HEPA Bypass Systems for HVAC: How They Work and Whether You Need One

Why HEPA does not go in the furnace slot

HEPA filters are rated at 99.97% efficiency for 0.3 micron particles, and EPA says HEPA-designated home filters usually perform comparably to MERV 16, though there is no widely accepted definition of HEPA in consumer products. EPA's technical summary says existing residential systems may not have enough fan capacity for higher pressure-drop filters, which can reduce airflow until heating or cooling capacity is lost and raise the risk of component failure. A HEPA filter in a standard 1-inch slot is the extreme case.

How a bypass unit works

Instead of forcing all the system's air through HEPA media, a bypass unit connects to the return duct, draws a portion of the air through its own pre-filter and HEPA filter with its own blower, and returns it upstream. The main furnace filter stays in place. Fantech's HERO HS300 manual describes:

From Fantech's HERO HS300 installation and operation manual.

Spec	Figure
Mounting	Return air plenum, or wall or floor with ducting
Airflow	208 cfm low, 280 cfm high
Power	134 W, 120 VAC
Filters	Pre-filter with carbon, then HEPA
Furnace filter	Not replaced; still needs regular maintenance
Installation	Recommended by a professional contractor

What it costs to run

At 134 watts, running a unit like the HS300 continuously would use 134 x 24 x 365 / 1,000 = about 1,174 kWh a year, or about $202 at the EIA's 2025 US average of 17.2 cents per kWh. Running it only when the furnace fan runs would cost less but filter less. Add replacement HEPA and pre-filters to the total, and the installation itself.

Is it better than MERV 13?

On paper, HEPA captures more of the smallest particles than MERV 13's minimum of 50% for 0.3 to 1 micron. In practice, EPA's technical summary says runtime limits any in-duct filter, and that experimental data and predictions indicate most MERV 13 to 16 filters are likely to be almost as effective as HEPA at reducing indoor particle concentrations. EPA also notes that field studies have not yet confirmed that central systems run long enough to improve health outcomes. A bypass unit with its own fan can run independently of heating and cooling, which is its real advantage.

Options for better whole-house particle filtration, based on EPA's technical summary and the products named.

Option	Particle capture	Install	Airflow risk to the system
1-inch MERV 13	At least 50% of 0.3 to 1 µm	None	Some systems
4-inch MERV 13 media cabinet	At least 50% of 0.3 to 1 µm	Ductwork change	Lower, thanks to more media
Bypass HEPA unit	HEPA on the share of air it treats	Professional	Uses its own fan
Portable HEPA purifier	HEPA in one room	None	None

When a bypass HEPA makes sense

• Someone in the home has a medical reason for the highest filtration and the household wants a whole-house solution, after talking to their doctor.
• The HVAC system cannot take MERV 13 and a media cabinet is not an option.
• You want filtration that runs independently of heating and cooling.

When it does not

• Your system handles MERV 13: EPA says that gets close to HEPA results.
• The concern is one or two rooms: a portable HEPA purifier sized by smoke CADR is cheaper and runs continuously. See the best air purifiers.
• Your home has no central ducted system.

Installation notes

Fantech recommends installation by a professional contractor familiar with indoor air systems, and says the unit must be installed in conformance with local codes. EPA's technical summary says in-duct air cleaners should be installed so that bypass airflow around filters is prevented, and that ducts should be well sealed where filters sit at return grilles. Ask the installer to check that the main system's airflow is unchanged afterward.

For deep-media alternatives, see the best whole-house air filters; for the MERV scale, MERV ratings explained; and for the upgrade decision, is a MERV 13 upgrade worth it?