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Routine

Weak airflow from the vents

When air barely comes out of the vents, the cause is somewhere between the blower and the register: a clogged filter or coil, a blower problem, or ductwork that is crushed, disconnected, leaking or too small. If every vent is weak, start at the filter and the air handler; if only some are, the problem is usually in the ducts that feed them.

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New air handler installed in a hallway closet with louvered doors

Likely causes, most common first

Pick one to see how likely it is, who can deal with it, and what it means.

Choose a likely cause

Common

You can check this

Clogged air filter

The filter is the first restriction air meets. A loaded filter cuts airflow to every room at once.

Air filtration

Common

Technician fix

Crushed, kinked or disconnected flex duct

Flex runs in the attic get stepped on, buried under storage, pinched around framing or pulled off their collars. Rooms on those runs get a trickle of air, and a disconnected run cools the attic instead.

Duct repair

Common

Technician fix

Dirty coil or blower wheel

Dust and growth on the indoor coil or packed into the blower wheel's blades choke the whole system.

AC Maintenance

Occasional

Technician fix

Leaking joints and plenums

Air escaping at loose connections, torn duct jackets or a deteriorated plenum never reaches the rooms.

Plenums

Occasional

Technician fix

Undersized ducts or too little return air

If the duct system is too small for the equipment, static pressure rises and airflow drops everywhere. Too few or too small returns are a common cause.

Airflow & duct design

Occasional

Technician fix

Blower motor or capacitor problem

A blower running slowly because of a weak capacitor or a failing motor moves less air even with clean ducts.

AC Repair

Follow the air from filter to vent

Weak airflow is a restriction or a leak somewhere along this path.

How a ducted AC system moves airSchematic of a house cross-section: the air handler pushes cooled air up into a supply plenum, through a trunk line and flexible duct runs in the attic to ceiling registers in each room. Air returns through a return grille and return duct back to the air handler.ATTICAIRHANDLER12345
Parts of the diagram

A clogged filter is the most common restriction and the easiest to fix. A dirty coil or blower wheel, or a blower motor that's slowing down, has the same effect.

More on air handler, filter and coil

Air has to come back before it can go out. Too few or too small returns, or furniture over the grille, choke the whole system.

More on return path

Leaking joints at the plenum lose air before it reaches any room, and the loss shows up at every vent.

More on supply plenum

Crushed, kinked or disconnected flex in the attic starves the rooms it serves. In hot attics the runs sag and flatten over time.

More on flex duct runs

An undersized or leaky trunk shortchanges the rooms at its far end first.

The measurement that settles it

Static pressure tells a technician whether the system is fighting a restriction, and roughly where.

Total external static pressure

in. w.c.

The resistance the blower is pushing against, compared with the maximum on the air handler's rating plate. Above it, delivered airflow drops.

Supply vs. return static

in. w.c.

Split into its two sides, it shows whether the restriction is on the intake side (filter and returns) or the supply side.

Temperature split

°F

With too little air, the coil gets colder and the split widens, and eventually the coil can freeze.

Airflow at each register

CFM

Separates a whole-system problem from one weak run.

ENERGY STAR estimates that airflow problems can reduce a system's efficiency by up to 15 percent.

When to stop and call

Turn the system off and call a technician if:

  • There's ice on the refrigerant lines or the coil. Turn cooling off and run the fan to thaw it.
  • A burning smell from the air handler or a vent.
  • The blower hums but doesn't move air.
  • Water dripping from ceiling grilles or ductwork.

Checks you can safely do first

Tick them off as you go. Don't walk on the ceiling between framing in the attic.

Done all of these and it’s still not right?

That points to a part that needs a technician’s meter, not another reset.

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What our visit looks like

A technician measures the system's static pressure and the airflow at registers, inspects the blower and coil, then traces the ductwork in the attic for crushing, disconnects and leaks. The technician explains whether a repair fixes it or whether part of the duct system needs to be redesigned.

If it turns out to be the ductwork

Planning a duct repair or replacement? See a rough range for the project in a couple of minutes, then we confirm it on site.

The on-site duct evaluation is free.

Estimate My Ductwork Project

Questions

Will closing vents in unused rooms send more air to the others?

Usually not much, and it can make things worse. Closing registers raises the pressure the blower works against, which can reduce total airflow, freeze the coil and make ducts leak more. It's better to fix the duct or airflow problem itself.

What is static pressure, and why does it matter for airflow?

It's the combined resistance of the filter, coil and ductwork that the blower has to push against. When it's too high, the blower moves less air than the system was designed for. Measuring it is how a technician tells whether weak airflow comes from the ducts or from the equipment.

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Florida license CAC1821225

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