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Healthier Buildings - Mechanical Systems

July 20, 2020

Ductwork in an HVAC ventilation system


1. ASHRAE recommends increasing outside air and constant operation of HVAC systems in order to reduce airborne contaminants. While most systems may not be capable of increasing outdoor air ventilation on a summer design day, systems may be evaluated to utilize greater percentages of outdoor air while maximizing available capacity and maintaining dew point control limits.

ASHRAE Position Document

2. It is important to increase the outside air only to the level that does not adversely affect maintaining recommended indoor temperature and humidity ranges. Recommended ranges of 30% to 60% humidity should be maintained.

3. Disable Demand Control Ventilation (DCV) such that fresher air is delivered continuously.

4. Increasing filtration (to MERV-13 or better) will aid in reducing other contaminants (dust, pollen, etc.) that could inadvertently be introduced to the indoor environment when increasing the amount of outside air into the building.

5. Much of the time, increasing outside air is energy-intensive. Building owners need to understand that increasing outside air to mitigate indoor airborne contaminants will likely increase their utility bills, particularly in hot summer months or cold winter months.

6. Equipment capacity should be analyzed prior to implementation.

7. If the building has operable windows, windows can be opened during favorable outdoor conditions.


1. MERV Rating/HEPA

a. Summary:

Filter Media (Mechanical) & MERV Rating: Consists of media with porous fiber structures or stretched membrane material to remove particles from airstreams.

b. The fraction of particles removed from air passing through a filter is termed “filter efficiency” and is provided by the Minimum Efficiency Reporting Value (MERV) under standard conditions.

i. MERV ranges from 1 to 16; higher MERV = higher efficiency

ii. MERV ≥13 (or ISO equivalent) are efficient at capturing airborne viruses. MERV 13 is currently one of ASHRAE recommendations for more immediate airborne contaminant removal.

1. MERV 13 Effectiveness: low microbial removal rate

2. MERV 13 Approximate Installed Cost: $25 per 2” thick 24”x24” cartridge

3. MERV 13 Maintenance: 2x the filter change frequency of a standard MERV 8 filter. Proper PPE is recommended for filter changes.

4. MERV 13 Installation Requirements: Fits in standard 2” or 4” filter racks

iii. MERV 14 (or ISO equivalent) filters are preferred

c. Increased filter efficiency generally results in increased pressure drop through the filter. Ensure HVAC systems can handle filter upgrades without negative impacts to pressure differentials and/or airflow rates prior to changing filters.

d. Generally, particles with an aerodynamic diameter around 0.3 μm are most penetrating; efficiency increases above and below this particle size.

e. The overall effectiveness of reducing particle concentrations depends on several factors: i. Filter efficiency ii. Airflow rate through the filter iii. Size of the particles iv. Location of the filter in the HVAC system or room air cleaner


a. By definition, true HEPA filters are at least 99.97% efficient at filtering 0.3 μm mass median diameter (MMD) particles in standard tests.

b. Most penetrating particle size may be smaller than 0.3 μm, so the filtration efficiency of most penetrating particles can be slightly lower.

c. HEPA filter efficiency is defined as a minimum of MERV 17.

d. Due to high-pressure drops, HEPA filters may not be able to be retrofitted into HVAC systems.

e. To function properly, HEPA filters must be sealed properly in filter racks.

f. Filters are often delicate and require careful handling to prevent damage and preserve performance.

g. HEPA filters can be located in HVAC systems or in:

i. Portable HEPA Machines: application for creating a negative pressure isolation room.

ii. Pre-Assembled Systems: “point of use” applications for individual rooms or spaces.

iii. Ad Hoc Assemblies: “sidestream” application for recirculating systems.

3. UVC1,2

a. Summary: UVC lights are typically installed in an air handling unit, just downstream of the cooling coil. This keeps bacteria from growing on the outside of the cooling coil. Excessive UVC light can be dangerous for humans and will have a negative impact on mechanical components that are susceptible to UV degradation, so it must be deployed in enclosed, unoccupied areas, with required safety devices and warning signs. Recommended by ASHRAE.

b. Approximate Installed Cost: $10,000 per AHU (25,000 to 35,000 cfm).

c. Maintenance: More expensive due to annual UVC lamp replacement.

d. Installation requirement: Some space in the AHU and 120v power.

