HEPA filtration

What is HEPA Filtration?

HEPA stands for High-Efficiency Particulate Air. A true HEPA filter is a specific type of mechanical air filter that must meet a very high standard set by the U.S. government. To be classified as a true HEPA filter, it must be able to trap at least 99.97% of airborne particles that are 0.3 microns in size. Particles of this size are the most difficult to trap, so a filter that can capture them effectively can also capture both larger and smaller particles with even greater efficiency.

HEPA filters are used in applications where extremely clean air is critical, such as in hospitals, laboratories, and aircraft cabins. They are also found in standalone air purifiers for homes.

What is MERV?

MERV stands for Minimum Efficiency Reporting Value. It's a rating system that measures how effectively an air filter captures particles of different sizes. The MERV scale ranges from 1 to 20, with higher numbers indicating better filtration.

A filter's MERV rating tells you what percentage of particles in a specific size range it can trap:

• MERV 1-4: Traps less than 20% of particles between 3 and 10 microns (pollen, dust mites, carpet fibers).

• MERV 5-8: Traps 20-85% of particles between 3 and 10 microns, and up to 50% of particles between 1 and 3 microns (mold spores, dust). This is the standard filter for most homes.

• MERV 9-12: Traps over 90% of particles between 1 and 10 microns and up to 80% of particles between 0.3 to 1 micron (auto emissions, lead dust).

• MERV 13-16: Traps over 90% of particles between 0.3 and 1 micron (bacteria, cooking oil, virus carriers). This is the highest you'd typically want to install in a home's HVAC system without professional consultation.

• MERV 17-20: Used in surgical operating rooms and cleanrooms. A MERV 17-20 filter is essentially a HEPA filter.

HEPA vs. MERV in a Home HVAC System

While a true HEPA filter is superior at trapping tiny particles, it is not suitable for most home HVAC systems. The dense, tightly-woven material of a HEPA filter creates significant static pressure, which is the resistance to airflow. A typical residential HVAC system's blower fan is not powerful enough to overcome this pressure. Forcing the fan to work against this high resistance can lead to:

• Decreased Airflow: The AC or furnace won't be able to properly circulate air throughout the home, leading to poor heating and cooling performance.

• Increased Energy Consumption: The blower motor has to work harder and longer, using more electricity.

• System Damage: The stress can cause the motor to overheat and fail prematurely.

A MERV 13 filter is often considered the ideal compromise for a residential HVAC system. It provides excellent filtration of most allergens and bacteria without creating excessive static pressure. For a higher level of filtration, a professional HVAC technician might be able to install a special air cleaner that can handle the increased resistance.

How a Bypass HEPA System Works

A bypass HEPA system is a standalone unit that is plumbed (ducted) into a home's or building's main HVAC system. It has its own dedicated blower motor and a multi-stage filtration process to clean the air.

1. Diverting Air: The system connects to the main return air plenum (the large duct that pulls air from the house back to the air handler). A portion of the return air is diverted, or "bypassed," into the HEPA unit's separate housing.

2. Multi-Stage Filtration: Inside its housing, the air passes through a series of filters, which typically includes:

   • A pre-filter (often a MERV 8 or 10) to trap larger particles like dust, pollen, and pet dander. This protects and extends the life of the expensive HEPA filter.

   • An activated carbon filter to absorb odors, gases, and volatile organic compounds (VOCs).

   • The HEPA filter, which is the core of the system. It's a dense filter that is 99.97% efficient at capturing airborne particles as small as 0.3 microns in size.

3. Returning Clean Air: After passing through the filters, the now-cleaned air is returned back into to the main air handlers' return plenum or, in some cases, the supply plenum (the duct that sends conditioned air into the rooms). This ensures the purified air is mixed and distributed throughout the entire duct system.

This bypass configuration allows the HEPA unit's powerful, separate blower to overcome the significant airflow resistance of the HEPA media without putting any strain on the central HVAC blower.

Why a Bypass System is Essential

An air handler's blower motor is engineered to handle the specific airflow resistance (measured in static pressure) of a standard filter and the connected ductwork. Using a high-efficiency filter like a HEPA filter directly in the return air system would be problematic for several reasons:

• High Static Pressure: HEPA filters are very dense, creating a very high-pressure drop. This increased resistance would force the air handlers' blower motor to work much harder.

• Reduced Airflow: The high resistance would significantly reduce the airflow (CFM) across the heating and cooling coils. This can lead to a host of performance issues, such as a frozen air conditioner coil in the summer or a cracked heat exchanger in the winter, both of which are serious and costly problems.

• Inefficient Operation: A struggling blower motor would consume more energy and would not be able to effectively heat or cool the home, leading to higher energy bills and reduced comfort.

By having its own housing, filters, and blower, a bypass HEPA system operates independently. It can be run continuously, 24/7, or with a remote controller that monitors the indoor air quality of the space, to provide whole-house air filtration without ever engaging the main air handlers' heating or cooling functions. This allows for constant air purification while the central HVAC system cycles on and off as needed for temperature control, providing the best of both worlds: superior air quality and protected HVAC equipment.

How They Work

Like the whole house HEPA, they use a smaller blower to pull air across a filter pack including a HEPA module, but without connected ductwork.

Standalone vs. Whole-House HEPA

A standalone air purifier is designed for a single room, so its effectiveness is limited to the space it's in. To get a similar level of filtration throughout a house, you would need to buy multiple units and place them in each room. Whole-house HEPA systems, on the other hand, are installed directly in a home's ductwork. This setup allows them to filter all the air that circulates through the HVAC system, providing a more comprehensive solution.

Advantages of Standalone Units

• Flexibility & Cost: Standalone units are much more affordable upfront and can be moved from room to room as needed. This flexibility allows you to target areas with higher concentrations of pollutants, like a bedroom for a person with allergies.

• Simple Installation: They require no professional installation—you just plug them in.

• Dedicated Filtration: They can continuously clean the air in a single room without relying on the HVAC system to be running.

Disadvantages of Standalone Units

• Limited Coverage: A single unit is not designed to filter the air in an entire home. Pollutants from a dusty basement or a kitchen would not be filtered by a unit in a bedroom.

• Maintenance: Each unit has its own filter that must be regularly cleaned or replaced, which can be a recurring cost.

• Aesthetics & Noise: Multiple units can take up floor space and generate noise, which may be disruptive.

While a whole-house HEPA system offers a more comprehensive approach, portable HEPA air purifiers are an effective and practical alternative for improving air quality in specific areas of a home.