mounted filtration modules. This summary focused on the three (3) main technologies currently commercially available.
Two (2) of the technologies noted in this summary appear to be the most widely used and effective (UV lights and Bi-Polar Ionization) with the least amount of maintenance and potential byproducts.
The third (DHP) system type seems to be one of the few, or only, active space cleaning technologies currently available.
Air Cleaning Recommendations for All Systems
Regardless of the system type, we suggest extending HVAC unit run times to allow for greater filtration and/or disinfection while attempting to reduce pathogen levels in the air and spaces served, possibly early in the morning prior to occupancy and at the end of the school or workday for a designated period of time.
We recommend for new construction, and HVAC replacement projects, that the design accommodates continuous fan operation during occupancy.
Air Cleaning System Recommendations for Packaged Equipment
For packaged equipment (Split DX, RTUs, & Heat Pumps), ultra-high efficiency units that use an inverter-driven compressor to allow the unit to turn down to match the load and keep the unit running is an optimum solution. These units are typically more expensive but also provide additional humidity control and energy efficiency in addition to the benefit of allowing the air purification system to continue to operate for longer periods of time. There are alternates to the inverter compressor systems to allow continuous unit operation that include hot gas reheat coils or specialized refrigerant control valves.
Air Cleaning System Recommendations for Central Plant Systems
For centralized equipment, VAV air handlers or packaged VAV type Roof-Top Units with terminal boxes allow for the HVAC system to remain energized throughout the occupied period and have similar benefits to those noted for packaged A/C units with inverter-driven compressors.
Air Cleaning System Recommendations for Four-pipe HVAC Systems
For a four-pipe HVAC system with large VAV, constant volume, or multi-zone, air handlers, we recommend installing a UV light system in the air handler where you have the longest exposure available. These systems can be sized for a 99.9% kill rate per air pass, meaning that they effectively eradicate the airborne pathogen from the air that is circulated in the system. This type of centralized UV light system can effectively protect a large area of the building.
However, many air handlers and fan coil units do not have adequate space for a UV light system to be installed inside them. As a result of this, we would recommend installing bi-polar ionization in these units. As long as the space being served is not more than 100 feet downstream of the bi-polar ionizer, a needlepoint ionizer will suffice. For longer duct runs, we would recommend installing either a tube type bi-polar ionizer or multiple needlepoint units closer to the discharge of the ductwork. We would recommend a similar installation of bi-polar ionizers in rooftop air conditioning units. For a water source heat pump system (which would include geothermal systems), we would recommend having needlepoint bi-polar ionizers installed on each water source heat pump while providing a tube type bi-polar ionizer in the 100% outdoor air unit to continuously provide ions into the space for cleaning even when the water source heat pump switches off.
Air Cleaning System Recommendations for Special Use Cases
There are some spaces within buildings that require special attention. The spaces we have identified as special use cases generally fall into two categories. The first are spaces with a high amount of non-occupant or visitor traffic. The other category is spaces where occupants may not wear face coverings due to the activity that happens in that space.
Some examples of special use case spaces are:
- Cafeterias or dining spaces in K-12 schools, higher education campuses, or office buildings
- Health clinics
- Administrative or lobby spaces in government buildings, schools, or offices
- Fitness centers, studios, or gyms within schools or office buildings
- Locker rooms or spas with shower facilities
- Shared housing spaces, such as dormitories on higher education campuses
In these special use case spaces, we would recommend providing space mounted DHP devices. This will provide additional coverage for these higher risk areas in order to provide the maximum effectiveness while minimizing the cost.