4. Bipolar Ionization

a. Summary: Bipolar Ionization devices actively emit positive and negative ions that react with other airborne contaminants and act as catalysts for changing the molecular structure of VOCs, assist with particle conglomeration, and have been shown in some independent tests to kill viruses. Laboratory tests are somewhat limited, however, and there is no consensus as to how to best conduct tests or what the “ion concentration” needs to be for the technology to be effective.  The extent of the effectiveness likely depends on the ion density in the airstream and the air circulation throughout occupied spaces. ASHRAE has not currently made any recommendations regarding this technology. The HVAC industry has not developed standardized testing of this technology.

b. This technology is available in various forms, but most systems are some form of “needlepoint” or “tube” technology. Needlepoint technology needs annual cleaning and inspection. Tube technology requires replacement of tubes every 2 years or so. Systems can be provided with “ion counters” to track whether the system is operating appropriately per the installation.

c. This technology is in the early stages of testing and industry acceptance in the United States. ICT will continue to monitor and evaluate forthcoming news and information regarding this technology; its applications and its effectiveness.

d. If implemented, care should be taken to install any bipolar ionization system according to the manufacturer’s instructions, and also to review the manufacturer’s data to make sure that the product does not produce Ozone.

e. Approximate Installed Cost: $9,000 per AHU (25,000 to 35,000 cfm).

f. Maintenance: Less expensive due to annual cleaning only. (Does this mean that MERV rated air filters require replacement less often.)

g. Installation requirement: Some space in the supply duct near the AHU and 120v power or 24v power (depending on system size)

5. Electronic Air Filters

a. Summary: Electronic filters include a wide variety of electrically-connected air cleaning devices that are designed to remove particles from airstreams. Electronic air filters fall into three major categories: i) Electric plate-type precipitators: ii) electrically-enhanced air filtration and iii) Dynamic type. 

i. Electric plate–type precipitators: Precipitators use electrostatic precipitation to remove and collect particulate contaminants on plates. The air cleaner has an ionization section and a collecting plate section. Maintenance: Must be cleaned periodically. Effectiveness: No national standard to rate effectiveness.

ii. Electrically-enhanced air filtration: Electrically-enhanced air cleaners incorporate an electrostatic field to charge contaminants before capture in a high-efficiency pleated filter.

iii. Dynamic Filtration: Manufacturers claim longer life than disposable or passive electrostatic filters. These are MERV static filter equivalent but have very low-pressure drops and long media life compared with traditional filter media. Maintenance: Filter change frequency less than similar MERV rated static filter. Installation Requirements: Operates on 24VAC- wired to unit control transformer.

6. Ozone Generation2

a. Summary: Ozone (O3) is a reactive gas that can disinfect air and surfaces by killing viruses, bacteria, and fungi. Ozone is harmful to health and exposure to ozone creates risk for a variety of symptoms and diseases associated with the respiratory tract. ASHRAE’s Environmental Health Committee issued an emerging issue brief suggesting “safe ozone levels would be lower than 10 ppb” and that “the introduction of ozone to indoor spaces should be reduced to ‘as low as reasonably achievable’ (ALARA) levels.” Ozone should only be considered for disinfection in unoccupied spaces; it should never be used in occupied spaces.

b. Effectiveness: Available scientific evidence shows that, at concentrations that do not exceed public health standards, ozone is generally ineffective in controlling indoor air pollution.

c. Approximate Cost: Varies by building envelope size.

d. Maintenance: N/A since it’s a portable application.

e. Installation Requirements: The unit is mounted in a room and connects to 120V Outlet.


1. Increase restroom exhaust to above code/ASHRAE standards.

2. Check that the amount of air exhausted via mechanical means does not exceed the amount of outside air introduced to the building via mechanical means. Maintaining a building’s slightly positive pressure is important in order to control indoor contaminants.

3. Lower restroom exhaust intakes can be implemented to reduce airborne contaminants in the breathing zone.

Retro-Commissioning/Systems Verification

1. Verify that systems are operating as originally designed through thorough systems functional performance testing and commissioning.

2. Verify HVAC and water filtration quality.

3. Verify outdoor air quantity and building pressurization.

4. Verify control systems are accurately modulating building systems to match building loads.


1. IES Committee Report: Germicidal Ultraviolet (GUV) - Frequently Asked Questions

2. ASHRAE Filtration/Disinfection: Ultraviolet Energy (UV-C) -