HVAC · Lesson 7 of 13Intermediate9 min read

Ducts, Airflow and Filters

The furnace or heat pump makes the heating and cooling; the blower and ducts deliver it. A great furnace on bad ducts still leaves rooms too hot, too cold or dusty, and wastes a good share of the energy you pay for.

A loop of air

A forced-air system moves air in a loop. Return grilles pull room air into the return ducts. The air passes through the filter, then the blower, then across the furnace’s heat exchanger or the cooling coil. It leaves through the supply plenum, a box on top of the unit, and travels down a trunk duct that feeds smaller branch ducts. Each branch ends at a register in a room, often with adjustable louvers. Then the air finds its way back to a return grille, and the loop repeats.

Every bit of air pushed into the rooms has to come back. If a room gets plenty of supply air but has no easy way back to a return, for example when its door is closed, it can end up stuffy, and the system moves less air overall.

A forced-air duct system. Every room’s supply air has to find its way back to a return. Sources: ENERGY STAR: Duct Sealing; U.S. Department of Energy, Energy Saver: Air-Source Heat Pumps.

How much air

A cooling coil or heat pump needs a steady stream of air to work as designed. The Department of Energy puts it at about 400 to 500 cubic feet per minute (cfm) for each ton of capacity, so a 3-ton system moves roughly 1,200 to 1,500 cfm. Too little air hurts: ENERGY STAR says airflow problems can reduce a system’s efficiency by up to 15 percent, and poor airflow can leave the air overly damp. DOE notes that technicians can raise the airflow by cleaning the indoor coil or increasing the fan speed.

Static pressure: what the blower pushes against

The blower makes a small pressure difference that pushes air through the filter, the coil, the ducts and the registers. Each of them resists the flow, and technicians measure the total resistance as static pressure, in inches of water column. One inch of water is about 249 pascals, about a quarter of one percent of the air pressure around you, which shows how gentle these pressures are.

Resistance adds up along the path. A clogged filter, a dirty coil, crushed flexible duct, closed registers and blocked return grilles all raise it, and the blower then moves less air. The widget below shows the effect.

An illustrative model, not a measurement of any real system: pressure losses grow with the square of the airflow, and the blower pushes less air against more resistance. Real numbers depend on the blower, the ducts and the filter. The 400 to 500 cfm per ton band is from the Department of Energy.

Filters and MERV ratings

The filter protects the blower and coil from dust and cleans the air you breathe, but only while the fan is running. Filters are rated by MERV, the Minimum Efficiency Reporting Value, from a national consensus test (ASHRAE Standard 52.2) that reports values from MERV 1 to MERV 16. A higher MERV removes more of the small particles. Flat panel filters typically rate MERV 1 to 4; pleated filters typically rate MERV 5 to 13 or higher, and only MERV 13 and above must catch at least half of the smallest particles in the test (0.3 to 1 micrometer).

EPA suggests choosing at least a MERV 13 filter, or the highest-rated filter your system’s fan and filter slot can handle, and asking an HVAC technician to be sure it will work, because a denser filter can resist airflow more. It should fit snugly so air doesn’t sneak around it. EPA says manufacturers typically recommend replacing filters every 60 to 90 days; ENERGY STAR says to check the filter every month and change it at least every three months, or sooner if it looks dirty.

Leaky ducts

ENERGY STAR estimates that in a typical house, about 20 to 30 percent of the air moving through the ducts is lost through leaks, holes and poorly connected ducts. When those ducts run through an attic, crawlspace or garage, the heated or cooled air is simply lost outside the living space. Leaky return ducts can also pull in dusty or humid air from those spaces. Signs include high bills, rooms that are hard to heat or cool, and tangled or kinked flexible ducts.

ENERGY STAR says sealing ducts can improve a system’s efficiency by as much as 20 percent. It recommends sealing leaks with mastic sealant or metal tape, never cloth “duct tape,” which doesn’t last, and insulating the ducts you can reach in attics, crawlspaces, unfinished basements and garages. A contractor can test the whole system for leaks and repair the ones out of reach.

Balancing and zoning

Balancing means adjusting how much air each room gets. Many branch ducts have a manual damper, a metal plate inside the duct turned by a lever on the outside, and registers have louvers. A zoned system goes further, with motorized dampers and a thermostat for each zone, so the upstairs can cool while the downstairs rests. Ductless mini-splits are zoned by design.

Keep supply registers and return grilles open and clear of furniture, rugs and curtains. Every closed or blocked opening adds resistance for the whole system, as the widget shows.

Air distribution parts and what they do
Part	What it does	Signs of trouble
Blower	The fan in the furnace or air handler that moves all the air.	Weak airflow everywhere; squealing or grinding.
Filter	Catches dust before the blower and coil.	Gray and caked; airflow drops; ice on the coil in summer.
Supply plenum and trunk	Carry conditioned air from the unit to the branches.	Leaks at seams; sweating in summer.
Branch ducts	Smaller ducts, rigid or flexible, to each room.	Crushed, kinked or disconnected runs; weak flow in one room.
Registers	Grilles with louvers where air enters each room.	Closed, blocked or dusty.
Return grilles and ducts	Bring room air back to the blower.	Blocked by furniture; whistling; dust streaks.
Dampers	Plates inside ducts that balance airflow between rooms or zones.	Rooms that never get enough air.
Duct insulation	Slows heat flow and prevents condensation on ducts in attics and crawlspaces.	Torn or missing; sweating ducts.

You canWhat you can safely do

Check the filter monthly and change it on schedule (Change a Furnace Air Filter).
Open and clear every register and return grille.
Look at the ducts you can reach for disconnected joints, crushed flex duct and torn insulation. ENERGY STAR says you can seal accessible joints with mastic or metal tape and insulate ducts in unconditioned spaces.

StopWhen to call a pro

Weak airflow everywhere with a clean filter, or a blower that squeals or grinds: blower and coil work is inside the equipment.
Before moving to a much denser filter, ask a technician whether your blower can handle it, as EPA advises.
Duct design, leakage testing and new duct runs. Ducts connected to fuel-burning equipment also affect how its exhaust vents, so changes there are a job for a pro.

Check yourself

Tap an answer to check it, or open “Show the answer.”

What does the return duct system do?
Show the answer
Brings room air back to the blower, through the filter. Return grilles and ducts carry room air back to the blower to be filtered, heated or cooled, and sent out again.
About how much of the air in a typical duct system is lost to leaks, according to ENERGY STAR?
Show the answer
20 to 30 percent. About 20 to 30 percent, through leaks, holes and poorly connected ducts.
Which is NOT recommended for sealing duct leaks?
Show the answer
Cloth duct tape. ENERGY STAR says never use duct tape; it doesn’t last.
Why should you check with a technician before switching to a much higher-MERV filter?
Show the answer
A denser filter can resist airflow more than your blower can handle. EPA advises using the highest MERV your fan and filter slot can accommodate, and consulting a pro to be sure.
DOE says a heat pump or air conditioner needs about how much airflow per ton?
Show the answer
400 to 500 cfm. About 400 to 500 cubic feet per minute for each ton of capacity.

Go deeper

ENERGY STAR: Duct Sealing ENERGY STAR on finding and sealing duct leaks.
U.S. EPA: Guide to Air Cleaners in the Home EPA’s consumer guide to air cleaners and furnace filters.
U.S. EPA: Residential Air Cleaners: A Technical Summary, 3rd edition (2018) EPA’s technical summary, with how MERV is measured.
Being an Adult 101: Change a Furnace Air Filter Step-by-step filter change.

Sources

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A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.